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The molecular geometry, the normal mode frequencies and corresponding vibrational assignments, natural bond orbital analysis and the HOMO-LUMO analysis of 7-Amino-2,4-dimethylquinolinium formate in the ground state were performed by B3LYP levels of theory using the 6-31G(d) basis set. The optimised bond lengths and bond angles are in good agreement with the X-ray data. The vibrational spectra of the title compound which is calculated by DFT method, reproduces vibrational wave numbers and intensities with an accuracy which allows reliable vibrational assignments. The possibility of N-H⋯O hydrogen bonding was identified using NBO analysis. Natural bond orbital analysis confirms the presence of intramolecular charge transfer and the hydrogen bonding interaction.

Vibrational spectral analysis was carried out for 4-methoxy-2-methyl benzoic acid (4M2MBA) by using Fourier transform infrared (FT-IR) (solid, gas phase) and FT-Raman spectroscopy in the range of 400-4000 and 10-3500 cm −1 respectively. The effects of molecular association through O-H· · ·O hydrogen bonding have been described by the single dimer structure. The theoretical computational density functional theory (DFT) and Hatree-Fock (HF) method were performed at 6-311++G(d,p) levels to derive the equilibrium geometry, vibrational wavenumbers, infrared intensities and Raman scattering activities. The scaled theoretical wavenumbers were also shown to be in good agreement with experimental data. The first-order hyperpolarizability (β 0 ) of this novel molecular system and related properties (β, α 0 and α) of 4M2MBA are calculated using the B3LYP/cc-pvdz basis set, based on the finite-field approach. A detailed interpretation of the infrared and Raman spectra of 4M2MBA is reported. The theoretical spectrograms for FT-IR and FT-Raman spectra of the title molecule were also constructed and compared with the experimental one.

The Fourier-transform infrared spectrum of 3-hydroxy-2-naphthoic acid hydrazide (3H2NAH) was recorded in the region 4000-400 cm −1 . The Fourier-transform Raman spectrum of 3H2NAH was also recorded in the region 3500-10 cm −1 . Quantum chemical calculations of energies, geometrical structure and vibrational wavenumbers of 3H2NAH were carried out by density functional theory (DFT/B3LYP) method with 6-31G(d,p) as basis set. The difference between the observed and scaled wavenumber values of most of the fundamentals is very small. The values of the electric dipole moment (µ) and the first-order hyperpolarizability (β) of the investigated molecule were computed using ab initio quantum mechanical calculations. The UV spectrum was measured in ethanol solution. The calculation results also show that the 3H2NAH molecule might have microscopic nonlinear optical (NLO) behavior with non-zero values. A detailed interpretation of the infrared and Raman spectra of 3H2NAH is also reported based on total energy distribution (TED). The calculated HOMO and LUMO energies shows that charge transfer occur within the molecule. The theoretical FT-IR and FT-Raman spectra for the title molecule have also been constructed.

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