FT-Raman and infrared spectra and vibrational assignments for 3-chloro-4-methoxybenzaldehyde, as supported by ab initio, hybrid density functional theory and normal coordinate calculations

Fourier-transform laser Raman (3500–50 cm−1) and infrared (4000–400 cm−1) spectral measurements have been made for the solid 3-chloro-4-methoxybenzaldehyde. The electronic structure calculations – ab initio (RHF) and hybrid density functional methods (B3LYP and B3PW91) – have been performed with 6-31G* and 6-311G* basis sets. Molecular electronic energies, equilibrium geometries, IR and Raman spectra have been computed. Potential energy distribution (PEDs) and normal mode analysis have also been performed. A complete assignment of the observed spectra has been proposed. Investigation of the relative orientation of the aldehydic oxygen and chlorine atom with respect to the methoxy group has shown that two forms, O-cis and O-trans exist, with O-trans form being more stable. The energy difference between O-cis and O-trans forms is 0.057 kcal/mol (21 cm−1) with B3LYP/6-31G*, which is less than the calculated torsional vibrational frequencies of the aldehyde and methoxy group. In the CH (O) aldehydic stretching region five observed bands are probably due to multiplet Fermi resonance. An infrared doublet near 1700 cm−1 with nearly equal intensities has been ascribed to the Fermi resonance: the two bands at 1696 and 1679 cm−1 arise due to the interaction between the CO stretching fundamental and a combination of O–CH3 and CC stretching vibrations.