The crystal magnification, characterization, X-ray single crystal structure, thermal behavior, and computational studies of the 2,4,6-trimethylpyridinium picrate European Journal of Chemistry 13 (4) (2022) 468-477 European Journal of Chemistry ISSN 2153-2249 (Print) / ISSN 2153-2257 (Online) – Copyright © 2022 The Authors – Atlanta Publishing House LLC – Printed in the USA. This work is published and licensed by Atlanta Publishing House LLC – CC BY NC – Some Rights Reserved. https://dx.doi.org/10.5155/eurjchem.13.4.468-477.2349 European Journal of Chemistry View Journal Online View Article Online The crystal magnification, characterization, X-ray single crystal structure, thermal behavior, and computational studies of the 2,4,6-trimethylpyridinium picrate Nahide Burcu Arslan 1 and Fatma Aydin 2,* 1 Department of Computer Education and Instructional Technology, Faculty of Education, Giresun University, 28200, Giresun, Turkey 2 Department of Chemistry, Faculty of Arts and Sciences, Canakkale Onsekiz Mart University, 17100, Canakkale, Turkey * Corresponding author at: Department of Chemistry, Faculty of Arts and Sciences, Canakkale Onsekiz Mart University, 17100, Canakkale, Turkey. e-mail: faydin@comu.edu.tr (F. Aydin). 10.5155/eurjchem.13.4.468-477.2349 Received: 23 September 2022 Received in revised form: 06 October 2022 Accepted: 24 October 2022 Published online: 31 December 2022 Printed: 31 December 2022 A crystal of organic salt, 2,4,6-trimethylpyridinium picrate (TMPPc), was synthesized and magnified by slow evaporation in a polar aprotic solvent and characterized by 1H NMR, 13C NMR, and FT-IR spectroscopic methods. X-ray diffraction analysis of the crystal structure of the compound TMPPc showed the presence of a monoclinic space group with a = 4.0174(4) Å, b = 27.863(3) Å, c = 13.9247(17) Å, β = 95.741(4)°, V = 1550.9(3) Å3, Z = 4, T = 296 K, μ(MoKα) = 0.123 mm-1, Dcalc = 1.500 g/cm3, 62749 reflections measured (5.88° ≤ 2Θ ≤ 57.058°), 3911 unique (Rint = 0.0536, Rsigma = 0.0226) which were used in all calculations. The final R1 was 0.0569 (I > 2σ(I)) and wR2 was 0.1710 (all data). Detailed investigation of molecular packing of the TMPPc molecule indicated the presence of intermolecular hydrogen bond between N4-H44···O1 and C13-H13B···O4 that generates C22(14) chain running parallel to the [001] direction. The infrared and Raman spectra of the prepared TMPPc compound were recorded and discussed. The thermal stability of the obtained TMPPc crystal was analysed by TGA/DTG technique and revealed that the crystal was stable up to 162 °C. Density functional theory calculations such as the value of the HOMO and LUMO energy gap, the parameters of the molecular electrostatic potential, the global reactivity and thermodynamic properties of the compound TMPPc were also performed using the DFT/B3LYP method with the level of the 6-311G (d, p) basis set. Picric acid 2,4,6-Collidine Thermal properties X-ray structure determination Molecular electrostatic potential 2,4,6-Trimethylpyridinium picrate Cite this: Eur. J. Chem. 2022, 13(4), 468-477 Journal website: www.eurjchem.com 1. Introduction 2,4,6-Trimethylpyridine, as named 2,4,6-collidine, is a pyridine derivative containing three methyl groups. It is useful as a base for a variety of reactions, such as dehydrohalogenation reactions [1]. Besides, it is a sterically hindered base (pKa = 7.43) and is also used as solvent in direct tritylation reactions of weakly acidic compounds as acetone and acetonitrile. It is used as the fluorinating reagent for organo-transition metal alkyls via 2,4,6-trimethyl-pyridine-bis-hydrofluoride [2], as also the electrophilic N-F reagents in the preparations of the pharmaceuticals containing fluoro-aliphatic, aromatic and heterocyclic units [3,4]. It is also important for the synthesis of the bis(2,4,6-trimethyl-pyridine)iodine hexafluorophosphate and bis(2,4,6-trimethyl-pyridine)bromine hexafluorophosphate reagents [5,6]. On another hand, picric acid (PA), as known as 2,4,6- trinitrophenol (TNP), is a phenol derivative containing three nitro groups and has the great possibility of resonance between the nitro groups with the negative charge of phenolic oxygen, thus it is really one of the strongest organic acids (pKa = 0.38) [7]. For organic nonlinear optics (NLO) materials, picric acid is known as an acidic ligand because it tends to form salts, particularly with aromatic or aliphatic amines [8-11]. Picric acid has been shown to act as an acceptor to form various π- stacking complexes with some aromatic molecules. It has been observed that it can form charge transfer complexes with some aromatic hydrocarbon groups, such as naphthalene, anth- racene, etc. [12-14]. In addition, picric acid is widely used in picro dye reactions, in the manufacture of matches, electric batteries, colored glass, disinfectants, and explosives as a component of rocket fuel [15,16]. Moreover, it has been widely used in leathers, pharmaceuticals, agriculture, etc. [17]. Previously, the crystal structure of picrates of various pyridine derivatives were synthesized and grouped, and the geometry and possible H-bond correlations of the pyridinium- picrate ion pair were investigated [18]. In this study, one of these compounds was examined in more detail and a new pers- pective was tried to be created for such compounds. Therefore, 2,4,6-trimethylpyridinium picrate was resynthesized, its single crystal was magnified by slow evaporation in THF solvent and characterized by elemental analysis, FT-IR, 1H NMR, and 13C NMR techniques. Its crystal properties were investigated by X- ray diffraction analysis. ABSTRACT RESEARCH ARTICLE KEYWORDS https://dx.doi.org/10.5155/eurjchem.13.4.468-477.2349 https://www.eurjchem.com/ https://dx.doi.org/10.5155/eurjchem.13.4.468-477.2349 mailto:faydin@comu.edu.tr http://www.eurjchem.com/ https://crossmark.crossref.org/dialog/?doi=10.5155/eurjchem.13.4.468-477.2349&domain=pdf&date_stamp=2022-12-31 Arslan and Aydin / European Journal of Chemistry 13 (4) (2022) 468-477 469 2022 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.13.4.468-477.2349 N CH3 H3C CH3 NO2 NO2O2N OH + N H CH3 CH3H3C O NO2 NO2 O2N THF Scheme 1. Synthesis pathway of the 2,4,6-trimethylpyridinium picrate. The optimized molecular geometry, the molecular electro- static potential (MEP) map, the highest occupied molecular orbital energy (HOMO), the lowest unoccupied molecular orbital energy (LUMO) and the thermodynamic properties were calculated by using density functional theory (DFT) at the B3LYP/6-311G(d,p) level. UV-Visible, infrared, and Raman spectra were recorded and discussed in detail. The thermal behavior of the organic TMPPc salt was also investigated by thermogravimetric analysis (TG/DTG). 2. Experimental 2.1. Material and methods 2,4,6-Trinitrophenol (picric acid), 2,4,6-trimethylpyridine (2,4,6-collidine), ethanol and tetrahydrofuran were purchased from Sigma-Aldrich and Merck Chemical Company. The melting point of the title compound was determined on the Electro- thermal 9100® apparatus. FT-IR analysis was performed using a PerkinElmer Spectrum-100 FT-IR instrument with an ATR apparatus in the range 4000-650 cm−1. The Raman spectrum was recorded in the region of 3500-150 cm-1 on a WitecAlpha 300RA FT-Raman spectrometer by using a 532 nm green laser. The 1H NMR and 13C NMR spectra were recorded on a JEOL ECX- 400 FT-NMR spectrometer operating at 400 and 100 MHz, respectively, using TMS as an internal standard and DMSO-d6 as solvent. UV-Vis measurements were carried out with a Perkin Elmer WinLab-25 series spectrophotometer in quartz cells of 1 cm path length. The crystal structure analysis of the title compound was carried out using a Bruker APEX-II CCD X-ray diffractometer (MoKα radiation, 0.71073 Å). PerkinElmer TGA 8000 was used for the TG/DTG analysis of the title compound. The 8.60 mg sample was heated at 10 ℃/min from 30 to 800 ℃ in a nitrogen atmosphere. 2.2. The synthesis of the 2,4,6-trimethylpyridinium picrate Picric acid (moistened with H2O, ≥98%, Sigma-Aldrich) was dried over two days at room temperature and recrystallized in ethanol, (Note: Aqueous solution of picric acid was purchased due to the explosive nature of it). Crystallized picric acid (1.145 g, 5 mmol) was dissolved in dry tetrahydrofuran (20 mL) and 2,4,6-trimethylpyridine (0.605 g, 5 mmol) was dropped to the above solution. The mixture was refluxed with stirring for 30 min. After then, the obtained yellow precipitate was filtered by using Whatman filter paper and dried in a vacuum desiccator. The obtained product (picrate salt) was crystallized by slow evaporation from THF as a solvent (Scheme 1). 2,4,6-Trimethylpyridinium picrate (TMPPc): M.p.: 159-160 ℃. FT-IR (ATR, ν, cm-1): 3281, 3067, 2921, 2792, 2721, 1612, 1556, 1481, 1329, 1259, 1151, 908, 709. 1H NMR (400 MHz, DMSO-d6, δ, ppm): 2.57 (s, 3H, CH3), 3.29 (s, 6H, CH3), 7.52 (s, 2H, Pry-H), 8.54 (s, 2H, Ph-H). 13C NMR (100 MHz, DMSO-d6, δ, ppm): 161.38 (1C, C6), 159.16 (1C, C9), 152.34 (2C, C7, C11), 142.33 (2C, C2, C4), 125.68 (2C, C8, C10), 125.57 (1C, C9), 124.68 (2C, C1, C5), 21.83 (2C, C12, C14), 19.38 (1C, C13). 2.3. X-ray crystallography A single crystal of 2,4,6-trimethylpyridinium picrate was grown by slow evaporation of the solution of product in tetrahydrofuran. The diffraction data of it were collected on a Bruker APEX-II CCD diffractometer using MoKα radiation. The cell parameters and crystal structure were solved and refined by SHELXS-97 and SHELXL-97 programs, respectively [19,20]. The refinement was carried out by the full-matrix least-squares method on the positional and anisotropic temperature parame- ters of the non-hydrogen atoms, or equivalently corresponding to 227 crystallographic parameters. The atomic numbering scheme with displacement ellipsoids of the crystal structure drawn with ORTEPIII was depicted at the 30% probability level for clarity (Figure 1). The details of crystal data, experimental condition and structural refinement are listed in Table 1. 2.4. Computational studies The molecular structure of 2,4,6-trimethylpyridinium picrate was optimized using DFT in the ground state by the B3LYP method with the 6-311G(d,p) basis sets included in Gaussian 09 program [21]. The molecular structure of the salt optimized by 6-311G(d,p) basis set displayed frontier mole- cular orbitals, and the electrostatic potential was simulated. The frontier molecular orbital and energy gap between the highest occupied and the lowest unoccupied molecular orbitals were calculated with density functional theory. In addition, thermodynamic parameters (i.e., heat capacity, enthalpy, and entropy values) for the title molecule were also calculated. 3. Results and discussion 3.1. Crystal structure and optimized geometry The 2,4,6-trimethylpyridinium picrate crystal, magnified by slow evaporation in a polar aprotic solvent (THF), is in the monoclinic form with space group P21/c with Z = 4 in the unit cell, similar to the literature [18]. The title compound consists of the 2,4,6-trimethylpyridinium and 2,4,6-trinitrophenolate moieties. The molecular packing diagram of the prepared compound is located along the a-axis (Figure 2). Due to the functional groups of the compound, the b-axis and the c-axis 27.863(3) and 13.9247(17) Å, respectively, are larger than the a-axis 4.0174(4) Å in the crystal dimensions. Crystallographic data and details of the X-ray diffraction study of the title compound are shown in Table 1. The bond lengths and bond angles of the 2,4,6-trimethylpyridinium picrate are given in Table 2, while the hydrogen bond details are compiled in Table 3. 470 Arslan and Aydin / European Journal of Chemistry 13 (4) (2022) 468-477 2022 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.13.4.468-477.2349 Table 1. Crystal data, details of the structure refinement parameters for the 2,4,6-trimethylpyridinium picrate. Empirical formula C14H14N4O7 Formula weight 350.29 Temperature (K) 293(2) Crystal system Monoclinic Space group P21/c a, (Å) 4.0174(4) b, (Å) 27.863(3) c, (Å) 13.9247(17) β (°) 95.741(4) Volume (Å3) 1550.9(3) Z 4 ρcalc (g/cm3) 1.500 μ (mm-1) 0.123 F(000) 728.0 Crystal size (mm3) 0.18 × 0.15 × 0.13 Radiation MoKα (λ = 0.71073) 2Θ range for data collection (°) 5.88 to 57.058 Index ranges -5 ≤ h ≤ 5, -37 ≤ k ≤ 37, -18 ≤ l ≤ 18 Reflections collected 62749 Independent reflections 3911 [Rint = 0.0536, Rsigma = 0.0226] Data/restraints/parameters 3911/0/227 Goodness-of-fit on F2 1.063 Final R indexes [I≥2σ (I)] R1 = 0.0569, wR2 = 0.1513 Final R indexes [all data] R1 = 0.0816, wR2 = 0.1710 Largest diff. peak/hole (e.Å-3) 0.33/-0.29 Computer programs BrukerAPEX2, BrukerSAINT, SHELXT 2014/4, SHELXL2016/6 Figure 1. ORTEP III diagram of the 2,4,6-trimethylpyridinium picrate. Figure 2. Molecular packing diagram of the 2,4,6-trimethylpyridinium picrate is displaced along the a-axis. The 2,4,6-trinitrophenolate moiety of the compound has a closely planar configuration and the maximum deviation from the mean plane belongs to the O2 oxygen atom with 0.288 Å. The picrate group twists slightly due to the consistency of the trimethyl group. The torsion angle values about this twist are - 16.3(3)° on the C6/C1/N1/O2 atom group and 164.98(19)° on the C6/C1/N1/O3 atom group. The molecular packing of the crystal consists of an N-H···O type intramolecular hydrogen bonding and a weak C-H···O interaction. The N2-H2···O1 intramolecular hydrogen bonding occurs between the N2 atom of the 2,4,6-trinitrophenolate group and the O1 atom of the 2,4,6-trimethylpyridinium group with 2.776(2) Å distance of D···A. The C13-H13B and N4-H44 groups in the molecule act as hydrogen-bond donors to atoms O4i and O1, respectively (Symmetry code, i: x, y, z+1.) forming a C22(14) chains running parallel to the [001] direction (Figure 2). Arslan and Aydin / European Journal of Chemistry 13 (4) (2022) 468-477 471 2022 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.13.4.468-477.2349 Table 2. Comparison of the optimized and calculated geometry (bond lengths (Å) and bond angles (°) of the 2,4,6-trimethylpyridinium picrate. Parameters Experimental Calculated B3LYP/6-311G(d,p) Parameters Experimental Calculated B3LYP/6-311G(d,p) Bond lengths (Å) C1-C2 1.369(3) 1.3836 C8-C9 1.388(3) 1.3967 C1-C6 1.455(3) 1.4529 C9-C10 1.393(3) 1.3990 C1-N1 1.457(3) 1.4554 C9-C13 1.497(3) 1.5039 C2-C3 1.378(3) 1.3865 C10-C11 1.375(3) 1.3858 C3-C4 1.389(3) 1.3959 C11-N4 1.354(3) 1.3508 C3-N2 1.449(3) 1.4601 C11-C14 1.492(3) 1.4966 C4-C5 1.367(3) 1.3740 N1-O2 1.215(3) 1.2356 C5-C6 1.461(3) 1.4518 N1-O3 1.223(3) 1.2249 C5-N3 1.465(3) 1.4692 N2-O5 1.209(3) 1.2268 C6-O1 1.252(2) 1.2538 N2-O4 1.220(3) 1.2268 C7-N4 1.354(3) 1.3499 N3-O6 1.199(3) 1.2251 C7-C8 1.376(3) 1.3882 N3-O7 1.203(3) 1.2257 C7-C12 1.492(3) 1.4968 Bond angels (°) C2-C1-C6 123.92(19) 123.4756 C8-C9-C10 118.2(2) 118.1433 C2-C1-N1 115.61(18) 116.5486 C8-C9-C13 121.5(2) 121.1031 C6-C1-N1 120.47(18) 119.9489 C10-C9-C13 120.3(2) 120.7460 C1-C2-C3 120.12(19) 119.6391 C11-C10-C9 121.0(2) 120.5834 C2-C3-C4 120.89(19) 120.8270 N4-C11-C10 118.09(19) 118.4516 C2-C3-N2 119.69(19) 119.5960 N4-C11-C14 118.1(2) 117.3962 C4-C3-N2 119.4(2) 119.5748 C10-C11-C14 123.8(2) 124.1487 C5-C4-C3 119.0(2) 119.3309 O2-N1-O3 121.2(2) 123.3341 C4-C5-C6 124.73(19) 124.0346 O2-N1-C1 120.1(2) 118.1011 C4-C5-N3 115.56(19) 116.7237 O3-N1-C1 118.6(19) 118.5569 C6-C5-N3 119.71(18) 119.2403 O5-N2-O4 122.6(2) 124.6459 O1-C6-C1 124.1(2) 125.3451 O5-N2-C3 119.2(2) 117.6111 O1-C6-C5 124.60(19) 122.0752 O4-N2-C3 118.2(2) 117.7428 C1-C6-C5 111.28(17) 112.4751 O6-N3-O7 120.7(2) 124.3083 N4-C7-C8 118.28(19) 118.3672 O6-N3-C5 118.3(2) 117.4655 N4-C7-C12 118.57(19) 117.5172 O7-N3-C5 121.0(2) 118.1909 C8-C7-C12 123.1(2) 124.1106 C11-N4-C7 123.61(18) 123.8217 C7-C8-C9 120.8(2) 120.6323 Table 3. Hydrogen-bond geometry (Å, °) for the 2,4,6-trimethylpyridinium picrate. D-H···A D-H H···A D···A ∠ D-H···A N4-H44···O1 0.86 1.96 2.776 (2) 159 C13-H13B···O4i 0.96 2.30 3.233 (4) 163 Symmetry code: (i) x, y, z+1. The formation of salt appears due to intramolecular proton transfer from the oxygen atom in 2,4,6-trinitrophenol to the nitrogen atom in 2,4,6-trimethylprydine. The optimized geometry parameters (theoretical) for the title compound, namely, the bond lengths and bond angles, were calculated using the B3LYP/6-311G(d,p) method and are listed in Table 2. The correlation coefficient data between the calculated and experimental geometrical parameters (bond length and bond angle values) were calculated. The data obtained show that almost all optimized bond lengths and bond angles are slightly larger than the experimental values. The correlation coefficient values for the bond length and bond angles by experimental and theoretical (B3LYP) are 0.9944 and 0.8043, respectively. The molecular conformation described by the torsional angles obtained from the X-ray data is C1-C2-C3-N2 and N3-C5- C6-C1 178.8(2)°, -179.19288° and -177.65(18)°, 175.0860°, as obtained by the experimental method, are smaller than those determined by the B3LYP method, as observed in the reported research paper [22]. 3.2. 1H NMR and 13C NMR analysis The 1H and 13C NMR spectrum of 2,4,6-trimethylpyridinium picrate is obtained in DMSO-d6. The OH proton signal in free picric acid normally appears at δ 11.94 ppm [23]. There is no OH signal observed in the 1H NMR spectrum due to the migration of the phenolic proton to pyridine nitrogen during the formation of TMPPc. The singlet peak at δ 8.54 ppm is assigned to the protons of the aromatic carbon atoms (C2 and C4) in the picrate moiety. The singlet peak at δ 7.52 ppm is due to the protons at C8 and C10 carbons of the pyridinium moiety. The presence of two different methyl protons at δ 3.29 (s, 6H) and 2.57 ppm (s, 3H) is confirmed by the appearance of the signals of the cationic moiety as 2,4,6-trimethylpyridinium. In the 13C NMR spectra of the title compound, there are magnetically and chemically two different aromatic rings. The ipso carbon (C6) signal of the picrate group appears at δ 161.29 ppm. The observed peaks at δ 142.33 and 125.57 ppm of the crystal were due to ortho (C1-C5) and para (C3) carbon atoms that contain the NO2 group in the picrate moiety, respectively. The peak at δ 124.68 ppm is assigned to the meta carbons (C2 and C4) of the picrate moiety. The observed peaks at δ 159.16 and 152.34 ppm of the crystal were due to C7-C11 and C9 carbon atoms containing CH3 group in the pyridinium moiety, respectively. The peak at δ 125.68 ppm is assigned to the C8 and C10 carbons of the picrate moiety. The methyl groups in the structure of the pyridinium picrate derivatives were confirmed at δ 21.83 and 19.38 ppm, respectively (Scheme 2). 3.3. FT-IR and Raman spectroscopic analysis FT-IR spectra of the 2,4,6-trimethylpyridine, picric acid, and 2,4,6-trimethylpyridinium picrate are given in Figure 3. In the split spectrum, while some functional groups are disappeared, some functional groups are appeared due to picrate salts. In the FT-IR spectrum, the broad peak observed at 3274 cm-1 is assigned to the presence of a hydrogen-bonded phenolic group [24]. The FT-Raman spectrum of 2,4,6-trimethylpyridinium picrate is also shown in Figure 4. In the Raman spectrum, this phenolic group peak does not appear to be very pronounced. The C-H vibrations cover the asymmetric and symmetric stretching vibration modes of the benzene and pyridine rings, as well as the methyl group. 472 Arslan and Aydin / European Journal of Chemistry 13 (4) (2022) 468-477 2022 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.13.4.468-477.2349 7.52 ppm 7.52 ppm 2.57 ppm 3.29 ppm3.29 ppm 8.54 ppm 8.54 ppm N H CH3 CH3CH3 O N+ O O - N+ O - O N+ O - O 152.34 ppm 125.68 ppm 19.38 ppm 21.83 ppm21.83 ppm 125.57 ppm N H CH3 CH3CH3 O N+ O O - N+ O- O N+ O - O 152.34 ppm 125.68 ppm 142.33 ppm 124.68 ppm 159.16 ppm 161.38 ppm 124.68 ppm 142.33 ppm Scheme 2. 1H and 13C NMR chemical shifts of 2,4,6-trimethylpyridinium picrate in DMSO-d6. Figure 3. FT-IR spectrums of 2,4,6-trimethylpyridine (a), 2,4,6-trinitrophenol (b) and 2,4,6-trimethylpyridinium picrate (c). Figure 4. Raman spectrum of the 2,4,6-trimethylpyridinium picrate. Vibration bands at 3068 and 3027 cm -1 (νCarom-H), 1566 and 1556 cm-1 (νC=Carom) in FT-IR spectra were observed as the result of the vibration of pyridyl and phenyl rings, respectively. Asymmetric and symmetric stretching vibrations of alip- hatic C-H bonds of methyl groups normally appear in the range of 3000-2700 cm-1 [25]. There are two different methyl func- tional groups (-CH3) attached to the aromatic ring in the structure of the crystal molecule. The peaks observed at 2934, 2872, and 2723 cm-1 in FT-IR spectra and 2933, 2817 and 2762 cm-1 in Raman spectra, respectively, confirm the existence of aliphatic C-H stretching vibrations of the TMPPc compound. Strong C-H bending vibrations are also observed at 904 and 913 cm-1 in the FT-IR and the Raman spectra, respectively. Further- more, the peaks occurring at 1622 and 1648 cm-1 are defined as the C=N stretching vibrations of TMPPc in the FT-IR and Raman spectra, respectively, and are attributed to the presence of the pyridinium core. The absorption bands at 1481 and 1258, 1454 and 1272 cm-1 are assigned to C-C and C-N stretching vibrations in FT-IR and Raman spectra, respectively [26]. Arslan and Aydin / European Journal of Chemistry 13 (4) (2022) 468-477 473 2022 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.13.4.468-477.2349 (a) (b) (c) Figure 5. (a) UV-Vis spectrum, (b) the scheme of charge transfers intermolecular for the π → π* transitions and (c) Tauc’s plot of the 2,4,6-trimethylpyridinium picrate. The aromatic molecules containing nitro functional group show a strong band in the region 1570-1485 cm-1 and 1370- 1320 cm-1 due to the NO2 asymmetric and symmetric stretching vibrations, respectively [27]. In the title organic salt, the sharp bands appearing at 1361, 1329 cm-1 and 1347, 1314 cm-1 in FT- IR and Raman spectra are assigned to the asymmetric and symmetric stretching vibrations of the NO2 group of the picrate anions, respectively. Other characteristics are observed at 868, 787, and 741 cm- 1 in the FT-IR spectra and at 825, 763, and 719 cm-1 in the Raman spectra, which are caused by out-of-plane bending (- NO2), in scissoring (-NO2) vibration mode of the nitro groups, respectively, and these assignments are very characteristic of the picrate anion [28]. 3.4. UV-Vis spectra and energy band gap The UV-Vis spectrum of the 2,4,6-trimethylpyridinium picrate (Figure 5a) showed an absorption maximum at 262 nm due to its charge transfer intermolecular π → π* transitions (Figure 5b). The absorbance at 380 nm was assigned to the transitions that take place in the 2,4,6-trimethylpyridine and picric acid moiety due to the shift of electrons from the non- bonding orbital to the anti-bonding orbital (n → π*). The optical band gaps (Eg) of 2,4,6-trimethylpyridinium picrate were calculated using Tauc’s plot [29]. The optical band gaps were found by extrapolating the curve (α×h×ν)2 = 0 (Inset of Figure 5c) and the obtained values are 2.7 and 4.3 eV, respectively. All this evidence supports the suggested structure and charac- teristic features of 2,4,6-trimethylpyridinium picrate. 3.5. HOMO and LUMO analysis The frontier molecular orbitals (FOMs) of a molecule are the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Molecular orbitals and their properties like energy are very useful in understanding and interpreting nature. Their frontier electron density is used to predict the most reactive position in the π-electron system and explains several types of reactions in conjugated systems [30]. The electronic absorption corresponding to the transition from the ground state to the first excited state is mainly described by electron excitation from the HOMO to the LUMO. Moreover, the Eigenvalues of HOMOs (π-donor) and LUMOs (π- acceptor) and their energy gaps show the charge transfer interaction taking place within the molecule and reflect the chemical activity [31]. Theoretical structural studies were carried out for the title crystal by optimizing the structure in the gas phase using B3LYP functional with 6-311G(d,p) basis set. The electronic properties of the molecule are characterized by the analysis of the frontier orbitals. The energy values of the HOMO and HOMO-1, LUMO and LUMO+1 frontier molecular orbitals with the corres- ponding ΔEHOMO-LUMO relative energy gap is shown in Figure 6. The positive and negative phase are represented in red and green color, respectively. As can be seen in Figure 6, the plot reveals that HOMO is primarily composed of the trinitrophenyl moiety corresponding to the benzene ring whereas the LUMO is spread over to the pyridinyl moiety of the molecule. The orbital energy level analysis for the title compound showed that HOMO and HOMO-1 values are -6.44 and -7.25 eV, while the values of LUMO and LUMO+1 are -2.73 and -2.58 eV, respectively. 474 Arslan and Aydin / European Journal of Chemistry 13 (4) (2022) 468-477 2022 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.13.4.468-477.2349 Figure 6. Frontier molecular orbital diagram for 2,4,6-trimethylpyridinium picrate. Figure 7. Molecular electrostatic potential map calculated at B3LYP/6-311G(d,p) level for the 2,4,6-trimethylpyridinium picrate. The energy gap values of the title compound calculated at the DFT level are 3.71 and 5.16 eV, respectively. It can be said that the title compound, which has large HOMO-LUMO gaps 3.71 and 5.16 eV, respectively, implies high kinetic stability and low chemical reactivity. 3.6. Molecular electrostatic potential (MEP) The MEP is a colored plot mapped onto the isosurfaces of electron density, which can be experimentally measured with X-ray diffraction and calculated by using the B3LYP functional with 6-311G(d,p) basis set. The different values of the electro- static potential on the surface are represented by different colors. Potential increases in order red < orange < yellow < green < blue. The color code of these maps is in the range between -5.744 a.u. (deepest red) and 5.744 a.u. (deepest blue) in the compound, where blue shows the strongest attraction and red shows the strongest repulsion. Regions of negative V(r) are usually associated with the lone pair of electronegative atoms [32]. To investigate the possible reactive sites of the structure to interact with the molecular electrostatic potential surface was generated [33]. It is known that nitro and methyl substituents in the aromatic ring have negative and positive inductive effects, respectively. As can be seen from the MEP map of the 2,4,6-trimethylpyridinium picrate compound (Figure 7), while the regions having the negative potential are over the electronegative atoms (oxygen atoms in the 2,4,6- trinitrophenyl ring), the regions having the positive potential are over the 2,4,6-trimethylpyridine ring. Consistent with the above results, the α-methyl, nitro, and amine parts play a crucial role in the chemical and intermolecular interactions. Arslan and Aydin / European Journal of Chemistry 13 (4) (2022) 468-477 475 2022 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.13.4.468-477.2349 Table 4. The calculated molecular electronic properties of the 2,4,6-trimethylpyridinium picrate (vacuum). Parameters Value EHOMO (eV) -6.4494 ELUMO (eV) -2.7303 Energy band gap, ΔE = EHOMO - ELUMO (eV) -3.7191 Ionization energy, I = -EHOMO (eV) +6.4494 Electron affinity, A = -ELUMO (eV) +2.7303 Chemical hardness, η = (I-A)/2 (eV) 1.8596 Chemical softness, ζ = 1/2η (eV) 0.2688 Nucleophilicity, ε = 1/ω (eV) 0.1765 Chemical potential, μ = - (I+A)/2 (eV) -4.5898 Electrophilicity index, ω = (μ²/2η) (eV) 5.6646 Electronegativity, χ = (I+A)/2 (eV) 4.5898 Dipol moment (Debye) 15.4277 Electronic energy (a.u.) -1287.5674 Figure 8. TGA-DTG thermograms of 2,4,6-trimethylpyridinium picrate. Figure 9. Thermodynamic properties of the 2,4,6-trimethylpyridinium picrate at different temperatures at B3LYP/6-311G(d,p) level. The ionization energy (I), chemical potential (μ), hardness (η), softness (S), electronegativity (χ), and electrophilicity index (ω) of the molecule are calculated and depicted in Table 4. 3.7. Thermal properties The thermal stability of the 2,4,6-trimethylpyridinium picrate was studied in a nitrogen atmosphere from 30 to 800 ℃ at a heating rate of 10 ℃/min and the obtained TG/DTG diag- ram is given in Figure 8. TG curve indicates that there was no weight loss from 30 to 162 °C for the 2,4,6-trimethylpyridinium picrate compound. The molecule is stable up to 160 ℃. The first peak at 160 ℃ is assigned to the dehydration of hygroscopic water (about 2.4%). After the dehydration of hygroscopic water, the 2,4,6-trimethylpyridinium picrate compound under- goes two stages of decomposition. The first decomposition starts at ca. 162 °C and ends at 278 °C with the loss of weight (63.65%). This major weight loss coincides exactly with the peak (278 °C) of the DTG curve. This may be due to the evolution of hydrocarbon gases followed by ring rupture. The second weight loss occurs between 278 and 738 °C in the TG curve. This may be due to the liberation of various gaseous like volatile substances, ammonia, and nitrogen dioxide gases [34]. After this temperature, the curve begins to flatten out and the crystal decomposes completely without any residue. The 2,4,6- trimethylpyridinium picrate is stable up to 162 °C and hence, this crystal can be used for several applications. 476 Arslan and Aydin / European Journal of Chemistry 13 (4) (2022) 468-477 2022 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.13.4.468-477.2349 3.8. Thermodynamic properties The statistically thermodynamic parameters, such as the standard heat capacity of constant pressure (C°p,m), the enthalpy (ΔH°m), and the entropy (S°m), were calculated using the B3LYP/6-311G(d,p) method in the ground state by increasing from 100 to 1000 K in the gas phase. Figure 9 shows that C°p,m, S°m, and ΔH°m of the title compound increase from 100 to 1000 K, which is caused by the rise of molecular vibration intensity with the increasing temperature [35]. Quadratic formulas were used to fit the correlation equations between different parameters, such as heat capacity, enthalpy, entropy, and temperature; R being the corresponding fitting factor for these thermodynamic parameters. The corres- ponding fitting equations for the title compound are as presented below: 𝐶𝐶𝑝𝑝,𝑚𝑚 ° = 12.8958 + 0.2937 T -1.2688×10-4 T2 (R2 = 0.9997) (1) 𝑆𝑆𝑚𝑚° = 72.8082 + 0.3725 T – 1.0422×10-4 T2 (R2 = 0.9971) (2) 𝛥𝛥𝛥𝛥𝑚𝑚0 = 167.1457 + 0.0454 T +7.7029×10-5 T2 (R2 = 0.9994) (3) 4. Conclusions The 2,4,6-trimethylpyridinium picrate crystal obtained from 2,4,6-trimethylpyridine and picric acid was magnified by the slow evaporation technique in THF solvent. When it was investigated by a single crystal X-ray diffraction study, the crystal structure of the 2,4,6-trimethylpyridinium picrate compound has the feature of monoclinic space group and was found to be similar to literature data. A N-H···O type intra- molecular hydrogen bonding and a C-H···O intermolecular weak interaction were observed in the crystal packing. The FT- IR, 1H NMR, and 13C NMR spectroscopic analyses confirm the crystallinity and purity of the material. When the crystal of the 2,4,6-trimethylpyridinium picrate compound is examined by X- ray analysis and spectroscopic data, it confirms the formation of proton transfer from picric acid to 2,4,6-collidine. The FT-IR spectrum, especially the +NH stretching vibration at 3327 cm-1, was substantial to the presence of functional groups. The theoretical values of the bond length and bond angle agree well with the experimental values. The FOMs, the molecular electro- static potential and the global reactivity descriptors were also calculated by using DFT methods and discussed. The identified HOMO-LUMO energy gap was discovered to be 3.71 eV, which explains the kinetic stability of the molecule. The molecular electrostatic potential map showed that the electron density of the picrate anion moiety was higher than that of the 2,4,6- trimethylpyridinium moiety. It was also observed the rising of molecular vibration intensity with the increasing temperature. The TG/DTG analysis showed that the title crystal is stable under 162 °C, and it starts to decompose without leaving residue. Acknowledgements This work was supported by the Scientific Research Coordination Unit of Çanakkale Onsekiz Mart University, Çanakkale, Turkey, (Project no: FYL- 2016-672). Supporting information CCDC-1548802 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by e-mailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44(0)1223-336033. Disclosure statement Conflict of interest: The authors declare that they have no conflict of interest. Ethical approval: All ethical guidelines have been adhered. Sample availability: Sample of the compound is available from the author. CRediT authorship contribution statement Conceptualization: Fatma Aydin, Nahide Burcu Arslan; Methodology: Fatma Aydin, Nahide Burcu Arslan; Software: Nahide Burcu Arslan; Synthesis - Characterization: Fatma Aydin; Validation: Fatma Aydin, Nahide Burcu Arslan; Formal Analysis: Fatma Aydin, Nahide Burcu Arslan; Investigation: Fatma Aydin, Nahide Burcu Arslan; Data Curation: Fatma Aydin, Nahide Burcu Arslan; Writing - Original Draft: Fatma Aydin, Nahide Burcu Arslan; Writing - Review and Editing: Fatma Aydin, Nahide Burcu Arslan; Visualization: Fatma Aydin, Nahide Burcu Arslan; Funding acquisition: Fatma Aydin. ORCID and Email Nahide Burcu Arslan burcu.aslan@giresun.edu.tr https://orcid.org/0000-0002-1880-1047 Fatma Aydin faydin@comu.edu.tr https://orcid.org/0000-0002-7219-6407 References [1]. Sherman, A. R. 2,4,6-Collidine. Encyclopedia of Reagents for Organic Synthesis 2001. [2]. Herzog, A.; Roesky, H. 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This is an open access article distributed under the terms and conditions of the CC BY NC License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited without any further permission from Atlanta Publishing House LLC (European Journal of Chemistry). No use, distribution or reproduction is permitted which does not comply with these terms. Permissions for commercial use of this work beyond the scope of the License (http://www.eurjchem.com/index.php/eurjchem/pages/view/terms) are administered by Atlanta Publishing House LLC (European Journal of Chemistry). http://www.eurjchem.com/index.php/eurjchem/pages/view/terms http://creativecommons.org/licenses/by-nc/4.0 http://www.eurjchem.com/index.php/eurjchem/pages/view/terms 1. Introduction 2. Experimental 2.1. Material and methods 2.2. The synthesis of the 2,4,6-trimethylpyridinium picrate 2.3. X-ray crystallography 2.4. Computational studies 3. Results and discussion 3.1. Crystal structure and optimized geometry 3.2. 1H NMR and 13C NMR analysis 3.3. FT-IR and Raman spectroscopic analysis 3.4. UV-Vis spectra and energy band gap 3.5. HOMO and LUMO analysis 3.6. Molecular electrostatic potential (MEP) 3.7. Thermal properties 3.8. Thermodynamic properties 4. Conclusions Acknowledgements Supporting information Disclosure statement CRediT authorship contribution statement ORCID and Email References PrintField10: PrintField11: PrintField12: PrintField13: PrintField14: PrintField15: PrintField16: PrintField17: PrintField18: PrintField19: PrintField20: PrintField21: PrintField22: PrintField23: PrintField24: PrintField25: PrintField26: PrintField27: PrintField28: PrintField29: