Hydrogen bond, dimerization and vibrational modes in 2-chloro-3-hydroxybenzaldehyde and 3-chloro-4-hydroxybenzaldehyde from vibrational and ab initio studies

Ab initio conformers and dimers have been computed at RHF and B3LYP/6-31G* levels for isomers 2-chloro-3-hydroxybenzaldehyde and 3-chloro-4-hydroxybenzaldehyde to explain the observed infrared absorption and Raman vibrational spectral features in the region 3500-50 cm ^-1. The position of the chlorine in ortho position with respect to aldehyde group in 2-chloro-3-hydroxybenzaldehyde yields four distinct conformers; whereas the chlorine in meta position in 3-chloro-4-hydroxybenzaldehyde yields effectively only three conformers. Major spectral features as strong absorptions near 3160-80 cm ^-1, down-shifting of the aldehydic carbonyl stretching mode and up-shifting of hydroxyl group's in-plane bending mode are explained using ab initio evidence of O-H⋯O bond-aided dimerization between the most stable conformers of each molecule. Absorption width of about 700 cm ^-1 (∼8.28 kJ/mol) of O-H stretching modes suggests a strong hydrogen bonding with the ab initio bond lengths, O-H⋯O in the range of 2.873-2.832 Å. A strong Raman mode near 110-85 cm ^-1 in each molecule is interpreted to be coupled vibrations of pseudo-dimeric trans and cis structures.