Vibrational spectra and first-order molecular hyperpolarizabilities of p-hydroxybenzaldehyde dimer D. Sajan a,c,⇑ , Y. Erdogdu b , Thomas Kuruvilla a , I. Hubert Joe c a Department of Physics, Bishop Moore College, Mavelikara, Alappuzha 690 110, Kerala, India b Department of Physics, Ahi Evran University, 40040 Kirsehir, Turkey c Centre for Molecular and Biophysics Research, Department of Physics, Mar Ivanios College, Thiruvananthapuram 695 015, Kerala, India article info Article history: Received 13 May 2010 Received in revised form 31 July 2010 Accepted 2 August 2010 Available online 6 August 2010 Keywords: NIR-FT-Raman spectroscopy FT-IR spectroscopy DFT ICT abstract Single crystals of p-hydroxybenzaldehyde (PHBA) were grown by the slow evaporation technique and vibrational spectral analysis was carried out using near-IR Fourier transform Raman and Fourier trans- form IR spectroscopy. The density functional theoretical (DFT) computations were also performed at the B3LYP/6-311++G(d,p) level to derive the equilibrium geometry, vibrational wavenumbers and inten- sities. The detailed interpretation of the vibrational spectra has been carried out with the aid of normal coordinate analysis (NCA) following the scaled quantum mechanical force field methodology. The various intramolecular interactions that is responsible for the stabilization of the molecule was revealed by nat- ural bond orbital analysis. Vibrational analysis based on the NIR-FT-Raman, FT-IR and computed spec- trum reveals that the CH in-plane bending of the aldehyde group interacts with its stretching mode via Fermi Resonance and evidence for intermolecular interaction can be well identified as two CH bands in IR spectra at 2740 and 2804 cm 1 aldehyde group of the p-hydroxybenzaldehyde dimer. The red shift of the O–H stretching wavenumber is due to the formation of strong O–HO hydrogen bonds by hyper- conjugation between the carbonyl oxygen lone electron pairs and the O–H r  anti-bonding orbitals. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction The search for new materials with non-linear optical (NLO) properties has been the subject of intense research due to their application in a wide range of technologies such as optical comput- ing and optical communication [1,2]. In the past years much atten- tion has been paid to organic NLO materials due to their promising applications in optoelectronics technology [3,4], their large non- linear response, extremely fast switching time and convenient optimization routes through molecular engineering compared to the currently studied inorganic materials [5]. It has been generally understood that for a material to have useful and highly efficient NLO properties, the constituting molecules need first to exhibit large molecular hyperpolarizabilities, which are generally charac- terized by a highly extended p-conjugated chain with strong elec- tron donor–acceptor pairs at the ends (D–p–A) [6]. Since a large molecular hyperpolarizability b is the basis of a strong second har- monic generation (SHG) response; organic molecules with long conjugation systems that usually exhibit large b values are cer- tainly candidate molecules for NLO materials. The benzaldehyde and substituted benzaldehydes have been subjected to various spectroscopic studies [7–24]. Mono-, halo-, methoxy and ethoxy-substituted benzaldehydes, among others, have attracted the attention of the spectroscopists. o- and m-Chlo- robenzaldehydes have been shown to have trans and cis conform- ers by Matrix Isolation IR spectroscopy [25]. Vibrational spectral studies of the molecules can provide deeper knowledge about the relationships between molecular architecture, non-linear re- sponse, and hyperpolarizability and support the efforts towards discovery of new efficient materials for technological applications. NIR-FT-Raman spectroscopy combined with quantum chemical computations have recently been used as effective tools in the vibrational analysis of drug molecules , biological compounds natu- ral products and NLO active compounds [26–30], since fluores- cence 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 sys- tems. The present work deals with detailed vibrational spectral investigation of p-hydroxybenzaldehyde (PHBA) dimer (Fig. 1) molecules using NIR-FT-Raman and FT-IR spectra, along with den- sity functional theoretical computations to study the structural and bonding features, nature of hydrogen bonding and vibrational 0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2010.08.003 ⇑ Corresponding author at: Department of Physics, Bishop Moore College, Mavelikara, Alappuzha 690 110, Kerala, India. Tel.: +91 9495043765; fax: +91 4792303230. E-mail address: dsajand@gmail.com (D. Sajan). Journal of Molecular Structure 983 (2010) 12–21 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc