Vibrational spectral studies and the non‐linear optical properties of a novel NLO material L‐prolinium tartrate

Vibrational spectral analysis of the novel non-linear optical (NLO) material, L-prolinium tartrate (LPT) was carried out using NIR-FT-Raman and FT-IR spectroscopy. The density functional theoretical (DFT) computations have been performed at B3LYP/6–31G (d) level to derive equilibrium geometry, vibrational wavenumbers, intensities and first hyperpolarizability. The reasonable NLO efficiency, predicted for the first time in this novel compound, has been confirmed by Kurtz–Perry powder second-harmonic generation (SHG) experiments. The charge-transfer interaction between the pyrrolidine ring and the carbonyl group of the tartrate anion through the intramolecular ionic hydrogen bonds is confirmed by the simultaneous activation of ring modes in IR and Raman spectra. The splitting of the ring-breathing mode, pseudo-rotational ring puckering modes and the NH2 modes of the pyrrolidine ring lead to the conclusion that the pyrrolidine ring adopts a conformation intermediate between the envelope (bent) form and the half-chair (twisted) form, resulting in the lowering of symmetry from C2 to Cs. The lowering of the methylenic stretching wavenumbers and the enhancement of the stretching intensities suggest the existence of the electronic effects of back-donation in LPT. The positional disorder of the pyrrolidine ring, the presence of blue-shifting H-bonds as well as other non-bonded interactions in LPT, low frequency H-bond vibrations and the role of intramolecular charge transfer and the hydrogen bonds in making the molecule NLO active have been analysed on the basis of the vibrational spectral features. Copyright © 2006 John Wiley & Sons, Ltd.