4-Carboxyanilinium dihydrogen phosphate monohydrate, an organophosphate adducts of 4-amino benzoic acid: Structural, vibrational, thermal, and computational studies


European Journal of Chemistry 15 (1) (2024) 1-16 

 

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4-Carboxyanilinium dihydrogen phosphate monohydrate,  
an organophosphate adducts of 4-amino benzoic acid:  
Structural, vibrational, thermal, and computational studies 
 
Lata Panicker   *  
 

Protein Crystallography Section, Bioscience Group, Bhabha Atomic Research Center, Trombay, Mumbai 400085, India 
 
* Corresponding author at: Protein Crystallography Section, Bioscience Group, Bhabha Atomic Research Center, Trombay, Mumbai 400085, India.  
e-mail: lata@barc.gov.in (L. Panicker). 

 
 

  

   

   

 
 10.5155/eurjchem.15.1.1-16.2484 

 
Received: 22 October 2023 
Received in revised form: 10 December 2023 
Accepted: 26 December 2023 
Published online: 31 March 2024 
Printed: 31 March 2024 

 
4-Carboxyanilinium dihydrogen phosphate monohydrate (4-CAH2PO4·H2O), an 
organophosphate adduct, was synthesized and characterized by single-crystal X-ray 
diffraction, Fourier transform infrared (FTIR), Differential scanning calorimetry (DSC) and 
computational analysis performed using CrystalExplorer 21, Gaussian 09W and Multiwfn 
3.7 software. The complex 4-CAH2PO4·H2O crystallized in the triclinic space group, P-1, with 
two molecules each of 4-carboxyanilinium (4-CA) cations, H2PO4– anions, and water, 
respectively, in an asymmetric unit. Crystal data for C7H12NO7P: triclinic, space group P-1, 
a = 8.5238(2) Å, b = 8.9068(2) Å, c = 14.4976(4) Å, α = 106.456(2)°, β = 90.195(2)°, γ = 
92.811(2)°, V = 1054.13(5) Å3, Z = 4, T = 293 K, μ(Cu Kα) = 2.587 mm-1, Dcalc = 1.595 g/cm3, 
18182 reflections measured (6.358° ≤ 2Θ ≤ 146.396°), 4149 unique (Rint = 0.1018, Rsigma = 
0.0521) which were used in all calculations. The final R1 was 0.0584 (I > 2σ(I)) and wR2 was 
0.1712 (all data). The organic layer containing 4-CA cations and the inorganic layer 
containing phosphate anions and water molecules in 4-CAH2PO4·H2O crystals are connected 
through a three-dimensional network of strong charge-assisted N–H···O and C-OH···O 
hydrogen bonds. The fingerprint plot of 4-CAH2PO4·H2O obtained indicated that the most 
prominent interaction corresponds to the short O···H contact, followed by the H···H and H···C 
contacts. The intermolecular interaction topology of 4-CAH2PO4·H2O has been quantitatively 
analyzed. The 4-CAH2PO4·H2O complex was optimized by density functional theory (DFT) 
with B3LYP/6-31G basis set and the theoretical IR vibrational spectra determined. The 
noncovalent interaction (NCI) and quantum theory of the atom in the molecule (QTAIM) 
analysis were done using Multiwfn 3.7 software. 4-CAH2PO4·H2O complex structure and its 
computational analysis are also compared with that of 4-carboxyanilinium dihydrogen 
phosphate (4-CAH2PO4). 

 
DSC 
FTIR 
DFT 
X-ray diffraction 
Single crystal structure 
4-Carboxyanilinium dihydrogen phosphate 
monohydrate 

Cite this: Eur. J. Chem. 2024, 15(1), 1-16 Journal website: www.eurjchem.com 

 
1. Introduction 
 

The organic-inorganic hybrid compounds due to their 
interesting properties and potential applications such as 
electric, magnetic, non-linear optical, ferroelectric, and catalytic 
properties have attracted the scientific community [1-3]. The 
organic-inorganic hybrid compounds have the combined 
properties of organic and inorganic compounds within one 
single compound, giving rise to interesting crystal structures 
and properties. The different components of these hybrid 
crystals are held together by a strong hydrogen bond, van der 
Waals interaction, and most likely electrostatic interactions. 
The synthesis of cocrystals of important pharmaceutical 
compounds has led to a significant improvement in its solubility 
and bioavailability [4,5]. Phosphate being biocompatible, ionic 
cocrystals of organophosphates have been used in drug 
products. Dihydrogen phosphate salt was found to exhibit 
superior stability compared to its free base [6]. 

Benali-Cherif et al. [7,8] has reported the crystal structure 
of p-carboxyphenylammonium dihydrogenmonophosphate 

monohydrate and 4-carboxyanilinium dihydrogen phosphate, 
respectively. 4-Carboxyanilinium dihydrogen phosphate (4-
CAH2PO4) crystalizes in space group monoclinic, P21/n, Z = 4 
and the asymmetric unit contains one 4-carboxyanilinium 
cation and one dihydrogen phosphate anion [8]. In the crystal 
structure, each p-carboxyanilinium cation is connected to five 
dihydrogen monophosphate anions via hydrogen bonds. As 
expected, the O atoms of H2PO4– act as proton acceptors or as 
proton donors, giving rise to a three-dimensional network. The 
H2PO4 tetrahedra are linked together in pairs by strong O-H···O 
hydrogen bonds. Thus, the three-dimensional hydrogen 
bonding framework stabilizes the crystal structure of 4-
CAH2PO4.  

The synthesis, characterization, and computational analysis 
of the 4-carboxyanilinium dihydrogen phosphate monohydrate 
(4-CAH2PO4·H2O) as an organophosphate adduct is presented 
in this article. The single crystal structure, thermal parameters, 
and vibrational frequency of 4-CAH2PO4·2H2O were determined 
using single crystal X-ray diffraction, DSC, and FTIR spectros-
copy, respectively, and the results are reported.  

ABSTRACT RESEARCH ARTICLE 

KEYWORDS 

https://dx.doi.org/10.5155/eurjchem.15.1.1-16.2484
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2 Lata Panicker / European Journal of Chemistry 15 (1) (2024) 1-16 
 

 
2024 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.15.1.1-16.2484 

 
Table 1. Crystal data and structure refinement parameters for 4-CAH2PO4·H2O at 293 K. 
Empirical formula C7H12NO7P  
Formula weight (g/mol) 253.15  
Temperature (K) 293  
Crystal system Triclinic  
Space group P-1  
a, (Å) 8.5238(2)  
b, (Å) 8.9068(2)  
c, (Å) 14.4976(4)  
α (°) 106.456(2)  
β (°) 90.195(2)  
γ (°) 92.811(2)  
Volume (Å3) 1054.13(5)  
Z 4  
ρcalc (g/cm3) 1.595  
μ (mm-1) 2.587  
F(000) 528.0  
Crystal size (mm3) 0.6 × 0.4 × 0.2  
Radiation Cu Kα (λ = 1.54184)  
2Θ range for data collection (°) 6.358 to 146.396  
Index ranges -10 ≤ h ≤ 10, -11 ≤ k ≤ 10, -17 ≤ l ≤ 17  
Reflections collected  18182  
Independent reflections  4149 [Rint = 0.1018, Rsigma = 0.0521]  
Data/restraints/parameters  4149/0/304  
Goodness-of-fit on F2  1.061  
Final R indexes [I≥2σ (I)]  R1 = 0.0584, wR2 = 0.1647  
Final R indexes [all data]  R1 = 0.0624, wR2 = 0.1712  
Largest diff. peak/hole (e.Å-3) 0.66/-0.43  

 

O

OH

H2N

P

O

H2O

OH

OH
+H3N

O

OH

P
O OH

OH-
O

H
O

H
+

H2O

 
 

Scheme 1. Synthesis of 4-carboxyanilinium dihydrogen phosphate monohydrate (4-CAH2PO4·H2O). 
 
Computational analysis obtained using CrystalExplorer 21 

is also reported. Density functional theory (DFT) calculation of 
4-CAH2PO4·H2O in gas phase was carried out with the B3LYP/6-
31G basis set. The calculation of noncovalent interaction (NCI) 
and quantum theory of atom in a molecule (QTAIM) was done 
using Multiwfn 3.7 software. 4-CAH2PO4·H2O complex structure 
and its computational analysis are also compared with that of 
4-carboxyanilinium dihydrogen phosphate (4-CAH2PO4). 
 
2. Experimental  
 
2.1. Crystal growth  
 

4-Carboxyanilinium dihydrogen phosphate monohydrate 
(4-CAH2PO4·H2O) single crystals were obtained by slow 
evaporation of the aqueous solution containing, 1:1 molar ratio 
of 4-amino benzoic acid (4-ABA) and phosphoric acid (H3PO4) 
(Scheme 1). The clear solution obtained by dissolving the 
ingredient was left undisturbed at 20 °C. Most of the time, first 
4-carboxyanilinium dihydrogen phosphate (4-CAH2PO4) 
crystals were obtained [8]. The 4-CAH2PO4 crystals were 
removed by filtering and the remaining solution was left 
undisturbed. Plate shaped single crystals of 4-CAH2PO4·H2O 
were obtained. Partially deuterated 4-CAH2PO4·H2O (i.e. d4-
CAH2PO4.H2O) and 4-ABA (i.e. d4-ABA) wear obtained by re-
crystallizing 4-CAH2PO4·H2O and 4-ABA, respectively, in D2O at 
least three times. In a partially deuterated compound, hydrogen 
atoms in the -NH2, -NH3+, H2PO4– and -COOH groups were 
replaced by deuterium atoms (-ND2, -ND3+, D2PO4– and -COOD). 
Almost 100% partial deuteration of 4-CAH2PO4·H2O and 4-ABA 
was achieved, as confirmed by the respective FTIR spectra. 

Partially deuterated compounds were synthesized for FTIR 
investigation.  FTIR spectra of the deuterated compounds (d4-
ABA and d4-CAH2PO4·H2O) were useful in assigning functional 
groups (-NH2 -NH3+, H2PO4– and -COOH) in 4-CAH2PO4·H2O. 
 
2.2. Single-crystal X-ray diffraction 
 

Single crystal X-ray diffraction data were collected at 
ambient temperature (293±2 K) on an Agilent Super Nova 
diffractometer equipped with a Titan CCD detector using a 
microfocus X-ray source, CuKα radiation of wavelength 
1.54184 Å. Diffraction data were collected at a crystal-to-
detector distance of 61 mm with scan width (ω) 1° oscillation 
per frame. Data collection, indexing, cell refinement, data integ-
ration, and reduction were carried out using the CrysAlisPro 
program [9]. The crystal structure was solved by the direct 
method using ShelXT [10], completed by difference Fourier 
syntheses, and refined by full-matrix least squares method 
using ShelXL [11] accessed by the Olex2 package [12]. The non-
H atoms were refined anisotropically. Hydrogen atoms were 
fixed in their calculated positions. The crystallographic data and 
refinement details are given in Table 1. 

 
2.3. Differential scanning calorimetry  
 

Mettler Toledo DSC 822 was used for thermal measure-
ments of the samples, with an empty aluminum pan as a 
reference. Temperature and enthalpy calibration of the 
instrument was done using cyclohexane and indium. The 
reported transition temperature, Tc, is the peak temperature.  

 



Lata Panicker / European Journal of Chemistry 15 (1) (2024) 1-16 3 
 

 
2024 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.15.1.1-16.2484 

 

 
 

Figure 1. The asymmetric unit of the 4-carboxyanilinium dihydrogen phosphate monohydrate (4-CAH2PO4·H2O) complex with the atom labeling scheme and 
hydrogen bond is shown as a dashed line. Displacement ellipsoids are drawn at 50% probability level and H atoms are shown as small spheres of arbitrary radius. 

 
The transition enthalpy of the endothermic curve was 

calculated using the Mettler Toledo software. The expected 
experimental errors in temperature and enthalpy values were 
±0.1 °C and ±5%, respectively. For measuring the low tempe-
rature transition, 25 to 30 mg of crystalline sample was taken, 
and for the crystal-to-isotropic melting, 4-5 mg of sample was 
used. The DSC experiments were repeated three times on the 
same sample to check the reversibility of the transitions, and 
fresh samples from different syntheses were also repeated to 
check the reproducibility of the DSC data.  
 
2.4. Fourier transform IR spectroscopy 
 

IR spectra were recorded in wavenumber regions of 4000 
to 650 cm-1 using a VERTEX 70 FTIR spectrometer (Bruker 
Optik GmbH, Germany) equipped with a BioATR cell II (Bruker 
Optik GmbH, Germany), which contains a multireflection silicon 
crystal. The DLaTGS detector was used to measure the 
vibrational bands. The spectral resolution was set to 4 cm−1, and 
for each spectrum 200 scans were recorded. The surface of the 
ATR crystal was cleaned with distilled water and isopropyl 
alcohol before loading the sample. The powder sample was 
placed on the silicon crystal and sealed with a lid. The 
temperature of the silicon crystal in the BioATR cell II assembly 
was controlled using an external water bath, Huber Ministate 
125 (Peter Huber, Kaltemaschinenbau, Germany). The sample 
was equilibrated at 298 K for 10 minutes before recording the 
spectrum. The sample chamber was flushed with dry air during 
the experiments. The spectrum of air was used as the 
background prior to each sample analysis. The FTIR spectra 
recorded were analyzed using OPUS software. 
 
2.5. Hirshfeld surface analyses  
 

CrystalExplorer 21 software [13-15] was used to analyze 
and visualize Hirshfeld surfaces (HS) [16] and their relative 2D 
fingerprint plots [17] using the final CIF file of 4-CAH2PO4·H2O. 
The normalized contact distance, dnorm, and 2D fingerprint plots 
were used to quantify, visualize, and decode the intercontact in 
the crystal packing. The dark-red spots on the dnorm surface 
arise as a result of the short interatomic contacts, while the 
other intermolecular interactions appear as light-red spots. The 
regions with blue color on the dnorm represent longer inter-
contacts, while the white color indicates the contacts around 
the van der Waals radius. HS analysis has become a very useful 
tool for explaining the nature of intermolecular interactions 
that affect the packing of molecules in crystals [15]. The 
parameters used in 2D fingerprint plots represent the distances 

to the Hirshfeld surface from the nuclei (di (inside) and de 
(outside)), with respect to the relative van der Waals radius 
(vdW). Two-dimensional fingerprint plots provide relevant 
information about the intermolecular contacts in the crystal. 
The dnorm surface was mapped with the color scale in the range 
-0.8010 a.u. (red) to 1.0648 a.u. (blue). The 2D fingerprint plots 
(di vs. de) were displayed using the expanded 0.6-2.6 Å range. 
 
2.6. 3D energy framework analysis  
 

CrystalExplorer 21 software [13,14] was used to calculate, 
visualize, and analyze the 3D energy framework along with the 
energies of intermolecular interaction in 4-CAH2PO4·H2O. The 
intermolecular interaction energies for the energy framework 
analysis have been estimated using the quantum level of CE-
HF/3-21G theory, as available in CrystalExplorer 21 by 
generating a cluster of 3×3×3 unit cell around the molecule. The 
density matrices for the neighboring molecules are generated 
by applying crystallographic symmetry operations to the 
density matrix of the central molecule [18,19]. For each compo-
nent of the interaction energy, the scale parameters have been 
optimized.  
 
2.7. Computational details for quantum chemical 
calculations 
 

In order to carry out the theoretical computation analysis, 
the molecular geometry of 4-CAH2PO4·H2O is taken from the 
crystallographically obtained structural data. Theoretical 
calculations were performed using the Gaussian 09W software 
package [20] and GaussView6 molecular visualization program 
[21]. The calculations were performed using density functional 
theory (DFT) with a hybrid function B3LYP (Becke’s three-
parameter hybrid functional using the LYP correlation 
functional) [22,23] at the 6-31G basis set in the gas phase. The 
compound, 4-CAH2PO4·H2O, was first optimized geometrically, 
and this optimized geometry was used to perform further 
calculations. The vibrational spectra, chemical reactivity, 
HOMO and LUMO orbital energies, molecular electrostatic 
potential, and thermodynamic properties of 4-CAH2PO4.H2O 
were also calculated using DFT. 
 
2.8. Non-covalent interaction and quantum theory of atom 
in molecule analysis  
 

The NCI and QTAIM analysis were performed with the 
DFT/B3LYP/6-31G optimized 4-CAH2PO4·H2O complex using 
Multiwfn 3.7 software [24].  



4 Lata Panicker / European Journal of Chemistry 15 (1) (2024) 1-16 
 

 
2024 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.15.1.1-16.2484 

   
Table 2. Bond lengths and angles for 4-carboxyanilinium dihydrogen phosphate monohydrate. 
Atom Atom  Length (Å)  Atom Atom  Length (Å) 
P1A O6A  1.5072(13)  C5 C4  1.379(3) 
P1A O5A  1.5703(15)  C5 C6  1.391(3) 
P1A O3A  1.5063(15)  C1 C2  1.489(3) 
P1A O4A  1.5695(15)  C2 C7  1.393(3) 
P1 O6  1.5086(13)  C2 C3  1.393(3) 
P1 O5  1.5689(15)  C2A C1A  1.493(3) 
P1 O3  1.5066(14)  C2A C3A  1.382(3) 
P1 O4  1.5712(15)  C2A C7A  1.393(3) 
O2 C1  1.305(3)  C5A C6A  1.383(3) 
O1 C1  1.221(3)  C5A C4A  1.379(3) 
O2A C1A  1.307(2)  C7 C6  1.382(3) 
N1 C5  1.456(2)  C4 C3  1.383(3) 
O1A C1A  1.215(3)  C3A C4A  1.383(3) 
N1A C5A  1.461(2)  C6A C7A  1.380(3) 
Atom Atom Atom Angle (°)  Atom Atom Atom Angle (°) 
O6A P1A O5A 106.05(8)  C3 C2 C1 121.42(18) 
O6A P1A O4A 108.27(9)  C3 C2 C7 119.36(18) 
O3A P1A O6A 114.79(9)  C3A C2A C1A 121.89(18) 
O3A P1A O5A 110.65(9)  C3A C2A C7A 119.56(18) 
O3A P1A O4A 109.95(8)  C7A C2A C1A 118.54(18) 
O4A P1A O5A 106.77(9)  C6A C5A N1A 118.78(17) 
O6 P1 O5 106.05(8)  C4A C5A N1A 119.90(18) 
O6 P1 O4 108.04(9)  C4A C5A C6A 121.32(19) 
O5 P1 O4 106.83(9)  C6 C7 C2 120.81(18) 
O3 P1 O6 115.04(9)  C5 C4 C3 119.53(18) 
O3 P1 O5 110.21(9)  C7 C6 C5 118.71(18) 
O3 P1 O4 110.27(8)  O2A C1A C2A 114.17(18) 
C4 C5 N1 119.37(17)  O1A C1A O2A 124.39(19) 
C4 C5 C6 121.33(18)  O1A C1A C2A 121.43(18) 
C6 C5 N1 119.29(17)  C2A C3A C4A 120.32(18) 
O2 C1 C2 114.40(17)  C7A C6A C5A 118.90(19) 
O1 C1 O2 124.58(19)  C4 C3 C2 120.23(18) 
O1 C1 C2 121.02(19)  C5A C4A C3A 119.34(19) 
C7 C2 C1 119.21(17)  C6A C7A C2A 120.53(19) 
  
Table 3. Inter-atomic bond distances (Å) and angles (°) in 4-CAH2PO4·H2O. 
D-H···A d(D-H) d(H···A) d(D···A) <D-H··A  
O4-H4B···O5  0.82 1.80 2.555(3)  152.7  1+x, +y, +z 
O4A-H4AA···O5A 0.82 1.78 2.544(3) 154.6 1+x, +y, +z 
N1-H1A···O5A 0.89 1.88 2.767(3) 171.6 1-x, 3-y, -z 
N1-H1B···O3 0.89 1.89 2.771(3) 168.6 - 
N1A-H1AC···O3A 0.89 1.92 2.786(3) 164.2 1-x, 3-y, -z 
N1A-H1AD···O5  0.89 1.87 2.754(3) 173.2 1+x, +y, +z 

 
3. Results and discussions 
 
3.1. Single crystal X-ray diffraction 
 

The single crystal structure of 4-carboxyanilinium 
dihydrogen phosphate monohydrate (4-CAH2PO4·H2O) was 
obtained at 293 K. The bond distance and angles of 4-
CAH2PO4·H2O are given in Table 2. The selected distances and 
angles of the interatomic bonds of 4-CAH2PO4.H2O are given in 
Table 3. The ORTEP drawing of the asymmetric unit of 4-
CAH2PO4·H2O with the numbering scheme and hydrogen bond 
is given in Figure 1 [25]. In the crystal structure, the transfer of 
a proton from the inorganic acid results in the protonation of 
the amino group (NH2) of 4-ABA and forms the 4-
carboxyanilinium cation (with +1 charge). Therefore, the 
chemical formula of 4-CAH2PO4·H2O is 2(NH3+C6H4COOH): 
2(H2PO4–):2H2O.  
 
3.1.1. 4-Carboxyanilinium dihydrogen phosphate 
monohydrate crystal structure and packing at 293 K 
 

4-CAH2PO4·H2O crystal structure belongs to the triclinic 
space group P-1, Z = 4, with two molecules each of 4-
carboxyanilinium cations, dihydrogen phosphate anions 
(H2PO4–) and water in an asymmetric unit (Figure 1). The 
crystal structure view of 4-CAH2PO4·H2O along the a-axis is 
shown in Figure 2a. It forms an alternate organic and inorganic 
layered structure that is parallel to the (100) plane (Figure 2). 
These layers contain crystallographically independent 4-
carboxyanilinium cations (in the organic layer) and dihydrogen 

phosphate anions (H2PO4–) and water molecules (in the 
inorganic layer) which are stabilized by an ionic and three-
dimensional network of hydrogen bonds between the 4-
carboxyanilinium cations and dihydrogen phosphate anions 
and water molecule. The phosphorus atom in dihydrogen 
phosphate (H2PO4–) anions is tetrahedrally bonded to two 
oxygen and two OH groups. The lengths of the P-O bonds where 
the P atom is connected to O are in the range 1.5063(15) Å to 
1.5086(15) Å. For P atom bond to OH group, the P-O lengths are 
in the range 1.5712(15) to 1.5689(14) Å. As expected, the 
protonated P-O link is significantly longer than that of the 
unprotonated bonds. The O-P-O bond angles of H2PO4– anions 
are in the range 106.04(8)° to 115.05(9)°. These lengths and 
angles of the bonds are in good agreement with those observed 
in similar compounds [7, 8]. 

In Figure 2a-c, it is observed that the H2PO4– anion in the 
inorganic layer forms a long chain by hydrogen bond interac-
tion between the P-OH and -P=O groups of adjacent H2PO4– 

anions in the same layer (Figure 2b). Each inorganic layer 
consists of two such chains of P–OH···O=P hydrogen bonds. 
These P–OH···O hydrogen bonds help stabilize the crystal 
structure. Therefore, the O atoms of the H2PO4– moiety act as 
proton acceptors or as proton donors, resulting in a three-
dimensional network. The alternate anions in these chains have 
the same orientation. Each H2PO4– anion forms a hydrogen bond 
with two molecules of water, two molecules of -4CA, and two 
molecules of H2PO4– (Figure 2c). The phosphate layers are 
interconnected by 4-CA ions of the organic layers through 
strong ionic and hydrogen bond interactions with both the              
-NH3+ and -COOH groups of two different 4-CA cations.  



Lata Panicker / European Journal of Chemistry 15 (1) (2024) 1-16 5 
 

 
2024 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.15.1.1-16.2484 

  

 
(a) 

 
(b) 

 
(e)  

(c) 
 

(d) 
 

Figure 2. The crystal structure of 4-CAH2PO4·H2O, (a) view along a-axis with 3-D projection of the layer parallel to the (100) plane (b) arrangement of H2PO4– 

anions in the inorganic layer (c) hydrogen bonding with H2PO4– anion (d) hydrogen bonding with 4-CA and (e) plane drawn using the cyclic acid dimer atoms 
(formed between the -COOH and -POOH groups). 

 

 
 

Figure 3. The crystal structure of 4-carboxyanilinium dihydrogen phosphate (4-CAH2PO4) (Reference [8] CCDC Deposition Number 647348) viewed along a-axis 
showing 3-D projection of the layer parallel to the (100) plane. 

 
The geometry of the organic cations in 4-CAH2PO4·H2O 

crystals is in good agreement with the structures reported for 
the 4-carboxyanilinium cations [26,27]. In 4-CAH2PO4·H2O 
crystal structure within the horizontal organic layer, the 
adjacent 4-carboxyanilinium ions are arranged in head-to-tail 
orientation (Figure 2a). The -COOH (-C=O and -C-OH) group of 
the 4-CA cation forms a hydrogen bond with the -P=O and -P-
OH groups of the same anion (Figure 2d). This leads to the 
formation of a cyclic arrangement and all atoms involved in 
hydrogen bonding lie in the same plane (Figure 2e). The 
interesting observation is that there is no hydrogen bond 
interaction between neighboring 4-carboxyanilinium cations. 
The -NH3 group forms hydrogen bonds with two water 
molecules and one H2PO4– anion, respectively (Figure 2d). The 
4-CA-cations are held in the organic layer by strong H-bonding 
and ionic interactions with anions and water on both sides. 
Such an extensive three-dimensional hydrogen-bonding 
framework stabilizes the crystal structure.  

4-Carboxyanilinium dihydrogen phosphate monohydrate 
(4-CAH2PO4·H2O) and 4-carboxyanilinium dihydrogen phos-
phate (4-CAH2PO4) (Figure 3) [8] both are organo-phosphate 
adducts of 4-amino benzoic acid. However, the crystal structure 
of 4-CAH2PO4·H2O is different from that of 4-CAH2PO4. In 4-
CAH2PO4, the inorganic layer has one chain of P-OH···O 
interlink, while in 4-CAH2PO4·H2O there are two such chains 
within the same layer. In the 4-CAH2PO4 crystals, four H2PO4– 
ions interact via a H-bond with the NH3+ group of three different 
4-carboxyanilinium cations, while in 4-CAH2PO4·H2O crystal, 
the -NH3 group forms hydrogen bonds with two molecules of 
water and one O atom of the H2PO4– anion. In both crystal 

structures, the -COOH group of the 4-CA cation forms a 
hydrogen bond with the -P=O and -P-OH groups of the same 
anion. This interaction leads to the formation of a cyclic 
arrangement of all atoms involved in hydrogen bonding. 
However, in the case of 4-CAH2PO4, these atoms do not lie in the 
same plane. Thus, the formation of different crystal structures 
from the same reaction mixture is an important property and 
characteristic of organic molecules. 
 
3.2. Differential scanning calorimetry 
 

The DSC heating thermograms of 4-CAH2PO4·H2O and 4-
ABA were obtained in the temperature range of 173 to 293 K 
and 293 to 480 K, respectively. The thermal parameters, 
transition temperatures, and transition enthalpies obtained 
from the DSC curves are given in Table 4. No transition is seen 
in the low temperature range (173 to 293 K). 4-CAH2PO4·H2O 
when heated gave endothermic transitions at 323.35, 374.16 
and 459.42 K, respectively, and the corresponding enthalpy 
associated with it was -10.80, -12.5 and -276.82 J/g, respect-
tively (Table 4, Figure 4). Thus, the heating thermogram of 4-
CAH2PO4·H2O obtained was different from that of 4-ABA, 
indicating formation of a new compound (Figure 4). The 
transitions seen at 323.35 and 374.16 K, which is irreversible, 
could be associated with the loss of crystallization water. This 
is supported by the observation that the clear crystal became 
opaque when heated to 374 K. Additionally, the sample had 
undergone weight loss. The broad isotropic melting transition 
is seen at 459.42 K (Figure 4). In 4-CAH2PO4, only a broad 
isotropic melting transition is seen at 466.33 K (Table 4). 



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Table 4. Transition temperatures and enthalpies of crystal to isotropic melting in 4-CAH2PO4·H2O, 4-CAH2PO4, and 4-ABA. 
Sample Isotropic melting 

Peak temperature (K)  Transition enthalpy (J/g) 
4-ABA  461.20  -186.80 
4-CAH2PO4·H2O   323.24 

374.16 
459.42 

 -10.80 
 -12.50 
 -276.82 

4-CAH2PO4 466.33 -253.53 
 

 
 
 

Figure 4. DSC heating thermograms at 5 °C/min of (a) 4-CAH2PO4·H2O and (b) 4-ABA in the temperature range of 293 to 480 K. The dotted curve gives the 
transition seen in 4-CAH2PO4·H2O from 300 to 410 K (expanded 16 times). 
 

 
 

Figure 5. FTIR spectra at 298 K of (a) 4-CAH2PO4·H2O, (b) d4-CAH2PO4.H2O, (c) 4-ABA and (d) d4-ABA in 3600-1800 cm-1. 
 

3.3. FTIR spectra 
 

FTIR spectra of 4-CAH2PO4·H2O, d4-CAH2PO4·H2O, 4-ABA 
and d4-ABA at 298 K were recorded in the 1800-3600 cm-1 and 
650-1800 cm-1 region, and are shown in Figures 5 and 6, 
respectively. The IR vibrational mode assignments for the 
different modes observed in these compounds are given in 
Table 5. The vibrational spectra and assignments of most of the 
modes of 4-ABA and d4-ABA obtained are consistent with the 
reported data [28-30]. The spectrum of the deuterated 
compound was analyzed and used for the assignment of 
functional groups (-NH2 -NH3+, -OH). When comparing the 4-
CAH2PO4·H2O spectra with those of the parent compound, 4-
ABA, it is clearly seen that the various vibrational bands of 4-
ABA are affected by the complexation (Figures 5 and 6) (Table 
5). This is due to the different types of intermolecular interac-
tions through hydrogen bonds existing in 4-ABA and 4-
CAH2PO4·H2O, respectively. Thus, the mode frequencies of the 
functional groups that participate in the hydrogen bonding are 
often altered; the extent of the shift depends on the strength of 
interaction. Therefore, the FTIR spectrum of 4-CAH2PO4·H2O, is 

significantly different from those of the parent component 4-
ABA. 
 
3.3.1. The vibrations of 4-carboxyanilinium cations and 
phosphate anions 
 

The compound 4-CAH2PO4·H2O is an ionic molecular crystal 
with the -NH3+, -COOH and H2PO4– groups. However, the 4-ABA 
molecule is neutral with -NH2 and -COOH groups. In the 4-ABA 
crystal structure pair of organic molecules, 4-ABA is linked 
together to form cyclic -COOH dimers through a strong O-H···O 
hydrogen bond arranged around a center of symmetry (Figure 
7a). The -NH2 group also forms a hydrogen bond with the                  
-COOH group (Figure 7a). However, in 4-CAH2PO4·H2O, (a) -
NH3+ forms hydrogen bond with H2PO4- and water molecules 
and (b) -COOH forms hydrogen bond with H2PO4– (Figure 7b). 
The differences in hydrogen bonding interactions observed in 
these crystals are clearly reflected in the FTIR spectra. The 
mode assignment of 4-CAH2PO4·H2O (d4-CAH2PO4·H2O) 
crystals was carried out by comparing the observed IR 
frequencies to those reported for the parent compound 4-ABA 
(d4-ABA) [28-30]. 



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Table 5. Assignment of the IR vibrational frequencies (cm-1) of the bands observed in 4-ABA, d4-ABA, 4-CAH2PO4·H2O and d4-CAH2PO4·H2O measured at 298 K 
and the DFT calculated frequencies of 4-CAH2PO4·H2O.  
4-ABA d4-ABA 4-CAH2PO4·H2O  d4-CAH2PO4·H2O Assignments * 4-CAH2PO4·H2O (DFT) 
3458 - - - νasNH2 - 
3361 - - - νsNH2 - 
3090 to 
2550 

- 3100 to 2500 - νOH 3725, 3348 (water), 3675, 3655 (H2PO4–), 3630 
(4-CA) 

- - 3226, 3067 
2660, 2628  

- νasNH3+ 
νsNH3+ 

3371, 2892, 2568 (4-CA) 

- 2590 - - νasND2 - 
- 2533, 2504, 2441 - - νsND2 - 
- 2323 to 1867 - - ν(O-D) - 
- - - 2441, 2387, 2229  νasND3+, νsND3+, ν(O-D) - 
- - - 2044, 1912 νsND3+ ν(O-D) - 
- - 1897 - - 1796, 1765 
1661 1658 1658s 1657s νC=O 1731 
1622 - 

 
- βasymNH2  - 

1597 1599 1612m 1603 8a (C-C stretching vibration) 1693 
1571 1570 1578 1568w 8b (C-C stretching vibration) 1652 
- - 1557m - βasymNH3+ 1693, 1652 
1520 1521 1512 m 1516w 19a (C-C stretching vibration) 1645 
1441 1435 1452 m 1432 19b (C-C stretching vibration) 1570 
1419 1420 1426 m 1422 9b (C-H in-plane bending vibration) 1485 
1419-1395 - 1419-1395 - βOH 1390 (-4CA) 
- 1364 - - βasymND2  -  

- - 1369 βasymND3+ - 
1311 1312 1321 m 1327w 14 (C-C stretching vibration) 1269 
1286 1288 1299 m 1293 m νC-OH / 14, βOD 1199   

1212 1215 δs(POHH), δas(POHH) 1172,1146, 1109 
1171 1175 1189 m  1175 9a (C-H in-plane bending vibration) 1186 
1128 1125 1132 m 1125m 7a/9a (C-H stretching vibration) 1057, 1085 
- - 1093m, 1019m  1059, 1043s νas(PO), νs(PO), βsymNH3+, νC-OD, 

βOH/γOH 
 1109, 1146, 1172 

- - 999 s, 958 s 1015s, 962 νas(POH), νs(POH) 1012, 1026 
1069 - - - βsymNH2  - 
- 1043, 1061 - - νC-OD - 
962w 963w -  - 17a (C-H out-of-plane vibration) 898 
922 - - - γOH  677 
889 888 887 899 5/ νC=O  879 
841m 842m 852v 844 17b (C-H out-of-plane vibration) 876 
817 817 839m 844 10a (C-H out-of-plane vibration) 862 
- - - - βsymND3+ - 
771 771 760m 772m 1 (radial skeletal vibration) 845 
- 746 - 752 γOD - 
698 700 687, 670  724, 701 4 (γ skeletal vibration), ρ(PO2) 782, 735, 725 (PO) 
* ν, stretching; β, in plane; γ, out of plane deformations; ρ, rocking; τ, twisting; δ, scissoring; s, strong; m, medium; w, weak. 
 

 
 

Figure 6. FTIR spectra at 298 K of (a) 4-CAH2PO4.H2O, (b) d4-CAH2PO4.H2O, (c) 4-ABA and (d) d4-ABA in 1800-650 cm-1. 

The FTIR spectra of 4-CAH2PO4·H2O exhibit bands 
corresponding to the vibration modes of the organic molecule 
4-carboxyanilinium (4-CA+) and the dihydrogen phosphate 
(H2PO4-) group. In 4-CAH2PO4·H2O, the asymmetrical (νasym) and 
symmetrical (νsym) stretching vibration bands of the N-H bonds 
(of the -NH3+ group) are observed as a broad band with multiple 
peaks in 3226 to 2628 cm-1. The broadened and decreased 
values of the N-H stretching frequency suggest the presence of 
a very strong N-H…O hydrogen bond in 4-CAH2PO4·H2O than in 
4-ABA (Table 5). From the crystallographic data, it is seen that 
in 4-ABA the -NH2 group forms a weak hydrogen bond with the 

C=O group, while in 4-CAH2PO4·H2O the -NH3+ group forms a 
strong hydrogen bond with the P=O groups and water 
molecules. On deuteration, the N-H stretch bands of 4-ABA and 
4-CAH2PO4·H2O, respectively, shift in the region 2044-2590    
cm-1, which corresponds to the N-D stretching band (Table 5). 
The broad band observed in the region 3100 to 2500 cm-1 is 
assigned to the symmetric stretching vibration bands of the OH 
bonds. 

In 4-CAH2PO4·H2O the C=O band is observed at higher 
frequency suggesting a weak hydrogen bond interaction in          
4-CAH2PO4.H2O as compared to that in 4-ABA (Table 5).  



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(a) (b) 
 
Figure 7. Unit cell representation of (a) 4-ABA and (b) 4-CAH2PO4·H2O. Crystallographic data reference numbers AMBNAC09 for 4-ABA and CCDC Deposition 
Number 2288228 for 4-CAH2PO4·H2O. 
 

 

 

(a) (b) 
 

Figure 8. Three-dimensional view of Hirshfeld surface in 4-CAH2PO4·H2O plotted over dnorm in the range -0.801 to 1.0648 a.u. using one 4-CA molecule in an 
asymmetric unit. HS with hydrogen bond interaction within the molecules in (a) an asymmetric unit and (b) in a unit cell. 

 
The bands in 650-1700 cm-1, whose frequencies are not 

affected by deuteration, are assigned to the ring modes 8a, 8b, 
19a, 19b, 9b, 14, 9a, 7a, 12, 17a, 5, 17b, 10a, 1 and 4, respect-
tively, consistent with the earlier work (Table 5) [28-30]. The 
band at 1557 cm-1 in 4-CAH2PO4.H2O, which shifts to a lower 
frequency 1369 cm-1 on deuteration, is assigned to βasymNH3+. 
The band at 1419-1395 cm-1 in 4-CAH2PO4·H2O, is assigned to 
βOH. In its deuterated counterpart, the βOD is seen at 1293 cm-1. 
The band observed at 1293 cm-1 in 4-CAH2PO4·H2O, can be 
assigned to νC-OH. On deuteration, the νC-OD band is seen at a 
lower frequency. 

The isolated PO43- tetrahedron has Td symmetry [31]. 
However, in the H2PO4– tetrahedron, the localization of two 
protons on two oxygen atoms reduces the ideal symmetry from 
Td to C2v. This gives rise to 15 internal vibrations active in IR 
and Raman, but exceptional has been the A2 mode, which is 
only Raman active [32,33]. However, in the crystal, the H2PO4– 
tetrahedron has a lower local symmetry C1, and therefore 
anisotropic crystal fields can increase the degeneracy and allow 
inactive modes to be active. The correlation of group to 
subgroup shows that these modes in H2PO4– can be divided into 
(2A1 + B1 + B2) stretching and (2A1 + A2 + B1 + B2) bending 
vibrations with C2v symmetry. H2PO4– anion interpretation is 
made in terms of internal modes of two atomic groups, PO2 
(O=P=O) and P(OH)2 (HO-P-OH). According to reference [34], 
the FTIR spectrum of H2PO4– exhibits five sharp bands at 1156, 
1077, 944, 879, and 521 cm-1, which is assigned to (two P-O 
stretches, two P-OH stretches, and O-P-O bending modes) 
νas(PO), νs(PO), νas(POH), νs(POH) and δ(OPO), respectively, 
These are two stretching vibrations, asymmetric and 
symmetric of PO2 group, and those related to P(OH)2. The 
bending modes of the H2PO4– group are observed at lower 
frequencies. The broad shoulder at 1213 cm-1 is assigned to the 

two P-OH-H bending modes bands, δs(POHH) and δas(POHH) 
[34,35]. The band at 748 cm−1 corresponds to the rocking 
ρ(PO2) vibrations. 

4-CAH2PO4·H2O compound consists of a dihydrogen 
phosphate ion ((HO)2PO2–). In the crystal structure, these ions 
exist as a polymeric chain by forming P=O···H-O-P hydrogen 
bond between the adjacent H2PO4– ions. The bending vibration 
of P-OH, δs(POHH) and δas(POHH) occur as abroad band at 1212 
cm-1. The vibrational bands observed at 1093 and 1019 cm-1 are 
attributed to the asymmetric (νas(PO)) and symmetric (νs(PO)) 
stretching vibrations of the P-O bond. The vibrational bands 
seen at 999 and 958 cm-1 are ascribed to the asymmetric 
(νas(POH)) and symmetric (νs(POH)) stretching vibrations of the 
P-OH bond. The vibration bands, νas(PO), νs(PO), νas(POH) and 
νs(POH) are not affected by deuteration (Table 5). But the 
vibrational bands, δs(POHH) and δas(POHH) shows shift to lower 
frequency on deuteration. When compared to IR spectra of neat 
H2PO4- ions, the νas(PO) and νs(PO) vibrational bands in 4-
CAH2PO4.H2O show a blue shift (decrease) of 63 and 58 cm-1, 
respectively, indicating a strong H-bond with P=O in the 
compound. The νas(POH) and νs(POH) vibrational bands show a 
red shift (increase) of 55 and 79 cm-1, respectively, indicating a 
reduced H-bonding interaction with P-OH in the compound 
than the neat H2PO4- ions. However, the bending vibration of P-
OH, δs(POHH), and δas(POHH) is not affected by the complex 
formation. The vibrational spectra and assignments of the 
modes of (HO)2PO2- obtained are consistent with those repor-
ted for dihydrogen phosphate compounds [32]. Therefore, most 
of the vibrational mode assignment of 4-CAH2PO4·H2O could be 
done by comparing the IR frequencies of its internal mode 
region with those observed in the parent compounds 4-ABA 
and H3PO4.  

 



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Figure 9. Two-dimensional fingerprint graphs of 4-CAH2PO4·H2O showing the contributions of (a) all intermolecular contacts and individual interactions: (b) 
O···H/H···O, (c) H···H, (d) H···C/C···H (e) O···C/C···O, (f) C···C and (g) O···O. The outline of the full fingerprint plot is shown in grey. HF surfaces highlight the 
relevant dnorm surface patches associated with the specific intermolecular contacts. The percentage contribution is specified for each contact. 
 
3.4. Hirshfeld surface analysis  
 

The intermolecular interactions in 4-CAH2PO4·H2O were 
investigated and visualized by HS analyzes performed using 
CrystalExplorer 21 [13,19]. One of the 4-CA molecules from the 
asymmetry unit was used to generate HS. The dnorm mapped on 
the HS shows short intermolecular contacts as red spots 
corresponding to C-OH···O, C=O···H and N-H··O hydrogen bonds 
(Figure 8). 

The fingerprint plot of 4-CAH2PO4·H2O showing all 
intermolecular contacts and the contribution of the individual 
O···H/H···O, H···H, H···C/C···H, O···C/C···O and O···O contacts, 
respectively, are illustrated in Figure 9 together with their 
relative contributions to the Hirshfeld surface. The most 
prominent interaction corresponds to the short O···H/H···O 
contacts; they contribute 41% to the overall surface contact and 
form a pair of spikes (Figure 9b). They are due to the 
intermolecular C-H···O/C=O···H hydrogen bond. H···H contacts 
contributing 28.4% to the overall surface are depicted in Figure 
9c. The presence of C-H···π interactions gives rise to a pair of 
characteristic wings in the fingerprint graph decomposed into 
H···C/C···H contacts (Figure 9d) contributing 18 % to the HS. 
The contributions of other contacts O···C/C···O, C··C, and O···O 
of 6.5, 3.6, and 2.5 %, respectively, to the Hirshfeld surface are 
negligible. Two-dimensional finger print plots of 4-CAH2PO4 
(Reference [8] CCDC deposition number 647348) showing 
contributions of (a) all intermolecular contacts and individual 
interactions, (b) O···H/H···O (47.7 %), (c) H···H (28.9 %) (d) 

H···C/C···H (8.6 %) (e) O···C/C···O (8.9 %) and (f) C···C (5.9 %) 
are shown in Figure 10. When comparing the individual 
interactions of 4-CAH2PO4·H2O with those of 4-CAH2PO4 as 
accepted, both show a similar type of interactions (Figure 11). 
However, the values were different, indicating the difference in 
intermolecular interaction (Figure 11).  
 
3.5. Energy framework analysis 
 

The 3D topology of the strong charge-assisted hydrogen 
bonds in 4-CAH2PO4·H2O was obtained by plotting the energy 
framework. To obtain the 3D topology, the interaction energies 
obtained are used to construct representative cylinders that 
connect the molecules, where the thickness of the cylinders is 
set proportional to the values of the interaction energy. The 
resulting 3D frameworks or topologies of interactions are 
termed energy frameworks [14]. Therefore, the energy 
frameworks allow us to examine ‘cooperative effects’ in 
intermolecular interactions across the crystal [13,14]. The 
energy frameworks allow one to find the electrostatic (red), 
dispersion (green) and total energy (blue for binding, gold for 
non-binding) terms separately. The total intermolecular 
interaction energy (Etot) given is the sum of four energy terms: 
electrostatic (Eele, red), polarization (Epol), dispersion (Edisp, 
green) and exchange repulsion (Erep) with scale factors of 1.019, 
0.651, 0.901 and 0.811, respectively (for CE-HF/3-21G 
quantum level of theory) [18,19].  

 



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Figure 10. Two-dimensional fingerprint graphs of 4-CAH2PO4 (Reference [8] CCDC Deposition Number 647348) showing contributions of (a) all intermolecular 
contacts and individual interactions: (b) O···H/H···O, (c) H···H, (d) H···C/C···H, (e) O···C/C···O and (f) C···C. The outline of the full fingerprint plot is shown in grey. 
HF surfaces highlight the relevant dnorm surface patches associated with specific intermolecular contacts. The percentage contribution is specified for each contact. 
 

 
 

Figure 11. Percentage contributions to the Hirshfeld surface area of various close intermolecular contacts for 4-CAH2PO4·H2O (red) and 4-CAH2PO4(blue) [8]. 
 

The relative strengths in the interaction energies in the 
individual directions of 4-CAH2PO4·H2O are represented by 
cylinder-shaped energy frameworks (Figure 12a-c). In 4-
CAH2PO4·H2O crystals, the intermolecular packing is mainly 
stabilized by the anion cation coulombic attraction, which is 
obvious from unusually high values of interaction energies 
ranging from -200 to -800 kJ/mol. This is evident from the 
predominant electrostatic contribution compared to the minor 
contribution of dispersion forces in the energy framework of 
the crystals (Figure 12). Although hydrogen-bonded organic 
framework structure, as demonstrated through the electro-
static framework, we can see a tight cohesive framework of 
dispersion interactions which also underpin the structure of 
both crystals. A similar type of 3D topology was observed in 4-
carboxyanilinium phosphite, bis(4-carboxyanilinium) sulfate, 
bis(4-carboxyanilinium) selenate trihydrate, and bis(4-
carboxyanilinium) selenate [36-38]. 3D topology plots of 4-
CAH2PO4 (Reference [8] CCDC deposition number 647348) 
showing electrostatic (red), dispersion (green) and total energy 
(blue for binding, gold for non-binding) contributions are 
shown in Figure 12d-f. The columbic attraction energies range 
from -100 to -500 kJ/mol in 4-CAH2PO4. 

3.6. DFT calculations 
 
3.6.1 Geometrical optimization of 4-CAH2PO4·H2O  
 

The geometric parameters of 4-CAH2PO4·H2O structure 
were optimized using the DFT / B3LYP level with the 6-31G 
basis set. The optimized molecular structure of 4-CAH2PO4·H2O 
with atom labeling scheme is shown in Figure 13a. The plot 
showing the correlation between the experimental and 
theoretical values of the bond distances obtained is shown in 
Figure 13b. When the experimental and theoretical values of 
bond length are compared, it is found that the theoretical bond 
lengths are slightly longer than those obtained from XRD data. 
The small discrepancies seen in the bond parameters are due to 
the fact that the theoretical calculations are carried out on an 
isolated molecule and the XRD data are obtained for the bulk 
molecules where the packing effects and intermolecular 
interactions play an important role in keeping the molecules 
together. The optimized C-C bond lengths of the phenyl ring 
were in the range of 1.3936 to 1.405 Å, which seems to be 
reliable and is in agreement with the experimental values 
obtained of the C-C bond lengths in the phenyl ring.  



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(a) (b) (c) 

   
(d) (e) (f) 

 
Figure 12. Energy framework diagrams for a 3×3×3 supercell of 4-CAH2PO4·H2O (a, b, c) and 4-CAH2PO4 (d, e, f) (Reference [8] CCDC Deposition Number 647348) 
crystal structures viewed along the crystallographic a axis. The cylindrical radius is proportional to the relative strength of the corresponding energy and was 
adjusted to a scale factor of 45 kJ/mol with a cut-off value of 5 kJ/mol for all energies. The electrostatic (a, d), dispersion (b, e), and total energy (c, f) terms are 
colored red, green, and blue, respectively. 
 

 
 

 
(a) (b) 

 
Figure 13. (a) The theoretically optimized molecular structure of 4-carboxyanilinium dihydrogen phosphate monohydrate with labeled atoms and (b) Plot 
depicting the graphical correlation between the experimental and theoretical values of the bond distances obtained by DFT/B3LYP level with 6-31G basis sets. 

 
The optimized P-O bond lengths were 1.5889 to 1.6284 Å 

for P=O bonds and 1.7554 to 1.7734 Å for P-O-H bonds, which 
seems to be reliable. The amine N-H···O hydrogen bond appears 
to be stronger in theoretical calculations due to the orientation 
of the phosphate anion and water molecule. The experimental 
and calculated values of the hydrogen bond in (i) N-H···O=P 
were 1.9312 Å (exp) and 1.5966 Å (opt), respectively, and (ii) 
N-H···O-H (water) were 1.9374 Å (exp) and 1.6947 Å (opt), 
respectively. From the theoretical values, it is observed that the 
most optimized bond lengths and bond angles are slightly 
larger than the experimental value, because the theoretical 
calculations were carried out on molecules in the gaseous 
phase, while the experimental results correspond to molecules 
in solid crystalline states [39,40]. 

The theoretical calculation and analysis of the frontier 
orbitals, HOMO (highest occupied molecular orbital) and LUMO 
(lowest unoccupied molecular orbital) of a molecule help to 

understand and determine its chemical reactivity and kinetic 
stability. To understand the chemical reactivity and kinetic 
stability of 4-CAH2PO4.H2O, the 3D plots of its HOMO and LUMO 
orbitals were obtained. The calculated energy gap, ΔE of 4-
CAH2PO4.H2O is 5.1114 eV (Figure 14). A molecule with a large 
HOMO-LUMO gap will be a hard molecule. However, if the 
molecule has a small HOMO-LUMO gap, it will be a soft 
molecule. The ionization energy can be correlated with the 
chemical reactivity of atoms and molecules. The higher the 
ionization energy, the higher the stability and chemical 
inertness and vice versa. The ionization energy (I) and the 
electron affinity (A) can be expressed through the orbital 
energies of HOMO and LUMO as I = - EHOMO = 7.2665 eV and A = 
- ELUMO = 2.1551eV. Using the value of I and A, various properties 
of the 4-CAH2PO4.H2O complex have been calculated and are 
reported in Table 6.  

 



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Table 6. Theoretically calculated I, A, ΔE, η, χ, μ, ω, S, and ΔNmax of 4-CAH2PO4.H2O using the orbital energies of HOMO and LUMO. 
Ionization energy (I) I = - EHOMO 7.2665 eV 
Electron affinity (A) A = - ELUMO 2.1551eV 
Energy gap (ΔE) ΔE = ELUMO-EHOMO 5.1114 eV 
Global hardness (η)  η = (I-A)/2  2.5557 eV 
Electronegativity (χ) χ = (I+A) /2  4.7111 eV 
Electronic chemical potential (μ)  μ = -(EHOMO+ELUMO)/2  4.7108 eV 
Global electrophilicity index (ω) ω = -(μ2/2η) -4.3394 eV 
Global softness (S)  S = (1/ η)  0.3913 eV-1 
Maximum amount of electronic charge that the electrophile system may accept (ΔNmax) ΔNmax = -μ/η 1.8432 
 

 
 

Figure 14.  Calculated molecular orbital surfaces, its contour diagram, and the band gap of the HOMO and LUMO orbital in 4-CAH2PO4.H2O at the B3LYP/6-31G 
level. 
 

  
(A) (B) 

 
Figure 15. Theoretically calculated IR spectra of (a) 4-CAH2PO4·H2O and (b) d4-CAH2PO4·H2O in (A) 4000-1850 cm-1 and (B) 1850-650 cm-1. The vibration mode 
assigned is done using Gaussian 09W. 
 
3.6.2. Vibrational spectra of 4-CAH2PO4·H2O  
 

The vibrational spectra of 4-CAH2PO4·H2O and d4-
CAH2PO4·H2O (the deuterated functional group) were obtained 
using the Gaussian 09 program with DFT/B3LYP level and 6-
31G basis set. The assignment of different vibration modes seen 
in the calculated IR spectra of 4-CAH2PO4·H2O is given in Table 
5. The experimentally and theoretically obtained spectra were 
quite similar but with a shift in the frequency value. However, 
the experimentally obtained spectra were broad as the 
measurement was done on the bulk sample, and the calculated 
IR was obtained assuming that the molecule was isolated and in 
the gas phase. Figures 15a and 15b show the theoretically 
calculated IR spectra of 4-CAH2PO4·H2O and d4-CAH2PO4·H2O 
with the vibration mode assigned using Gaussian 09W. The 4-
CAH2PO4·H2O molecule has five functional groups: a para-
substituted benzene ring, -NH3+, -COOH, -OH (water), and 
H2PO4–. The functional groups in d4-CAH2PO4·H2O were 
deuterated, -ND3+, -COOD, -OD (water), and -POD. In the 4000-
1850 cm-1 region, the prominent stretching modes of the 
functional groups -NH3+, -COOH, -OH, and H2PO4– are seen that 

get shifted to a higher frequency on deuteration (Figure 15A). 
Similar behavior is seen in the 1850-650 cm-1 region, where in 
the deuterated counterpart the frequency is shifted to a higher 
value. The vibration modes of the C=O and P=O and the benzene 
ring are hardly affected by deuteration. Thus, a very good 
correlation could be seen between the experimentally and 
theoretically obtained spectra of 4-CAH2PO4·H2O. 
 
3.7. Non-covalent interaction (NCI) and Quantum theory of 
atom in molecule (QTAIM) analysis 
 
3.7.1. Non-covalent interaction analysis 
 

The noncovalent interaction (NCI), also known as reduced 
density gradient (RDG), is a methodology to identify and 
visualize weak interactions such as van der Waals, hydrogen 
bonds, and steric clashes within molecules [41]. The RDG for the 
molecules can be calculated using the electron density and its 
first derivative, as given in Equation 1. Thus, the RDG(r) 
function is a dimensionless form of the electron density 
gradient norm function, used to describe the deviation from the 



Lata Panicker / European Journal of Chemistry 15 (1) (2024) 1-16 13 
 

 
2024 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.15.1.1-16.2484 

homogeneous electron distribution. For regions far from the 
molecule, the electron density decays exponentially to zero; 
hence, the reduced gradient will have very large positive values. 
However, in the region of covalent bonding and noncovalent 
interactions, the reduced gradient has very small values. 
Therefore, to study the noncovalent interactions in a molecular 
system, the plot of the reduced gradients, RDG(r) versus ρ(r) is 
plotted and examined in the low reduced gradient regions (i.e. 
for the region where ρ(r) is small). 
  
RDG(r) = 1

2(3𝜋𝜋2)
1
3

|𝛻𝛻𝛻𝛻(𝑟𝑟)|

𝛻𝛻(𝑟𝑟)
4
3

    (1) 

 
The second largest eigenvalue of the Hessian matrix of 

electron density, λ2 and ρ can be used to distinguish these 
noncovalent interactions. For which a real space function, the 
sign(λ2)ρ is defined and when this sign(λ2)ρ (a.u.) function is 
plotted against RDG(r), a few spikes are observed. On the basis 
of the sign(λ2) and ρ values, the following regions are defined. 
Strong attraction (halogen bond and hydrogen bond) if: (ρ > 0 
and λ2 < 0) blue. Van der Waals interaction if: (ρ ≈ 0 and λ2 ≈ 0, 
i.e. very near to 0) green color (>-0.02 to < 0.005) region 
indicated in Figure 16a. Strong repulsion (Steric effect in the 
ring and cage) if (ρ > 0 and λ2> 0) red. 

The sign(λ2)ρ function can also be represented using 
different colors based on its value, as mentioned above. The 
spikes obtained by plotting the sign(λ2)ρ (a.u.) function against 
RDG(r) are classified into three types, the blue spike region, the 
green spike region, and the red spike region. By mapping the 
cross-section area of the spikes at a particular value of RDG(r), 
the RDG isosurface can be constructed. From this pictorial 
representation of RDG isosurfaces, both the location of the weak 
interaction and the type of interactions present in the molecule 
can be easily identified. 

The noncovalent interactions present in 4-CAH2PO4·H2O 
complex were investigated using the Multiwfn 3.7 [24] and the 
graph obtained is shown in Figure 16a. From Figure 16a, it is 
evident that 4-CAH2PO4·H2O has steric, van der Waals and 
hydrogen bond interactions. There are spikes seen at very 
negative region of sign(λ2)ρ, indicating that the 4-CAH2PO4·H2O 
compound contains an attractive intermolecular interaction. 
There are also spikes in the positive region indicating the 
presence of a steric effect in 4-CAH2PO4·H2O. In Figure 16a, six 
spikes are seen, with their peak at position as per CP positions 
in AIM theory. The spikes around 0.020 and 0.010 a.u. represent 
the steric hindrance. The spikes around -0.010 a.u. represent 
the van der Waals force of attraction. The spikes seen around -
0.035 and -0.05 a.u. are pertaining to hydrogen bond formation. 
The spike seen around -0.025 a.u. could be due to strong van 
der Waals force of attraction, weak hydrogen bond or both. 
These different interactions seen in 4-CAH2PO4·H2O can be 
represented pictorially by generating RDG isosurfaces using 
two cube files generated by Multiwfn 3.7 and viewed in the VMD 
1.9.3 tool [24,42]. The structure of 4-CAH2PO4·H2O complex 
showing different isosurfaces of RDG is shown in Figure 16b. 
The red isosurfaces seen in the middle of the aromatic rings 
represent the presence of steric hindrance within the aromatic 
ring. The red and green isosurfaces seen near the -COOH region 
indicate the presence of both steric hindrance and van der 
Waals attraction in this region. In the cation: anion: water 
interface region, red, green, and blue-colored isosurfaces are 
seen, indicating the presence of steric hindrance, van der Waals 
attraction, and hydrogen bond interaction. The hydrogen bonds 
N-H⋅⋅⋅O (between NH3+ and water), and O⋅⋅⋅H-O (between 
H2PO4–and water) are strong, as indicated by the presence of 
blue isosurfaces of RDG. While the N-H⋅⋅⋅O (between NH3+ and 
H2PO4–) hydrogen bond interaction is relatively weak. 

 

3.7.2 Quantum theory of atom in molecule (QTAIM) analysis  
 

Bader’s topology analysis technique is used to find the 
electron density in 'atoms in molecules' (AIM) theory, which is 
also known as 'the quantum theory of atoms in molecules' 
(QTAIM) [43]. In topology analysis, the points where the 
gradient norm of the function value is zero (except at infinity) 
are called as critical points (CPs). The CPs are classified into 
four types, nuclear critical point (NCP) (3, -3), bond critical 
point (BCP) (3, -1), ring critical point (RCP) (3, +1) and cage 
critical point (CCP) (3, +3) according to how many eigenvalues 
of the Hessian matrix of real space function are negative. QTAIM 
has both geometrical and topological features of electron 
density and its derivatives. The bond critical points (BCPs) of 
curvature (3, -1) mark the topological saddle points along 'bond 
lines' of maximum density between the nuclei. Thus, QTAIM is 
a useful tool for characterizing the topological properties of 
chemical bonds. The presence of chemical bonds between 
atoms and interatomic interactions is revealed by the presence 
of bond critical points (BCPs). The QTAIM method provides 
information on the electron density of a system that governs the 
properties at BCPs. The QTAIM theory gives information on 
variations of electron density due to the formation of bonds or 
complexes [43]. 

The QTAIM data of 4-CAH2PO4·H2O complex was generated 
using Multiwfn 3.7 and viewed with the VMD 1.9.3 tool. In the 
pictorial plot of the 4-CAH2PO4·H2O complex generated, the 
nonbonded interactions are represented by the bond critical 
points of curvature (3, -1) as shown in Figure 17. From Table 7 
and Figure 17, it is observed that there are four BCPs between 
nonbonded atoms (a) N-H···O, (b) N-H···O, (c) O-H···O, and (d) 
N-H···O and four ring critical points (RCPs). The charge density 
(ρ), Laplacian of charge density (∇2ρ), ellipticity (ε), energy 
density H(r), and hydrogen bond interaction energy (EHB) 
calculated for these four BCPs obtained from QTAIM analysis 
are given in Table 7. The values of ρ(r) and ∇2ρ at BCPs were in 
the range 0.0253 to 0.0627 a.u. and 0.0421 to 0.1429 a.u., 
respectively (Table 7) and therefore satisfy the Koch and 
Popelier criterion [44]. Bond ellipticity (ϵ) at different BCPs is 
defined as ε =((λ1/λ2)-1), where λ1 and λ2 are eigen values of 
Hessian of the electron density at BCP. This quantity estimates 
the extent to which the electron density is deformed in one 
direction relative to another. Thus, bond ellipticity is a measure 
of anisotropy of the curvature of the electron density (ρ(r)) in 
the direction normal to bond [43,45]. Therefore, ellipticity 
provides a measure of π or σ character of the chemical bonds. 
Hence, the ellipticity factor reflects the stability of the bonds 
[45]. A high value of ellipticity (ε > 0.1) indicates a π character 
of the bond, while a lower value reflects an σ character of the 
bond. In the 4-CAH2PO4.H2O complex, the value of the four BCPs 
was in the range 0.0295 to 0.0963 indicate these interactions to 
have σ character (Table 7). The interaction energies at the BCPs 
are calculated using the Equation (2 and 3) proposed by 
Emamian et al. [46]. 
 
BE(kcal/mole) = -332.34 × ρ(rBCP)(a.u) - 1.0661   (2)  
(for charged complex) with a mean absolute percentage error 
(MAPE) of 10% 
 
BE(kcal/mole) = -223.08 × ρ(rBCP)(a.u) + 0.7423   (3)  
(for neutral complex) with a mean absolute percentage error 
(MAPE) of 14.7% 
 

4-CAH2PO4·H2O is a charged complex. The calculated 
interaction energies (kcal/mol) for BCP at a, b, c, and d were -
9.48739, -21.92024, -12.90466 and -17.95912, respectively 
(Table 7). Thus, the interaction energy for BCP at b > d > c > a. 
Therefore, the strongest non-bonded interaction (hydrogen 
bond) in the 4-CAH2PO4.H2O complex was for the BCP at N-H···O 
(b) having the value of -21.9202 kcal/mol.  



14 Lata Panicker / European Journal of Chemistry 15 (1) (2024) 1-16 
 

 
2024 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.15.1.1-16.2484 

 
Table 7. Parameters obtained from QTAIM analysis: charge density (ρ(r)), Laplacian of charge density (∇2ρ), ellipticity (ε), potential energy density V(r), energy 
density H(r) and calculated H-bond interaction energy (EHB) for the BCPs of the 4-CAH2PO4.H2O complex.  
 BCP 
(Interacting groups) 

Density of all electrons  
ρ(r) (a.u.) 

Laplacian of charge density  
∇2ρ (a.u.) 

Ellipticity of  
electron density, ε 

Energy density  
H(r) (a.u) 

 EHB 
(kcal/mol) 

(a) N-H···O, (-NH3+···H2PO4–) 0.02534  0.08930  0.029521 -0.0002 -9.48739 
(b) N-H···O, (-NH3+···H2PO4–) 0.06275  0.18648 0.061555 -0.0054 -21.92024 
(c) O-H···O, (Water···H2PO4–) 0.03562  0.11162 0.096310 -0.0023 -12.90466 
(d) N-H···O, (-NH3+···Water) 0.05083  0.15592 0.040958 -0.0032 -17.95912  
 

 

 

(a) (b) 
 

Figure 16. The non-covalent interaction in 4-CAH2PO4·H2O complex depicted by the presence of (a) spikes obtained by plotting the sign(λ2)ρ (a.u.) function 
against RDG(r) and (b) RDG isosurfaces obtained by plotting NCL iso-surface at 0.5 iso-value (blue : H-bond, green: van der Waals attraction and red: steric 
hindrance). 
 

 
 

Figure 17. QTAIM molecular graph of the optimized 4-CAH2PO4·H2O complex showing a graphical representation of bond critical points (small orange spheres), 
ring critical points (small gray sphere), and bond paths (black lines). 
 

The total energy of hydrogen bond interaction involved in 
the formation of the stable 4-CAH2PO4·H2O complex is -62.271 
kcal/mol. According to Rozas et al. [47] the interactions at the 
different BCPs can be classified as follows: for strong hydrogen 
bonds (∇2ρBCP < 0, HBCP < 0), for medium hydrogen bonds (∇2ρBCP 
> 0, HBCP < 0) for weak hydrogen bonds (∇2ρBCP > 0, HBCP > 0). 
From Table 7, it is seen that for 4-CAH2PO4·H2O complex all BCP 
fall in the medium hydrogen bonds category as ∇2ρBCP > 0, HBCP 
< 0 in all 4 BCPs. 
 
4. Conclusion 
 

4-Carboxyanilinium dihydrogen phosphate monohydrate 
(4-CAH2PO4·H2O) was synthesized and investigated using FTIR 
spectroscopy, single-crystal XRD, differential, scanning calori-
metry and computational analysis done using Gaussian 09W, 
CrystalExplorer 21, and Multiwfn 3.7 software. The crystal 
structure obtained belongs to the triclinic space group P-1, with 
two 4-carboxyanilinium cations, two dihydrogen phosphate 
(H2PO4–) anions, and two water molecules in an asymmetric 
unit. The H2PO4– anion in the inorganic layer is linked and forms 
a long chain by hydrogen bond interaction between P-OH and    
-P=O groups of the adjacent H2PO4-1 anions in the same layer. 
The phosphate layers are interconnected by the 4-CA cations of 
the organic layers by strong ionic and hydrogen-bond interac-
tions. The interesting observation is that there is no hydrogen 
bond interaction between neighboring 4-carboxyanilinium 
cations within the organic layer. The 4-CA-cations are 

connected through a three-dimensional network of strong 
charge-assisted N-H···O and C-OH···O hydrogen bonds with 
H2PO4- and water molecules. Such an extensive three-dimen-
sional hydrogen-bonding framework stabilizes the crystal 
structure. Most of the vibrational bands of 4-CAH2PO4·H2O 
could be assigned based on the known IR assignments of the 
parent compounds (4-ABA, d4-ABA, and H2PO4) and also by 
comparing with the spectra of deuterated 4-CAH2PO4·H2O. The 
fingerprint plot of 4-CAH2PO4·H2O obtained using Hirshfeld 
surface analysis indicated that the short O···H contacts were the 
most prominent interaction (41.1%) and this was followed by 
H···H (28.4%) and H···C (18%) contacts. The 3D topology of the 
strong charge-assisted hydrogen bonds in 4-CAH2PO4.H2O was 
obtained by plotting energy framework using CrystalExplorer 
21. High values of interaction energies ranging from -200 to           
-800 kJ/mol were obtained. Quantum chemical calculations 
were performed in the gas phase using DFT with a hybrid 
function B3LYP at 6-31G basis set. The calculated IR spectra of 
the DFT optimized 4-CAH2PO4·H2O complex were similar to its 
experimentally obtained spectra.  NCI analysis indicated that 
the 4-CAH2PO4.H2O complex has steric, van der Waals, and 
hydrogen bond interactions. The hydrogen bonds N-H···O 
(between NH3+ and water), and O···H-O (between H2PO4– and 
water) were strong as indicated by the presence of blue RDG 
iso-surfaces. From the QTAIM analysis of 4-CAH2PO4·H2O 
complex, four intermolecular hydrogen bonds were located. 
The hydrogen bond: N-H···O(-NH3+···H2PO4–) > N-H···O(-
NH3+···Water) > O-H···O(Water···H2PO4–) > N-H···O(-



Lata Panicker / European Journal of Chemistry 15 (1) (2024) 1-16 15 
 

 
2024 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.15.1.1-16.2484 

NH3+···H2PO4–) energies were calculated to be -21.92024,                 
-17.95912 -12.90466 and -9.48739 kcal/mol, respectively, and 
which according to Rozas et al. [47] fall in the medium hydrogen 
bonds category. Thus, the thermal, vibrational, and 
computational results corroborate well with the structural data 
of 4-CAH2PO4·H2O complex. 4-CAH2PO4·H2O complex structure 
and its computational analysis are also compared with that of 
4-carboxyanilinium dihydrogen phosphate (4-CAH2PO4). 
 
Acknowledgements 
 
The authors thank the Heavy Water Board, Bhabha Atomic Research Center, 
Trombay, Mumbai 400085, India, for supply of D2O (99.4% isotopic purity).  
 
Supporting information  
 
CCDC-2288228 contains the supplementary crystallographic data for this 
article (4-carboxyanilinium dihydrogen phosphate monohydrate (4-
CAH2PO4·H2O)). These data can be obtained free of charge via www.ccdc.cam 
.ac.uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.uk, or by 
contacting The Cambridge Crystallographic Data Center, 12 Union Road, 
Cambridge CB2 1EZ, UK; fax: +44(0)1223-336033. 
 
Disclosure statement  
 
Conflict of interest: The author declares that they have no conflict of interest. 
Ethical approval: All ethical guidelines have been followed. 
Sample availability: Samples of the compound are available from the author. 
 
Funding  
 
Research work is carried out at the Bhabha Atomic Research Center, 
Trombay, Mumbai 400085, India, which is funded by the Indian government. 
 
ORCID  and Email  
 
Lata Panicker  

 lata_panicker@yahoo.com  
 lata@barc.gov.in   

 https://orcid.org/0000-0001-5448-7502  
 
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mailto:data_request@ccdc.cam.ac.uk
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mailto:lata@barc.gov.in
https://orcid.org/0000-0001-5448-7502


16 Lata Panicker / European Journal of Chemistry 15 (1) (2024) 1-16 
 

 
2024 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.15.1.1-16.2484 

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	1. Introduction
	2. Experimental
	2.1. Crystal growth
	2.2. Single-crystal X-ray diffraction
	2.3. Differential scanning calorimetry
	2.4. Fourier transform IR spectroscopy
	2.5. Hirshfeld surface analyses
	2.6. 3D energy framework analysis
	2.7. Computational details for quantum chemical calculations
	2.8. Non-covalent interaction and quantum theory of atom in molecule analysis

	3. Results and discussions
	3.1. Single crystal X-ray diffraction
	3.1.1. 4-Carboxyanilinium dihydrogen phosphate monohydrate crystal structure and packing at 293 K

	3.2. Differential scanning calorimetry
	3.3. FTIR spectra
	3.3.1. The vibrations of 4-carboxyanilinium cations and phosphate anions

	3.4. Hirshfeld surface analysis
	3.5. Energy framework analysis
	3.6. DFT calculations
	3.6.1 Geometrical optimization of 4-CAH2PO4 H2O
	3.6.2. Vibrational spectra of 4-CAH2PO4 H2O

	3.7. Non-covalent interaction (NCI) and Quantum theory of atom in molecule (QTAIM) analysis
	3.7.1. Non-covalent interaction analysis
	3.7.2 Quantum theory of atom in molecule (QTAIM) analysis


	4. Conclusion
	Acknowledgements
	Supporting information
	Disclosure statement
	Funding
	ORCID  and Email
	References

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