Structural and electronic contributions to hyperpolarizability in methyl p-hydroxy benzoate

The single crystals of methyl p-hydroxy benzoate (MPHB) are grown from methanol solution by a low temperature solution growth technique. The SHG efficiency is tested using Q-switched Nd:YAG laser of wavelength λ at 1064 nm, which is approximately 1.2 times that of urea. Vibrational spectral analysis using NIR-FT Raman and FT-IR spectra is carried out to understand the structural and electronic contributions to hyperpolarizability in MPHB. The DFT computations are also performed at B3LYP/6-311G(d,p) level to derive equilibrium geometry, vibrational wavenumbers and intensities. The results of ab initio calculations at HF/6-311G(d,p) level show that the vibrational contribution for the second-order electro-optic coefficient in MPHB is about 19.5%. Vibrational spectral studies also provide evidence for the charge transfer interaction between the donors and the acceptor group through the π-system. The π-electron cloud movement from donor to acceptor can make the molecule highly polarized and the intramolecular charge transfer interaction must be responsible for the nonlinear optical properties of MPHB. The splitting of the carbonyl mode may be attributed to the intramolecular association based on CO⋯H type hydrogen bonding in the molecule. The conjugation and influence of intermolecular hydrogen bonding (CO⋯H) type network in the crystal results in lowered CO stretching wavenumber.