Synthesis, analysis of single crystal structure and computational studies on a novel picrate derivative: 2-(Pyridine-2-ylthio)pyridine-1-ium picrate European Journal of Chemistry 16 (2) (2025) 117-128 European Journal of Chemistry ISSN 2153-2249 (Print) / ISSN 2153-2257 (Online) – Copyright © 2025 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.16.2.117-128.2673 European Journal of Chemistry View Journal Online View Article Online Synthesis, analysis of single crystal structure and computational studies on a novel picrate derivative: 2-(Pyridine-2-ylthio)pyridine-1-ium picrate Fatma Aydin 1,* and Asli Ozturk Kiraz 2 1 Department of Chemistry, Faculty of Science, Çanakkale Onsekiz Mart University, 17100, Çanakkale, Turkey 2 Department of Physics, Computational Physics Laboratory, Faculty of Science, Pamukkale University, 20070, Denizli, Turkey * Corresponding author at: Department of Chemistry, Faculty of Science, Çanakkale Onsekiz Mart University, 17100, Çanakkale, Turkey. e-mail: faydin@comu.edu.tr (F. Aydin). 10.5155/eurjchem.16.2.117-128.2673 Received: 12 February 2025 Received in revised form: 18 April 2025 Accepted: 1 May 2025 Published online: 30 June 2025 Printed: 30 June 2025 A new organic salt, 2-(pyridine-2-ylthio)pyridine-1-ium picrate (C16H11N5O7S: 2-PyrTPPc), has been synthesized and characterized using various spectroscopic methods such as 1H NMR, 13C NMR, and FT-IR. The crystal structure of the title compound was analyzed using X- ray structure analysis, which revealed that it belongs to the monoclinic P21/c space group with a = 16.876(4) Å, b = 7.6675(18) Å, c = 13.846(3) Å, Z = 4, and V = 1766.9(7) Å3. The molecular packing of the compound showed the presence of several intermolecular hydrogen bonds between different atoms. The electronic properties of the crystal were investigated using density functional theory (DFT) with B3LYP/6-311G(d,p) level. Frontier molecular orbitals were drawn and related global quantities such as electronic chemical potential, chemical hardness-softness, electrophilicity, HOMO and LUMO energy eigenvalues, and the difference between HOMO and LUMO (∆E) were calculated and discussed. TG/DTG analysis revealed the thermal stability of the 2-PyrTPPc crystal. The single stage of decomposition and the sharpness of the peak in the temperature range of 157-224 °C illustrated the purity and good crystallinity of the grown crystal. The different vibration modes of the 2-PyrTPPc molecule were determined by analyzing the FT-IR and FT- Raman spectra. The detection of vibrations of the pyrNH+ and aromatic thioether moiety supports the confirmation of the di(pyridin-2-yl)sulfane structure through the intermediate formed by the transfer of the proton from picric acid to 2-mercaptopyridine. Thermal properties 2-Mercaptopyridine Density functional theory X-ray structure determination Molecular electrostatic potentials 2-(Pyridine-2-ylthio)pyridine-1-ium picrate Cite this: Eur. J. Chem. 2025, 16(2), 117-128 Journal website: www.eurjchem.com 1. Introduction 2-Mercaptopyridine is an organic sulfur compound that contains a mercapto(-SH) group in the ortho position of a pyridine ring. Due to its tautomerization reaction, the compound can exist in two forms: thione and thiol isomers [1,2]. 2-Mercaptopyridine is a Lewis base due to its base centers, such as pyridine and thiophenol. Both pyridine (pKa = 5.2) and thiophenol (pKa = 6.0) have Lewis base properties, but the thiophenol unit is more basic. Thiophenol can be used as a reagent for unsymmetrical disulfide derivatives [3], alkylating agents [4], and chelating ligands [5]. It can also be used as a selective glutathione (GSH) detection reagent [6]. With this powerful reducing agent, you can selectively oxidize it to 2,2'- (dipyridyl)disulfide [7]. Picric acid is an organic acid derived from phenol and contains three nitro groups. Due to its stabilization of the phenolate anion through the resonance of nitro groups, picric acid has become one of the strongest organic acids (pKa = 0.42). Picric acid, which is an electron acceptor, can form charge transfer complexes or ammonium salts with various aliphatic amines [8] and aromatic amines such as some aniline derivatives [9,10] and pyridine derivatives [11-13]. It is also known for its ability to form π-π* interactions with aromatic hydrocarbons [14,15]. Picric acid forms simple picrate salts from dimeric cations of the AA+ type, where A and A+ are amino acids in the Zwitter ionic and singly charged cationic states [16- 18]. Many molecular organic change transfer crystals of picric acid(donor) with organic acceptor molecules exhibit especially applications such as organic non-linear optical material (NLO) [19,20]. The purpose of this comment is to point out that the title of the article is 2-(pyridine-2-ylthio)pyridine-1-ium picrate (C10H8N2S·C6H3N3O7) and not 2-mercaptopyridinium picrate (C5H5NS·C6H3N3O7) as might appear when reading the article. Pyridine is an analog of benzene and can undergo an aromatic nucleophilic substitution reaction, similar to benzene, through an E1cB alkyne elimination reaction mechanism [21,22]. In this study, we successfully synthesized a novel picrate salt using a distinctive synthetic pathway, as illustrated in Scheme 1. The structure of the compound was thoroughly characterized through elemental analysis and a variety of spectroscopic techniques, including FT-IR, 1H NMR, and 13C NMR. The crystal structure was determined via single-crystal X- ray diffraction. To explore the electronic properties, global reactivity, thermal properties, and NBO analysis of the compound, we performed density functional theory (DFT) calculations at the B3LYP/6-311G(d,p) level. ABSTRACT RESEARCH ARTICLE KEYWORDS https://dx.doi.org/10.5155/eurjchem.16.2.117-128.2673 https://www.eurjchem.com/ https://dx.doi.org/10.5155/eurjchem.16.2.117-128.2673 mailto:faydin@comu.edu.tr http://www.eurjchem.com/ https://crossmark.crossref.org/dialog/?doi=10.5155/eurjchem.16.2.117-128.2673&domain=pdf&date_stamp=2025-06-30 118 Aydin and Kiraz / European Journal of Chemistry 16 (2) (2025) 117-128 2025 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.16.2.117-128.2673 Scheme 1. Reaction pathway for the synthesis of the title compound, 2-PyrTPPc. Figure 1. Photograph of the 2-PyrTPPc crystals. The exceptional features of both experimental and theoretical infrared and Raman spectra were meticulously examined in the article [23,24]. 2. Experimental 2.1. General remarks 2,4,6-Trinitrophenol (picric acid), 2-mercaptopyridine, ethanol, tetrahydrofuran, and chloroform were purchased from Sigma-Aldrich and Merck Chemicals. The melting point of the compound was determined using an Electrothermal 9100® apparatus. Elemental analyzes were carried out using a Carlo- Erba instrument. FT-IR analysis was performed on a Perkin Elmer Spectrum-100 FT-IR instrument with an ATR apparatus in the range of 4000-650 cm-1. The Raman spectrum was recorded in the region of 3500-150 cm-1 on a WITEC ALPHA 300RA FT-Raman spectrometer using a 532 nm green laser. The 1H NMR and 13C NMR spectra were recorded on a Bruker GmbH NMR spectrometer using CDCl3 as a solvent and TMS as the internal standard. Crystal structure analysis of the compound was performed using a Bruker D8 QUEST X-ray diffractometer (MoKα radiation, λ = 0.71073). TG/DTG analysis of the compound was performed using a Perkin Elmer TGA8000 instrument. Thermogravimetric analysis (TGA) of the sample (8.899 mg) was carried out under a nitrogen atmosphere with a heating rate of 20 °C/min from 30 to 800 °C on a PerkinElmer TGA 8000 analyzer. 2.2. Synthesis of 2-(pyridine-2-ylthio)pyridine-1-ium picrate, 2-PyrTPPc Commercial picric acid (1% in H2O) purchased from Sigma- Aldrich was crystallized using ethanol. The resulting crystallized picric acid (2.29 g, 10 mmol) was dissolved in chloroform (20 mL), and 2-mercapto pyridine (1.11 g, 10 mmol) in chloroform (15 mL) was added dropwise to the solution. The mixture was refluxed with stirring for four hours. After that, the reddish oil product obtained was crystallized by slow evaporation of chloroform. The reddish seed crystal of the title compound was grown by the slow evaporation method, after dissolving the crystallized salt in THF at room temperature. After 20 days, a good quality single crystal was harvested and is shown photographically in Figure 1. 2- (Pyridine-2-ylthio)pyridine-1-ium picrate (2-PyrTPPc): Color: Reddish. Yield: 78%. M.p.: 157-158 °C. FT-IR (ν, cm-1): 3472, 3442, 3369, 3214, 3097, 1636, 1598, 1564, 1471, 1422, 1385, 1334, 1316, 1246, 1153, 1071, 1067, 981, 928, 908, 823, 772, 701. 1H NMR (400 MHz, CDCl3, δ, ppm): 14.07 (brs, 1H, NH+), 7.71, 7.70, 7.68, 7.59, 7.46, 7.44, 7.42, 7.33, 6.86, 6.84 (10 H, Ar- H). 13C NMR (100 MHz, CDCl3, δ, ppm): 176.45 (NH=C-S), 156.10 (C-Ophenoxy), 149.33 (N=C-S), 137.87, 126.10, 122.18 (Cphenoxy), 137.76, 133.20, 119.93, 114.56 (Cprydinyl). Anal. calcd. for C16H11N5O7S: C, 46.05; H, 2.66; N, 16.78; S, 7.68; Found: C, 46.12; H, 2.70; N, 16.69; S, 7.65%. 2.3. X-ray crystallography X-ray diffraction data for a single crystal were collected using MoKα radiation at a wavelength of 0.71073 Å at a temperature of 273(2) K. The structure was determined using SHELXS-97 and refined using SHELXL-97 least squares on F2 [25]. Absorption correction was performed using the multi- scan method. These data are available free of charge from the Cambridge Crystallographic Data Center at www.ccdc.cam.ac.uk/data_request/cif. http://www.ccdc.cam.ac.uk/data_request/cif Aydin and Kiraz / European Journal of Chemistry 16 (2) (2025) 117-128 119 2025 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.16.2.117-128.2673 Table 1. Crystal data, data collection, and refinement parameters for the title compound, 2-PyrTPPc. Empirical formula C16H11N5O7S Formula weight (g/mol) 417.36 Temperature (K) 273(2) Crystal system Monoclinic Space group P21/c a (Å) 16.876(4) b (Å) 7.6675(18) c (Å) 13.846(3) α (°) 90 β (°) 99.543(7) γ (°) 90 Volume (Å3) 1766.9(7) Z 4 ρcalc (g/cm3) 1.569 μ (mm-1) 0.237 F (000) 856.0 Crystal size (mm3) 0.4 × 0.27 × 0.18 Radiation MoKα (λ = 0.71073) 2Θ range for data collection (°) 2.4 to 53.48 Index ranges -21 ≤ h ≤ 21, -9 ≤ k ≤ 9, -17 ≤ l ≤ 17 Reflections collected 30487 Independent reflections 3741 [Rint = 0.0387, Rsigma = 0.0313] Data/restraints/parameters 3741/0/306 Goodness-of-fit on F2 1.018 Final R indexes [I≥2σ(I)] R1 = 0.0469, wR2 = 0.1039 Final R indexes [all data] R1 = 0.0768, wR2 = 0.1221 Largest diff. peak/hole (e.Å-3) 0.33/-0.28 Measurement Bruker Kappa APEXII CCD Programs system SHELXTL-97 Structure determination SHELXS-97 CCDC deposition number 2335699 2.4. Computational studies Quantum chemical calculations were carried out in the neutral ground state of the title compound (charge: 0, multiplicity: 1) using Gaussian16 [26] and GaussView 6.0 [27]. The Gaussian package is a tool that uses model chemistry to determine the shapes and energies of orbitals in a given molecule. However, density functional theory (DFT) methods describe the interaction between electrons and the total electron density. The most commonly used DFT method is the Becke3 parameter, which calculates the molecular energy for overlapping orbitals. Basis sets are used to increase the number of basis functions per atom. The split-valence triple-zeta basis set 6-311G(d,p) is a popular choice because it allows orbitals to change size but not shape. This basis set has various origins and is recommended by Foresman and Frisch for atoms in the periodic table from hydrogen to bromine [28]. For modeling, the initial guess of picrate was obtained first from the X-ray coordinates and transformed into the Z-matrix format with the Babel program [29]; the model starting geometry of picrate was obtained and applying ab initio optimization at B3LYP/6- 311G(d,p) level. 3. Results and discussion 3.1. Synthesis Aliphatic and aromatic amine picrate complexes are generally formed from the interactions of various amine compounds with picric acid. However, due to the difference in Lewis basic properties of thiols and amines, the proton transfer region in picrate complexes changes. In our study using the 2- mercaptopyridine compound, it was observed that it was not 2- mercaptopyridinium picrate (C5H5NS·C6H3N3O7) but 2- (pyridin-2-ylthio)pyridin-1-ium picrate,(C10H8N2S·C6H3N3O7) was formed via a different mechanism due to the structure and region of the proton transfer center. The C-H protons, which are consistent with the protons of the pyridinium ring, resonate around δ 6.84-7.71 ppm from TMS. The N-H protons of the pyridinium salt are responsible for the broad signal observed at δ 14.07 ppm from TMS [30]. The ipso carbon (C19) of the picrate moiety is responsible for the faint carbon signal at δ 176.45 ppm. The aromatic carbons (C21 and C24) of the pyridinium group bonded to the sulfur atom appear at δ 156.10 ppm. At δ 137.87 ppm, the aromatic carbon atoms C17 and C18 of the same type appear in the picrate moiety. The carbon atoms C15, C16, C14 and C20-C29 in the other aromatic moieties of the complex were assigned to the peak that appeared at δ 149.93, 137.76, 133.20, 126.30, 122.18, 116.93, and 114.56 ppm, respectively. 3.2. Crystal structure and optimized geometry The crystal structure of 2-(pyridine-2-ylthio)-pyridine-1- ium picrate is monoclinic with the space group P21/c with Z = 4 in the unit cell. The asymmetric unit contains one pyridinium and one picrate ion. Due to the functional groups of the title compound, the b-axis and the c-axis (7.6675(18) and 13.846(3) Å, respectively) are larger than the a-axis (16.876(4) Å) in the crystal dimensions. All non-hydrogen atoms were refined anisotropically; all other hydrogen atoms were calculated to their idealized positions and a riding model was used. Information on the crystal data, experimental conditions and structural refinement can be found in Table 1. The ORTEP view of the molecular structure of the picrate crystal is shown in Figure 2. Furthermore, the optimized geometric structure of the title compound obtained from the DFT/B3LYP/6-311G(d,p) method is shown in Figure 3 with the atom numbering scheme. Table 2 shows the final positional parameters of the title compound. The molecule is composed of six-membered two rings of pyridine and one benzene ring. In the six-membered pyridine rings C20/C23/C29/C28/C24/N12 and C21/C22/C26 /C27/C25/N7 are planers having total puckering amplitudes QT of 0.0124(3) Å [φ2 = -120.67(13.01)°, θ2 = 108.51(11.17)°] and 0.0099(3) [φ2 = -63.49(17.23)°, θ2 = 85.31(16.39)°], respec- tively. Also, the six-membered benzene ring C14/C15/C18/ C19/C17/C16 is almost coplanar with the parameters QT = 0.0497(3) Å, [φ2 = 98.65(2.64)°, θ2 = 92.44(2.72)° [31]. The bond lengths, bond angles and dihedral angles are obtained from the geometry optimization which used starting geometries taken from the X-ray structure determination. 120 Aydin and Kiraz / European Journal of Chemistry 16 (2) (2025) 117-128 2025 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.16.2.117-128.2673 Table 2. Selected experimental and optimized structural parameters of the title compound, 2-PyrTPPc. Parameters B3LYP/6-311G(d,p) Exp. (Present work) Parameter B3LYP/ 6-311G(d,p) Exp. (Present work) Bond lengths (Å) S1-C21 1.768 1.749(3) N9-O13 1.235 1.220(3) S1-C24 1.793 1.779(3) N9-O10 1.227 1.229(3) O2-N5 1.229 1.233(2) N9-C18 1.457 1.456(3) O3-N5 1.230 1.231(2) N12-C24 1.334 1.337(3) O4-N8 1.227 1.225(3) N12-C20 1.343 1.347(3) N5-C14 1.453 1.447(3) C14-C15 1.392 1.382(3) N7-C21 1.351 1.342(3) C14-C16 1.396 1.389(3) N7-C25 1.354 1.346(3) C15-C18 1.378 1.370(3) N8-O11 1.231 1.218(2) C16-C17 1.375 1.361(3) N8-C17 1.461 1.463(3) C17-C19 1.466 1.453(3) Bond angles (°) O2-N5-C14 117.88 118.20(19) N9-C18-C19 119.90 119.42(19) O3-N5-O2 124.19 123.39(19) O10-N9-C18 118.44 118.6(2) O3-N5-C14 117.93 118.40(19) O11-N8-O4 123.45 122.6(2) O4-N8-C17 118.03 118.64(19) O11-N8-C17 118.48 118.6(2) O6-C19-C17 123.72 123.8(2) N12-C20-C23 122.91 123.1(3) O6-C19-C18 124.35 125.5(2) N12-C24-C28 122.79 123.1(2) N7-C21-C22 119.59 118.1(3) N12-C24-S1 122.06 120.39(19) N7-C21-S1 122.70 122.25(19) O13-N9-O10 122.99 122.4(2) N7-C25-C27 120.18 120.3(3) O13-N9-C18 118.53 119.0(2) C15-C14-N5 119.58 120.00(19) C15-C14-C16 120.78 120.77(19) C15-C18-N9 116.52 116.4(2) C16-C17-N8 116.76 115.9(2) C15-C18-C19 123.57 124.2(2) C16-C17-C19 123.83 125.4(2) C16-C14-N5 119.61 119.23(19) C17-C16-C14 119.65 119.0(2) C15-C14-N5 119.58 120.00(19) C17-C19-C18 111.85 110.70(18) C16-C17-N8 116.76 115.9(2) C18-C15-C14 119.79 119.7(2) C21-S1-C24 107.04 107.63(11) C19-C17-N8 119.41 118.70(18) C21-N7-C25 122.30 122.3(2) C22-C21-S1 117.70 119.6(2) C24-N12-C20 118.48 117.6(2) C28-C24-S1 115.15 116.5(2) Dihedral angles (°) S1-C21-C22-C26 177.99 179.0(2) O11-N8-C17-C16 149.77 137.8(2) S1-C24 -C28-C29 -179.30 177.7(2) O11-N8-C17-C19 -31.06 42.4(3) O2-N5-C14-C15 0.92 172.3(2) N12-C20-C23-C29 0.12 1.5(4) O2-N5-C14-C16 0.92 8.4(3) N12-C24-C28-C29 0.02 0.6(4) O3-N5-C14-C15 -0.87 8.6(3) O13-N9-C18-C15 -152.32 154.0(2) O3-N5-C14-C16 -179.12 170.7(2) O13-N9-C18-C19 28.51 26.6(3) O4-N8-C17-C16 -28.06 38.4(3) C14-C15-C18-N9 176.67 175.99(19) N5-C14-C15-C18 -178.81 179.22(19) C14-C15-C18-C19 -4.19 4.7(3) N5-C14-C16-C17 178.80 176.07(19) C14-C16-C17-C19 4.23 5.2(3) N7-C21-C22-C26 -0.92 0.0(4) C14-C16-C17-N8 -176.64 174.94(19) N7-C25-C27-C26 -0.44 0.2(4) C16-C17-C19-C18 -8.06 2.3(3) N8-C17-C19-O6 -10.15 1.8(3) C15-C14-C16-C17 0.57 3.2(3) N8-C17-C19-C18 172.83 177.91(18) C15-C18-C19-O6 -168.97 176.9(2) N9-C18-C19-O6 10.13 2.4(3) C15-C18-C19-C17 8.03 2.8(3) . N9-C18-C19-C17 -172.87 177.88(19) C16-C14-C15-C18 -0.58 1.5(3) O10-N9-C18-C15 25.52 25.0(3) C20-N12-C24-C28 0.14 0.5(4) O10-N9-C18-C19 -153.64 154.4(2) C20-N12-C24-S1 179.41 177.64(18) C20-C23-C29-C28 0.05 1.4(4) C22-C26-C27-C25 -0.22 0.7(4) C21-S1-C24-N12 3.62 5.2(2) C24-S1-C21-N7 -4.24 7.6(2) C21-S1-C24-C28 -177.05 173.05(19) C24-S1-C21-C22 176.89 173.5(2) C21-N7-C25-C27 0.42 1.0(4) C24-N12-C20-C23 -0.21 0.5(4) C21-C22-C26-C27 0.89 0.8(4) C24-C28-C29-C23 -0.12 0.4(4) C25-N7-C21-C22 0.28 0.9(4) C25-N7-C21-S1 -178.58 179.9(2) Figure 2. ORTEP III drawing of the title compound, 2-PyrTPPc with the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level [30]. To compare the theoretical results with the experimental one, some selected geometrical parameters are gathered in Table 2. The calculated S-C, N-C, and O-N bond lengths agree well with the experimental bond lengths, while the O4-N8, N8- C17, N9-O10 bond lenghts are nearly 0.002 Å and the N9-C18 bond length is nearly 0.001 Å longer. The calculated C-C and C=C bond lengths are also close to those from the X-ray data. The calculated O-N-C, O-C-C, N-C-S, N-C-C, O-N-O, C-C-S, C-S-C, and C-C-C bond angles are consistent with the experimental data except for the N12-C24-S1 bond angle. The theoretical value of this angle is slightly different from the experimental value, with the N12-C24-S1 bond angle being theoretically 122.06° and experimentally 120.39(19)°. This difference in angle is due to the limitations of the DFT method [32]. Aydin and Kiraz / European Journal of Chemistry 16 (2) (2025) 117-128 121 2025 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.16.2.117-128.2673 Table 3. Hydrogen bond between some atom (Å, °) for the compound, 2-PyrTPPc. D-H···A D-H (Å) D···A (Å) H···A (Å) D-H···A (o) N7-H40···N12i 0.85(3) 2.630(3) 1.85(3) 150(3) C16-H30···O2ii 0.96(2) 3.255(3) 2.59(2) 127.12(17) C15-H31···O10i 0.93(2) 2.703(3) 2.38(2) 100.1(16) C26-H32···O6ii 0.98(3) 3.128(4) 2.36(3) 135(2) C25-H34···O6iii 0.95(3) 3.252(3) 2.35(3) 159(2) C25-H34···O11iii 0.95(3) 3.097(3) 2.41(3) 128(2) Symmetry codes: (i) x, y, z; (ii) -x, 1-y ,1-z; (iii) x, 3/2-y, -1/2+z. Figure 3. The ground state optimized geometry for the title compound, 2-PyrTPPc. Figure 4. Intramolecular and intermolecular H-bonds in the dimer. The dotted lines represent C–H∙∙∙O and N-H∙∙∙N hydrogen interactions of the title compound, 2-PyrTPPc. The crystal structure is an impressive display of molecular architecture, with inter- and intramolecular hydrogen bonding playing a crucial role. The arrangement of these bonds has been carefully planned to ensure optimal balance and stability within the structure. This systematic design provides a comprehensive overview of the structural dynamics involved. These balance and stability are also evident from the NBO analysis parameters. The stabilization energies between the donor S1(LP) and the acceptor C24-N12 bond were found to have a low value of 4.31 kcal/mol. The orientation of the pyridine and benzene rings is also defined by torsion angles. The compound demonstrates intramolecular and intermolecular N-H∙∙∙N and C-H∙∙∙O hydrogen bonds in the crystal (Table 3). This type of C-H∙∙∙X (X =O, N) hydrogen bond has been well reviewed in the literature [33]. The notable properties of the crystal arise from an intricately organized and balanced structural framework. This framework is formed through complex interactions that involve both intermolecular and intramolecular hydrogen bonding. Intermolecular hydrogen bonds occur between separate molecules, whereas intramolecular bonds form within a single molecule. The synergistic effects of these bonding interactions enhance not only the stability of the crystal but also the ability to significantly influence its various physical and chemical properties. A comprehensive analysis of these interactions and their implications is detailed in Table 3. The title compound consists of dimeric units that are formed by hydrogen bonds between the C-H∙∙∙O and N-H∙∙∙N in the picrate rings. These dimeric units are then connected by paired C-H∙∙∙O and N-H∙∙∙N hydrogen bonds, forming sheets along the a-axis, as shown in Figure 4. 3.3. FT-IR and Raman spectral analysis FT-IR spectra confirm the presence of various functional groups and chemical bonds such as hydrogen bonds in the compounds. In the FT-IR spectrum, some functional groups have disappeared, while others have appeared due to picrate salts (Figure 5). 122 Aydin and Kiraz / European Journal of Chemistry 16 (2) (2025) 117-128 2025 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.16.2.117-128.2673 Figure 5. FT-IR spectrum of 2-(pyridine-2-ylthio)pyridine-1-ium picrate (2-PyrTPPc), 2-mercaptopyridine, 2,4,6-trinitrophenol (picric acid) (red, blue, and green line, respectively). Figure 6. RAMAN spectrum of the title compound, 2-PyrTPPc. The experimental characteristic FT-IR and Raman spectra along with the calculated (DFT, B3LYP method at 6-311G(d,p) level) in the gas-phase transmission spectra of the title compound are shown in Figures 5-7. The calculated wavenumbers corresponding to the normal modes of the FT-IR spectra are listed in Table 4. Protonation of pyridine nitrogen to form the pyridinium salt gives rise to several bands from the N+-H stretching between 3300 and 1950 cm-1 that have moderate IR intensity, but are weak in the Raman spectrum. Due to the strong intermolecular hydrogen bond between the nitrogen atoms of the other pyridine, the characteristic peak of the N+-H group was observed in the vibrational spectra of the pyridinium moiety at 3471 and 3443 cm-1 [34]. In the Raman spectrum, these peaks do not appear very prominent (Figure 7). The C-H vibrations cover the asymmetric and symmetric stretching modes of two different pyridine rings in the phenolic group. Absorption above 3000 cm-1(3369, 3314, and 3213 cm-1) is attributed to the stretching vibrations of ν(Cring-H) of the aromatic rings, such as pyridinium rings and phenolic rings [35]. The calculated infrared stretching vibrations of the C-H bond of the aromatic ring are around 2970-2880 cm-1, whereas these peaks of the experimental spectrum appear around 3085-2999 and 3000 cm-1, respectively [36]. In addition, the symmetric and asymmetric stretching vibrations of the C-S bond of the aromatic thioether moiety of the title compound, which are experimentally found at 771, 738, and 707 cm-1, are attributed to the calculated peaks at 734, 712, and 486 cm-1, respectively [37]. On the other hand, the asymmetric and symmetric stretching vibrations of the NO2 group of the picrate anion are responsible for the sharp bands that appear experimentally at 1487-1422 cm-1 in the FT-IR and at 1481 and 1446 cm-1 in the calculated spectra [38]. The comparison between Raman and IR spectra of the vibrational modes in the title compound is more easily emphasized. Other characteristic peaks are observed at 987, 732 cm-1 in the Raman spectra, which are caused by the out-of-plane bending(O-N=O) and scissoring (O-N=O) vibration modes of the nitro groups, respectively [39]. These assignments are very characteristic of the picrate anion. These assignments are very characteristic of picrate anions [40]. The experimental and the calculated vibrational frequencies agree well and confirm the formation of the structure of the title compound. 30 40 50 60 70 80 90 100 65010501450185022502650305034503850 % T ra ns m itt an ce Wavenumber (cm-1) Aydin and Kiraz / European Journal of Chemistry 16 (2) (2025) 117-128 123 2025 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.16.2.117-128.2673 Table 4. Vibrational wavenumbers of the title compound, 2-PyrTPPc obtained from B3LYP/6-311G(d,p) in cm-1, assignment with TED (total energy distribution). Modes Unscaled Scaled a IR Assignment [TED] b ≥ 10% 1 9.898 9.570 0.816 νCC(28%)+δCCN(15%) 2 15.90 15.37 0.931 νCC(21%)+δCCN(47%)+νON(16%) 3 22.51 21.77 0.640 νCC(16%)+δNCC(17%)+δCNC(11) 4 30.96 29.94 0.111 νNC(11%) 5 38.59 37.32 2.793 νCC(21%)+τHCCN(13%) 6 54.16 52.37 5.390 δCNC(12%) 8 61.55 59.52 1.857 νCC(19%) 9 63.78 61.68 0.671 νCH(20%) 10 74.91 72.43 3.314 νCH(10%)+νCC(13%)+δCCC(11%) 11 95.56 92.41 3.257 νCH(16%) 12 107.5 104.0 6.585 νCH(14%) 13 135.1 130.6 6.337 νCH(12%)+νCC(10%) 14 147.0 142.2 28.17 νNC(32%)+νSC(20%) 15 156.2 151.0 1.578 νCH(16%)+νCC(13%)+δNCC(16%) 16 178.2 172.3 0.501 νCC(14%)+δCCC(29%) 17 183.0 176.9 0.378 νCH(16%)+τCCOH(11%) 19 198.1 191.5 0.017 νNC(17%) 20 262.3 253.6 5.084 νCC(23%)+δCCC(33%) 21 292.2 282.6 4.636 νCC(20%)+τCNCC(13%)+τNCCC(11%)+γSCNC(15%) 22 298.5 288.7 3.311 νCH(13%)+δHCC(37%) 23 308.3 298.1 1.567 τCNCC(16%) 24 328.6 317.7 2.192 νCC(29%)+δCCC(14%) 25 353.6 341.9 8.371 νCC(20%)+γNCCC(12%) 26 365.3 353.2 1.216 νCC(72%) 27 387.4 374.7 1.851 τHCCC(13%) 28 394.0 381.0 0.646 τHCCC(30%) 29 410.9 397.3 1.929 δHCC(15%) 30 455.8 440.8 4.056 νCC(12%)+δHCC(33%) 31 474.2 458.6 0.695 νCH(43%)+δHCC(34%) 32 478.3 462.5 9.969 νCH(18%)+τHNCC(16%)+τCCC(16%) 33 486.7 470.6 1.674 νSC(31%) 34 492.2 476.0 0.451 νCH(12%)+τHCCC(11%) 36 525.3 508.0 5.228 νNH(10%)+δHCC(11%) 37 552.3 534.0 4.109 νCH(32%)+δHCC(19%) 38 631.4 610.5 0.830 τCCCN(16%) 39 650.8 629.3 7.315 νNH(11%)+δHNC(44%) 40 689.0 666.25 12.71 δCCC(16%)+τCCCC(30%)+τNCCC(10%) 41 693.2 670.30 12.33 νCC(11%)+γCNSC(12%) 42 712.7 689.18 4.685 γCSCC(12%) 43 724.4 700.47 26.86 νCC(11%) 44 729.7 705.62 3.423 δCCC(11%)+τCSCC(11%) 45 734.2 709.92 69.77 γSCNC(10%) 46 737.8 713.42 28.24 τONCC(13%)+τNCSC(11%) 47 744.8 720.17 0.281 νNC(13%) 48 746.5 721.9 18.53 νNC(17%) 49 776.2 750. 6 80.03 νNC(12%) 50 780.9 755.2 0.681 νONC(20%)+νCC(20%) 54 848.8 820.8 0.218 δCCC(10%) 56 927.7 897.1 43.01 νCC(24%)+δCCC(13%) 57 942.2 911.1 9.093 δCCC(10%) 61 984.3 951.9 1.841 δONC(17%) 63 1011 977.3 7.917 τONCC(20%) 64 1021 987.1 0.151 νON(14%) 65 1022. 988.2 15.64 νON(46%)+δONO(10%) 66 1051 1017 29.22 δONC(31%) 67 1056 1021 10.14 τONCC(13%) 68 1066 1031 7.753 νOC(13%) 69 1078 1042 1.680 τONCC(47%)+γOCON(17%) 70 1086 1051 105.7 δONC(19%) 71 1110 1073 24.81 νON(11%) 72 1120 1083 15.68 δHCC(23%) 73 1141 1103 55.57 δONC(11%)+δNCC(18%) 76 1177 1138 95.07 νCC(12%) 77 1186 1146 5.565 γOCON(14%) 78 1198 1158 20.47 νON(16%) 79 1268 1226 47.88 νCC(23%) 82 1310 1267 29.38 γOCON(12%) 84 1347 1302 4.089 δCOH(10%) 85 1347 1303 658.7 νNC(12%) 86 1370 1325 67.27 νON(12%) 87 1390 1344 46.23 νCC(13%) 89 1451 1403 59.46 νNC(21%) 90 1468 1420 78.03 γNCCC(10%) 91 1479 1430 190.9 νON(19%)+νNC(10%)+δONO(12%) 92 1496 1446 2.036 δONO(15%) 93 1532 1481 5.956 νON(24%)+δONC(18%)+δONO(11%) 94 1556 1505 214.8 νON(12%)+δONO(12%)+γOCON(16%) 95 1557 1506 114.8 δONC(18%) 96 1574 1522 28.68 τHCCC(15%) 98 1609 1556 184.7 νCC(10%) 124 Aydin and Kiraz / European Journal of Chemistry 16 (2) (2025) 117-128 2025 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.16.2.117-128.2673 Table 4. (Continued). Modes Unscaled Scaled a IR Assignment [TED] b ≥ 10% 100 1628 1574 233.8 δHCC(13%) 101 1638 1574 85.83 δHCC(11%) 102 1660 1605 494.4 τHCCC(19%) 103 1691 1635 742.0 νCH(14%)+τHCCC(28%) 104 2998 2899 1022 τHCNC(31%)+τCOHC(13%) 105 3153 3049 323.3 τHCNC(12%)+τHCCC(14%) 106 3176 3071 7.722 νCH(20%)+τHCCC(10%) 107 3180 3075 209.6 δHCC(23%)+τHCNC(20%)+τHCCC(28%) 108 3186 3080 4.256 νCH(24%)+τHCCN(22%) 109 3198 3092 0.632 νCH(51%)+δHCC(10%)+τHCCC(10%) 110 3210 3105 3.884 δHCC(18%)+δHCN(43%)+τHCCN(11%)+τHCNC(12%) 111 3210 3105 2.035 δHCC(40%)+τHCCN(11%) 112 3224 3118 1.787 δHCC(50%)+τHCCC(10%) 113 3232 3125 11.67 νCH(84%) 114 3233 3126 11.52 νCH(60%)+τHCCC(13%) a Wavenumbers are scaled with 0.967 for B3LYP/6-311G(d,p) basis set. b TED: total energy distribution, ν; stretching, δ; in-plane-bending, γ; out-of plane bending, τ;torsion, ρ: rocking, w; wagging, t; twisting, sym; symmetric and asym; asymmetric. (a) (b) Figure 7. Calculated FT-IR (a) and FT-RAMAN (b) spectrums of the title compound, 2-PyrTPPc. 3.4. The frontier orbitals analysis To understand the stability and reactivity of chemical compounds on a global scale, it is essential to perform a detailed analysis of several key properties. Among these, frontier molecular orbitals (FMO) are vital to elucidate the electronic structure of molecules, as they represent the highest occupied orbitals (HOMO) and lowest unoccupied orbitals (LUMO) involved in chemical bonding and reactions [41]. The energy difference between these orbitals is critical, as it directly influences a compound’s reactivity; narrower gaps often correspond to increased reactivity due to the ease of electron excitation [42]. Hardness refers to the resistance of a molecule to deformation and chemical change, while softness indicates a tendency to engage in chemical interactions [43]. These characteristics provide valuable insights into the anticipated behavior of compounds in various chemical environments. Furthermore, the chemical potential is analyzed to understand the likelihood that a compound changes in response to variations in external conditions. This potential illustrates how a substance interacts with its environment, affecting its stability and reactivity. These comprehensive analyzes not only improve our understanding of the intrinsic properties of chemical compounds, but also help predict their behavior in real-world applications and reactions under various conditions [44]. The energy difference between the HOMO-LUMO orbitals and other global reactivates for the title compound was calculated in Table 5. As can be seen from the plot of the HOMO level, all negative regions are spread over the picrate anion of the title molecule. At the LUMO level, the negative regions are spread over the 2-(pyridine-2-ylthio)-pyridine-1-ium moiety. The atomic orbital compositions of the molecular orbitals are shown in Figure 8. The HOMO value is -5.70 eV, while the LUMO value is -3.15 eV, and the ΔE value calculated at the DFT level is 2.55 eV. This small energy value of the title compound explains the high chemical reactivity and the low kinetic stability due to the narrow energy space of electronic transitions [45,46]. 3.5. Molecular electrostatic potential (MEP) analysis MEP is used for prediction by highlighting the areas of nucleophilic and electrophilic attack depending on the electron densities (ED) on the molecule. It also identifies the hydrogen bond interaction sites [47]. The electrophilic and nucleophilic reactivity by the map of MEP are represented as different colors such as blue, green, and yellow or red from positive, neutral to negative regions, respectively. The red and blue colors in the MEP structure indicate more electron-rich regions and less electron-rich regions, respectively [48]. DFT calculations using the optimized structure with the B3LYP/6-311G(d,p) basis set were performed to determine the MEP surface of the title compound. Aydin and Kiraz / European Journal of Chemistry 16 (2) (2025) 117-128 125 2025 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.16.2.117-128.2673 Table 5. Calculated electronic parameters of the title compound, 2-PyrTPPc using the DFT method at the B3LYP/6-311G(d,p) level. Parameters Gas phase Electronic energy (a.u.) -1814.908 Excitation energy (eV) 2.065 EHOMO (eV) -5.70 ELUMO (eV) -3.15 Energy band gap [ΔE = ELUMO-EHOMO] (eV) 2.55 Ionization energy [I = -EHOMO] (eV) 5.70 Electron affinity [A = -ELUMO ] (eV) 3.15 Electronegativity [χ =(I+A)/2 ] (eV) 4.43 Chemical hardness [η =(I-A)/2)] (eV) 1.28 Chemical softness [ζ = 1/2η] (eV-1) 0.78 Chemical potential [µ = -(I+A)/2] (eV) -4.43 Electrophilicity index [ω = µ2 /(2η)] (eV) 7.68 Table 6. The second-order perturbation energies E(2)(kcal/mol) correspond to the most important charge transfer interaction(donor-acceptor) in 2-PyrTPPc by the DFT/B3LYP/6-311G(d,p) method. Donor NBO(i) Acceptor NBO(j) E(2)a E(j)-E(i)b a.u. F(i,j)c a.u. π(N7 -C21) LP(1) C22 13.46 0.27 0.069 π(N7 -C21) π*(C25 -C27) 19.40 0.39 0.079 π(N12 -C20) π*(C23 -C29) 11.03 0.34 0.056 π*(C24 -C28) 25.77 0.32 0.084 σ(C20 -H35) σ*(N12 -C24) 5.85 1.04 0.070 σ(C22 -H33) σ*(N7 -C21) 5.68 0.95 0.066 π(C23 -C29) π*(N12 -C20) 35.19 0.24 0.084 π*(C24 -C28) 20.28 0.26 0.065 π(C24 -C28) π*(N12 -C20) 17.25 0.26 0.061 π*(C23 -C29) 21.75 0.30 0.073 π(C25 -C27) LP*(1) C26 45.02 0.18 0.094 π*(N7 -C21) 14.17 0.21 0.053 σ(C28 -H37) σ*(N12 -C24) 5.25 1.04 0.066 LP(1) S1 σ*(N7 -C21) 5.18 1.07 0.067 LP(2) S1 π*(N7 -C21) 35.21 0.18 0.078 π*(C24 -C28) 19.21 0.25 0.063 LP(1) N12 σ*(N7 -H40) 25.07 0.75 0.124 σ*(C20 -C23) 7.76 0.93 0.078 σ*(C24 -C28) 9.43 0.90 0.085 LP(1) C22 π*(N7 -C21) 432.40 0.03 0.124 LP*(1) C26 π*(C25 -C27) 61.28 0.11 0.101 π*(N7 -C21) π*(C25 -C27) 31.87 0.08 0.069 π*(N12 -C20) π*(C23 -C29) 103.18 0.04 0.088 π*(C24 -C28) 159.65 0.02 0.076 π*(C24 -C28) π*(C23 -C29) 178.71 0.02 0.084 a E(2) means the energy of hyperconjugative interactions. b Energy difference between donor and acceptor i and j NBO orbitals. c F(i,j) is the Fock matrix element between the i and j NBO orbitals. ELUMO = - 3.15 eV ∆E = 2.55 eV EHOMO = - 5.70 eV Figure 8. Frontier molecular orbitals (HOMO and LUMO) diagram for the title compound, 2-PyrTPPc. Figure 9 shows the mapped MEP surface of the investigated compound. The color code of the title compound is in the range of -8.765e-2 to 8.765e-2. In the MEP mapping, the picrate anion moiety with three nitro groups is highlighted in red, while the 2-(pyridine-2-ylthio)pyridine-1-ium moiety is highlighted in blue. Therefore, it can be seen from Figure 9 that the anionic moiety is the electronegative region, while the cationic moiety is the electropositive region. 126 Aydin and Kiraz / European Journal of Chemistry 16 (2) (2025) 117-128 2025 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.16.2.117-128.2673 Table 7. Calculated thermodynamic parameters of the title compound Parameters B3LYP/6-311G(d,p) Temperature (K) 298.15 Pressure (atm) 1 The sum of electronic and zero-point Energies (a.u.) -1814.636376 Zero-point vibrational energy (kcal/mol) 170.16634 Rotational constants (GHz) 0.29215 0.06797 0.05720 E (Thermal) (KCal/Mol) 186.008 Specific heat (Cv) (cal /mol K) 91.777 Entropy (S) (cal/mol K) 183.977 Dipole moment (Debye) 24.6258 Table 8. The thermodynamic properties of the title compound, 2-PyrTPPc at different temperatures at B3LYP/6-311G(d,p) level. T (K) C°p,m (cal/mol.K) S°m (cal/mol.K) ΔH° (kcal/mol) 100 182.981 476.807 11.903 200 287.291 635.564 35.403 298.15 392.310 769.872 68.761 300 394.257 772.305 69.489 400 493.125 899.605 113.976 500 575.378 1018.809 167.550 600 640.781 1129.730 228.487 700 692.455 1232.541 295.248 800 733.703 1327.798 366.631 900 767.108 1416.211 441.728 1000 794.535 1498.499 519.854 Figure 9. Molecular electrostatic potential map calculated at B3LYP/6-311G(d,p) level for the title compound, 2-PyrTPPc. 3.6. Natural bond orbital (NBO) analysis NBO analysis provides sufficient fundamentals for the investigation of charge transfer or conjugative interactions in molecular systems and also provides abundant methods for studying intramolecular and intermolecular bonding and interaction among bonds [49]. The interacting stabilization energy, some electron donor orbital and acceptor orbital disturbance theory are declared [50,51]. The E(2) value exhibits the interaction energy between the electron acceptor and the electron donors, and this value is greater than the dependence of the electron donation from the donor to the acceptor. The delocalization of electron density between occupied Lewis type (bond or lone pair) NBO orbitals and formally unoccupied (antibonding or Rydberg) non-Lewis NBO orbitals corresponds to a stabling donor-acceptor interaction [52]. Table 6 displays the most potent interactions and electron delocalization values. This information is critical in understanding the underlying chemical mechanisms and can significantly enhance our ability to predict molecular behavior with greater accuracy. 3.7. Thermal analysis The TG curve of the title compound is shown in Figure 10. Important information about thermal stability, compound decomposition, and crystal purity can be obtained by thermo- gravimetry and differential thermogravimetry (TG/DTG) [53]. From the TG curve, it is clear that there is no weight loss between 30 and 157 °C and shows that the title compound is stable up to 157.46 °C and moisture-free. Furthermore, the crystal has a one-step decomposition. After 157.46 °C, there is no residue left due to decomposition resulting in the release of volatile substances such as CH4, NO2, NH3, CO, CO2 and H2S molecules [54]. The purity of the title compound crystal is also illustrated by the sharpness of the thermogram. Figure 10. TG/DTG thermograms of the title compound, 2-PyrTPPc. 3.8. Thermodynamic properties Statistically thermodynamic parameters such as specific heat capacity (Cp), enthalpy (H), entropy (S), zero-point Aydin and Kiraz / European Journal of Chemistry 16 (2) (2025) 117-128 127 2025 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.16.2.117-128.2673 vibration energy (ZPVE), rotation constants, and rotational temperature thermodynamic parameters were calculated using the B3LYP/6-311G(d,p) method in the ground state (Table 7). The determination of thermochemical parameters such as the standard heat capacity of constant pressure (Cp), enthalpy (H) and entropy (S) and identification of the behavior of chemical compounds under temperature is quite important [55]. Properties of interest exert a direct influence on the vibrational dynamics between the atoms within chemical compounds. Furthermore, they play a crucial role in the assessment of the energy and heat associated with both chemical reactions and physical transformations, as outlined in Table 8. 4. Conclusions Using a different route, we synthesized a new picrate salt. FT-IR, 1H NMR, and 13C NMR spectral techniques were used to characterize the molecular structure of the title compound. The single crystals were deliberately grown using a meticulous slow evaporation method with tetrahydrofuran as a solvent. The molecular form was determined conclusively by rigorous X-ray structure analysis, enabling the precise calculation of geometrical parameters. In general, while the formation of the picrate salt of the aromatic amine is expected, the formation of the picrate salt of the amine synthesized through the reactions of substituted benzyne(pyridyne)-type intermediates is assumed according to the spectral data. Experimental data such as X-ray, FTIR, and Raman with optimized geometric parameters, and the vibrational frequency data obtained as the theoretical calculations have been tested using the B3LYP/6- 311G (d, p) basis set. Furthermore, using these data, the analysis of HOMO and LUMO energy, electronegativity (χ), hardness (η), softness (S), MEP and NBO of the picrate compound were also calculated. The properties of interest directly affect the vibrational dynamics of atoms within chemical compounds. These measurements are essential for evaluating energy transfer and thermal changes during chemical reactions and physical transformations. A thorough analysis of these processes provides valuable insights into the interactions among substances, such as the mechanisms by which energy is absorbed or released. TG-DTG analysis demonstrated the thermal stability of the 2-PyrTPPc crystal and indicated its good crystallinity with single-stage decomposition. Acknowledgements We are also grateful to TUBITAK ULAKBIM, the High Performance and Grid Computing Center (TRUBA Resources). CRediT authorship contribution statement Conceptualization: Fatma Aydin; Methodology: Fatma Aydin; Software: Asli Ozturk Kiraz; Validation: Fatma Aydin, Asli Ozturk Kiraz; Formal Analysis: Asli Ozturk Kiraz; Investigation: Fatma Aydin; Resources: Fatma Aydin; Data Curation: Fatma Aydin, Asli Ozturk Kiraz; Writing-Original Draft: Fatma Aydin, Asli Ozturk Kiraz; Writing-Review And Editing: Fatma Aydin, Asli Ozturk Kiraz; Visualization: Fatma Aydin, Asli Ozturk Kiraz; Funding Acquisition: Fatma Aydin, Asli Ozturk Kiraz; Project Administration: Fatma Aydin. Disclosure statement Conflict of interest: The authors declare no financial interest/personal relationships which may be considered as potential competing interests. Ethical approval: All ethical guidelines have been adhered. Sample availability: Samples of the compounds are available from the author. Supporting information CCDC-2335699 contains the supplementary crystallographic data for the structure reported in this article. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/data_request/cif, 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:+44-1223-336033. Funding This work was financially supported by the Çanakkale Onsekiz Mart University (Grant Number: FBA-2016-672) and Pamukkale University (Grant numbers: 2018FEBE002 and 2020FEBE012). ORCID and Email Fatma Aydin faydin@comu.edu.tr https://orcid.org/0000-0002-7219-6407 Asli Ozturk Kiraz aslio@pau.edu.tr https://orcid.org/0000-0001-9837-0779 References [1]. Moran, D.; Sukcharoenphon, K.; Puchta, R.; Schaefer, H. F.; Schleyer, P. v.; Hoff, C. D. 2-Pyridinethiol/2-Pyridinethione Tautomeric Equilibrium. A Comparative Experimental and Computational Study. J. Org. Chem. 2002, 67 (25), 9061–9069. [2]. Jones, R. 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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 (https://www.eurjchem.com/index.php/eurjchem/terms) are administered by Atlanta Publishing House LLC (European Journal of Chemistry). https://doi.org/10.1007/s11224-023-02232-x https://doi.org/10.1007/s11224-023-02232-x https://www.eurjchem.com/index.php/eurjchem/terms http://creativecommons.org/licenses/by-nc/4.0 https://www.eurjchem.com/index.php/eurjchem/terms 1. Introduction 2. Experimental 2.1. General remarks 2.2. Synthesis of 2-(pyridine-2-ylthio)pyridine-1-ium picrate, 2-PyrTPPc 2.3. X-ray crystallography 2.4. Computational studies 3. Results and discussion 3.1. Synthesis 3.2. Crystal structure and optimized geometry 3.3. FT-IR and Raman spectral analysis 3.4. The frontier orbitals analysis 3.5. Molecular electrostatic potential (MEP) analysis 3.6. Natural bond orbital (NBO) analysis 3.7. Thermal analysis 3.8. Thermodynamic properties 4. Conclusions Acknowledgements CRediT authorship contribution statement Disclosure statement Supporting information Funding ORCID and Email References PrintField10: PrintField11: PrintField12: PrintField13: PrintField14: PrintField15: PrintField16: PrintField17: PrintField18: PrintField19: PrintField110: PrintField111: PrintField20: PrintField21: PrintField22: PrintField23: PrintField24: PrintField25: PrintField26: PrintField27: PrintField28: PrintField29: PrintField210: PrintField211: