Changes in the vibrational spectral modes by the nonbonded interactions in the NLO crystal vanillin

Abstract

The crystal of the famous flavoring agent vanillin is known for its nonlinear optical activity. Density functional theory at B3LYP/6-31G(d) level has been used to compute energies of different conformers of vanillin to find their stability, the optimized geometry of the most stable conformer and its vibrational spectrum. The conformer VAN1 with torsion angles 0°, 180°and 0°respectively for C 2 -C 1 -C 7 -O 14 , C 2 -C 1 -C 7 -H 13 and C 3 -C 4 -O 10 -H 19 is found to be most stable. Geometry predicted by DFT and XRD reveals that geometrical parameters of hydroxyl group are altered substantially because of intermolecular hydrogen bonding. Vibrational analysis based on the NIR FT Raman, FT IR and computed spectrum reveals that the CH in-plane bending of the aldehyde group interacts with its stretching mode via Fermi resonance, and evidence for intermolecular hydrogen bonding can be observed as the broadness of the band in the stretching region in IR spectrum. The unusual lowering of carbonyl stretching wave number is attributed to the electron-releasing effect of C O group in the acceptor subunit due to intramolecular charge transfer, leading to NLO activity, in addition to the intermolecular interaction and p-conjugation. Other vibrational spectral contributions of NLO activity such as violation of selection rule for tangential C-C stretching mode 8 and overlooking of complementarities are also observed. Nonbonded interactions affecting each vibrational mode are found, and the resulting shifting of band position is discussed. Firstorder hyperpolarizability, computed at HF/3-21G level, is used to analyze the conformation dependent NLO activity.

Molecular structure, vibrational spectroscopic (FT-IR, FT-Raman), UV and NBO analysis of 2-chlorobenzonitrile by density functional method

“…The FT-IR and NIR FT-Raman spectroscopy combined with Quantum Chemical computations have been recently used as an effective tool in the vibrational analysis of drug molecules [10], biological compounds and natural products [11]. Since fluorescence Raman spectra and the computed results can help unambiguous identification of various modes as well as the bonding and structural features of complex organic molecular systems.…”

Electron–phonon coupling and vibrational modes contributing to linear electro‐optic effect of the efficient NLO chromophore 4‐(N,N‐dimethylamino)‐N‐methyl‐4′‐toluene sulfonate (DAST) from their vibrational spectra

“…[22] Vibrational spectroscopy has been extensively used to understand the factors contributing to the linear electro-optic (LEO) effect from the vibrational modes in organic materials and to provide deeper knowledge regarding the intermolecular interactions and the relationship between molecular architecture, nonlinear response and hyperpolarizability. [1] Near-infrared Fourier transform (NIR-FT) Raman spectroscopy combined with quantum chemical computations has recently been used as an effective tool in the vibrational analysis of drug molecules, [23] biological compounds, [24 -27] natural products [28,29] and NLO active compounds, [30 -38] as fluorescence-free Raman spectra and computed results can help unambiguous identification of vibrational modes as well as the bonding and structural features of complex organic molecular systems. The present work describes the vibrational spectral investigations aided by density functional theoretical (DFT) methods to elucidate the correlation between the molecular structure and NLO property by investigating the intramolecular charge transfer (ICT) interaction, e/ph coupling, vibrational modes contributing to electro-optic effect, intramolecular ionic hydrogen bonding, effects of ethylenic bridge and static first hyperpolarizability of the prospective electro-optic crystal DAST.…”

An efficient and simple approach for the synthesis of pyranopyrazoles using imidazole (catalytic) in aqueous medium, and the vibrational spectroscopic studies on 6-amino-4-(4′-methoxyphenyl)-5-cyano-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazole using density functional theory

“…The asymmetric stretching mode of methyl group is expected to be around 2980 cm −1 and symmetric stretching at 2870 cm −1 [31,32]. In our molecule, C-H symmetric stretch of methyl group vibrations recorded at 2836 cm The symmetric and asymmetric bending vibrations of methyl group are normally expected in the regions 1465-1440 cm −1 and 1390-1370 cm −1 , respectively.…”