FT-IR
The FT-IR spectrum of the compound HNT is shown in Fig. 2, the vibrational spectrum of HNT, exhibits the characteristic bands of the 8-hydroxy-5-nitroquinolinum cation and ρ-toluenesulfonate anion. The peaks at 1500 cm− 1 and 1361 cm− 1 are characteristic of NO2 asymmetric and symmetric stretching vibrations respectively. The broad peak observed in the range 2700 − 2250 cm− 1 is due to the NH+ stretching vibration. The characteristic absorption at 1332 cm− 1 corresponds to CH3 rocking vibrations. The SO3 asymmetric and symmetric modes of vibrations of sulfonate group in the ρ-toluene- sulfonate ring is found at 1252 and 1021 cm− 1 respectively. The observed characteristic IR frequencies are listed in Table 1.
Table 1
The observed characteristic IR frequencies of HNT
Frequencies(cm− 1) | Assignments |
1500 | ν (NO2) |
1361 | νas (NO2) |
1252 | νas (SO3) |
1021 | ν(SO3) |
2700 − 2250 | NH+ amine salt |
1332 | CH3 |
(ν) symmetric stretching; (νas) asymmetric stretching |
Powder Xrd Analysis
The indexed powder XRD pattern of HNT along with simulated pattern is shown in Fig. 3. XRD profiles show that a sample is of single phase without detectable impurity. The well-defined Bragg’s peaks at specific 2θ angles show good crystallinity of the material. Most of the peak positions in powder XRD and simulated XRD pattern derived from single crystal XRD coincide. Relative intensities difference could be due to the preferred orientation of the sample used for diffractogram measurement. Also, the mosaic spread of powder and single crystal patterns may differ resulting in varied intensities.
Single Crystal Xrd
Single crystal X-ray diffraction analysis reveals that the compound HNT crystallizes in monoclinic centrosymmetric space group C2/c with Z = 8. An ORTEP of HNT shown at the 50% probability level (Fig. 4a) clearly indicates proton transfer. The asymmetric unit of the crystal contains one molecule of the cationic form of 8-hydroxy-5-niroquinolinium and one molecule of the anionic form of p-toluene sulfonate anion. The crystal data and structure refinement details of HNT are given in Table 2. In the packing diagram (Fig. 4b) the intermolecular hydrogen bonds are shown as dashed lines. The p-toluenesulfonate anion is bonded to the 8-hydroxy-5-nitroquinolinium cation thro’ O(4)-H(4A)…O(2)[D…A = 2.669(2)], N(2)-H(2A)…O(2)[D…A = 2.821(2)] and C(15)-H(15)…O(1)[D…A = 3.3363(3)], intermolecular hydrogen bonds. Strong hydrogen bond O(4)-H(4A)…O(2) with a short contact distance is observed between hydroxyl O atom and sulfonate O atom. Relatively weak hydrogen bond with a distance of > 3.4 Å is observed for N(2)-H(2A)…S(1)[(D...A) = 3.5802(18), H…A = 2.80(3), D-H…A = 141.3(19)°] (Table 3) forming an intermolecular hydrogen bond between quinolinium N atom with sulfur atom S. The N-H…S and C-H…O interactions are given in Figs. S1. Crystal three-dimensional packing showing molecular interactions along ‘a’, ‘b’ and ‘c’ axes are displayed in Fig. S2.
Table 2
Crystal data and structure refinement for HNT
Empirical formula | C16H14N2O6S |
Formula weight | 362.35 |
Temperature | 296(2) K |
Wavelength | 0.71073 Å |
Crystal system | Monoclinic |
Space group | C2/c |
Unit cell dimensions | a = 32.1365(13) Å | α = 90° |
b = 6.3720(2) Å | β = 97.370(2)° |
c = 15.6220(6) Å | γ = 90° |
Volume | 3172.5(2) Å3 |
Z | 8 |
Density (calculated) | 1.517 Mg/m3 |
Absorption coefficient | 0.242 mm-1 |
F(000) | 1504 |
Crystal size | 0.250 x 0.200 x 0.100 mm3 |
Theta range for data collection | 2.556 to 28.550°. |
Index ranges | -42 < = h<=42, -8 < = k<=8, -21 < = l<=20 |
Reflections collected | 39399 |
Independent reflections | 4034 [R(int) = 0.0481] |
Completeness to theta | 100.0% |
Max. and min. transmission | 0.7459 and 0.7049 |
Data / restraints / parameters | 4034 / 0 / 230 |
Goodness-of-fit on F2 | 1.065 |
Final R indices [I > 2sigma(I)] | R1 = 0.0436, wR2 = 0.1171 |
R indices (all data) | R1 = 0.0792, wR2 = 0.1443 |
Largest diff. peak and hole | 0.300 and − 0.309 e.Å-3 |
CCDC No | 1871548 |
Table 3
Hydrogen bonds for HNT [Å and °]
D-H...A | d(D-H) | d(H...A) | d(D...A) | <(DHA) |
C(15)-H(15)...O(1)#1 | 0.93 | 2.5 | 3.336(3) | 149 |
O(4)-H(4A)...O(1) | 0.82 | 2.57 | 3.081(2) | 122.2 |
O(4)-H(4A)...O(2) | 0.82 | 1.85 | 2.669(2) | 172.8 |
O(4)-H(4A)...S(1) | 0.82 | 2.69 | 3.444(15) | 153.9 |
N(2)-H(2A)...O(2)#2 | 0.94(3) | 1.91(3) | 2.821(2) | 163(2) |
N(2)-H(2A)...S(1)#2 | 0.94(3) | 2.80(3) | 3.580(18) | 141.3(19) |
Symmetry transformations used to generate equivalent atoms: |
#1 -x + 1,-y + 2,-z + 1 #2 -x + 1,-y + 1,-z + 1 |
Uv-vis-nir Diffuse Reflectance Spectrum
The optical absorption spectra of HNT are recorded and band gap energies are estimated. The optical absorption spectrum shows the absorption is minimum in the visible region and cut-off wavelength is 427 nm. The Kubelka–Munk theory provides a correlation between reflectance and concentration [36].
F(R)=(1-R)2/2R= α/s =Ac/s
where F(R) is the Kubelka–Munk function, R is the reflectance of the crystal, α is the absorption coefficient and S is the scattering coefficient, A is the absorbance and c is the concentration of the absorbing species. The direct band gap energy of the specimens is 2.85 eV respectively from the Tauc plot, [F(R) hʋ]2 versus hʋ (where hʋ is the photon energy in eV) (Fig. 5).
Thermal Analysis
The TG-DTA response curve is shown in Fig. 6. Thermogravimetric curve shows its complete decomposition in stages. A significant weight loss of 44% is observed in the temperature range 232 to 447 oC. In the DTA curve the endothermic peak observed at 237OC is attributed to melting of the HNT crystal and 49.78% of the sample remained as residue at 600 oC. The material has a good thermal stability up to 232 oC, without any decomposition before this temperature. Crystalline nature is good as evident from the sharpness of the peaks observed in DTA. No decomposition up to the melting point ensures the suitability of the material for application in lasers where the crystals are required to withstand high temperatures.
Optical Nonlinear Properties
The hyperpolarizability tensor β(s), the first-order molecular hyperpolarizability was computed and the values are given in Table 4. The high β value 285.454× 10− 30 esu (~ 750 times of urea) is associated with high charge transfer. As shown in the table βxxx is dominant and delocalization of electron cloud is more towards x-axis. Generally, high βtot value is ascribed to conjugation, molecular symmetry, hyperconjugation, substitution, aromaticity and charge transfer [37]. Intra- and intermolecular hydrogen bonding interactions and the resulting supramolecular assembly contribute mainly to observed large microscopic nonlinearity. Also, large β could be due to the dipole moment difference between the ground state and first π-electron excited state [38]. Large molecular hyperpolarizability leads to intensive dipole-dipole interactions [39] facilitating centrosymmetric crystallization of HNT (monoclinic, C2/c). Contrary to expectations, small second harmonic generation efficiency (SHG) is observed by the Kurtz and Perry technique [40] with KDP as reference material. Some of the earlier reported quinolinium derivatives exhibiting SHG efficiency are given in Table 5. Reactants and product HNT have different symmetry groups. 8-Hydroxy-5-nitroquinoline, C9H6N2O3 exhibits an acentric orthorhombic space group Fdd2 [41] whereas p-toluenesulfonic acid, belongs to centrosymmetric monoclinic space group P21/c [42]. Centrosymmetric structure of proton-transfer complex HNT was confirmed by X-ray structure analysis. Small SHG observed (0.2 < times of KDP) could be ascribed to the traces of noncentrosymmetric impurity, quite likely the reactant, 8-hydroxy-5-nitroquinoline. Also, the possibility of laser induced local noncentrosymmetry at the time of irradiation is not ruled out.
Table 4
The calculated dipole moment (in D), β components (a.u.) and βtot(esu)
First-order hyperpolarizability |
βxxx | 30359.464 |
βxxy | 5359.738 |
βxyy | 1388.148 |
βyyy | 1069.667 |
βxxz | -3568.716 |
βxyz | -616.136 |
βyyz | 13.373 |
βxzz | 425.921 |
βyzz | 149.881 |
βzzz | -92.526 |
βtot( X 10− 30) | 285.454 |
Dipole moment |
µx | 6.5185 |
µy | 1.2319 |
µz | -1.5413 |
µ | 17.3113 |
Table 5
Some quinolinium derivatives exhibiting SHG efficiency
Specimen | System | Centrosymmetric space group | SHG efficiency | Ref |
1, 6-dimethylquinolinium iodide | Monoclinic | P21/c | 3×10− 4 times of urea | 22 |
1-methyl quinoliniumiodide | Monoclinic | C2/c | 1.67 times of urea | 22 |
6-chloro-1-methylquinolinium iodide | Monoclinic | P21/c | 1.67 times of urea | 23 |
1-methyl-6-nitro quinoliniumiodide | Monoclinic | P21/c | 9 × 10− 3 times of urea | 23 |
8-hydroxy-5-nitro- quinolinium p-toluene sulfonate | Monoclinic | C2/c | 0.2 < times of KDP | Present work |
| | Non- centrosymmetric space group | | |
8-hydroxyquinolinium hydrogen squarate | Orthorhombic | P212121 | 2.6 times of KDP | 21 |
8-hydroxyquinolinium succinate | Monoclinic | P21 | 1.3 times of KDP | 18 |
2-amino-5-nitropyridinium p-phenolsulfonate | Orthorhombic | Pna21 | 22 times of KDP | 12 |
8-Hydroxyquinolinium hydrogen maleate | Orthorhombic | P212121 | 7 times of KDP | 20 |
Hirshfeld Surfaces Analysis
Hirshfeld surfaces of individual molecules are given in Fig. 7 for a better understanding of the molecular interactions. The Hirshfeld surface of HNT used for analyzing intermolecular interactions is displayed in Fig. 7 showing surfaces that have been mapped over a dnorm, curvedness, de, shape index and di [43–44]. Because of the symmetry between de and di in the expression for dnorm where two Hirshfeld surfaces trace, both display a red spot indistinguishable in colour, intensity as well as size and shape. The combination of de and di as 2D two-dimensional fingerprint plots, Fig. 8 provides a summary of intermolecular interactions in the crystal. The O···H (21%) & H…O (18.7%) and N…O (0.7%) & O…N (0.4%) interactions are characterized by a spike in the bottom area whereas the C···H (8.2%) & H…C (6.4%) interactions are signified by a spike in the top middle region in the fingerprint plot. The H···H (28.5%) interactions are spotted in the middle of fingerprint plot. At the top right corner in the fingerprint region, C…C (3.4%) interactions are distinguished. Dominant interactions occupy more space as shown in the finger print region. The measurement of molecular interactions, pie charts of HNT are given in Fig. S3.
Electrostatic Potential
Molecular electrostatic potential maps drawn on the Hirshfeld surfaces with potentials ranging from − 0.112 au (red) to + 0.235 au (blue) (Fig. 9). HNT crystallizes in the monoclinic space group C2/c with Z = 8. A clear separation of the electropositive and electronegative regions was observed. Positive ESP on the H2A atom (+ 0.227 au) interacts with negative ESP region over the O1 atom (-0.099 au) resulting in the formation of the O···H contact in the crystal packing (Fig. 10a). In the N…H contact (Fig. 10b) the electropositive region (+ 0.059 au) around H2A interacts with highly electronegative (-0.076 au) region around the N2 (Fig. 10b).
Fourier Molecular Orbital Analysis (Fmo)
The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of HNT are shown in Fig. 11. The red and green colors represent the positive and negative values for the wave function. The HOMO is the orbital that primarily acts as an electron donor and the LUMO is the orbital that mainly acts as an electron acceptor. HOMO is localized on SO3 sulfonate group and LUMO is localized on whole 5-nitro-8-hydroxyquinolinium molecule, while HOMO-1, HOMO-2, HOMO-3 and HOMO-4 are localized on whole benzene sulfonate molecule. Whereas, LUMO + 1, LUMO + 2 and LUMO + 3 are localized almost on the whole 5-nitro-8-hydroxyquinolinium molecule. The LUMO + 4 is localized on the pyridinium ring. The energy gap between HOMO (-0.2097) to LUMO (-0.1740) of the molecule is about 0.0357 a.u (0.97 eV). The theoretical energy band gap value obtained from FMOs is somewhat lower than the experimental value because the alienated HNT molecule is in the gas phase whereas the obtained experimental value is in the solid phase [45]. The HOMO and LUMO energy gap explain the eventual charge transfer interactions taking place within the molecule.
Molecular Electrostatic Potential (Mep)
The MEP plot of electrostatic potential mapped (DFT/6-311G(d, p)) on the constant electron density surface displaying electrostatic potential (electron + nuclei) distribution is shown in Fig. S4. The different values of the electrostatic potential at the surfaces are represented by different colors. Red represents regions of most negative electrostatic potential (preferred site for electrophilic attack), blue represents regions of most positive electrostatic potential (preferred site for nucleophilic attack) and green represents regions of zero potential. Red colours indicate the more electron rich and blue the more electron poor area. Furthermore, the polarization effect is clearly visible. The color code of this map is in the range between − 0.117e0 to + 0.117e0 for HNT. The negative region is mainly localized from sulfonate anion group whereas the positive region lies in the quinolinium cation system. Molecular shape, size and dipole moments of the molecule provide a visual method to understand the relative polarity.
Mulliken Population Analysis
Mulliken atomic charges are calculated by determining the electron population of each atom as defined by the basis function (DFT/6-311G(d, p)). Figure 12 shows the Mulliken atomic charges and plots of HNT. From the atomic charge values the oxygen (O1-O6), nitrogen (N2) and carbon (C1-C3, C5–C7, C9-C10, C12 and C14) in HNT had a large negative charge and behaved as electron donors. The remaining atoms (all H atoms and C4, C8, C11, C13, C15-C16, S1 and N1) are acceptors exhibiting positive charge. The negative charges on nitrogen/oxygen, which is a donor atom and net positive charge on hydrogen and sulfur atoms, which is an acceptor atom, suggest the presence of intra- and intermolecular hydrogen bonding interactions as visualized by Hirshfeld surface analysis. The Mulliken charge population values are given in Table 6.
Table 6
Atoms | Charges | Atoms | Charges |
C1 | -0.3555 | C14 | -0.279 |
C2 | -0.0394 | H11 | 0.2292 |
H1 | 0.2052 | C15 | 0.1983 |
C3 | -0.1514 | H12 | 0.2590 |
H2 | 0.1790 | C16 | 0.2323 |
C4 | 0.0018 | H10 | 0.2538 |
C5 | -0.1690 | C14 | -0.279 |
H3 | 0.1805 | N1 | 0.0250 |
C6 | -0.1007 | N2 | -0.7330 |
H4 | 0.2075 | O4 | -0.5690 |
C7 | -0.6629 | H13 | 0.4243 |
H5 | 0.2070 | O5 | -0.2750 |
H6 | 0.2165 | O6 | -0.2380 |
H7 | 0.2243 | H14 | 0.4106 |
O1 | -0.7040 | | |
O2 | -0.7440 | | |
O3 | -0.6160 | | |
S1 | 1.2298 | | |
C8 | 0.3041 | | |
C9 | -0.0140 | | |
H8 | 0.2540 | | |
C10 | -0.1300 | | |
H9 | 0.2401 | | |
C11 | 0.2469 | | |
C12 | -0.0510 | | |
C13 | 0.1032 | | |