Vibrational assignments and effect of aldehyde rotation on substituents in some trisubstituted benzaldehydes

Fourier-transform, laser Raman (3500–50 cm−1) and infrared (IR) (4000–400 cm−1) spectral measurements have been made for solid samples of 2-chloro-3,6-difluoro-, 3-chloro-2,6-difluoro- and 2-chloro-6-fluoro-3-methyl-benzaldehydes. The electronic structure calculations at RHF/6-311G* and B3LYP/6-311G* levels have been performed. Molecular electronic energies, equilibrium geometries, IR and Raman spectra have been computed. Potential energy distributions (PEDs) and normal modes, for the spectral data computed at B3LYP/6-311G* have also been obtained from a force-field calculations. A complete assignment of the observed spectra has been proposed. Investigation of the internal rotation of the aldehyde has shown two rotamers, O-cis and O-trans, with respect to aldehydic oxygen and chlorine. The energy difference between rotamers for three compounds is 226 cal/mol, 8 cal/mol and 167 cal/mol respectively.