Conformational analysis of halobenzaldehydes: a theoretical and spectroscopic study

Abstract

Rotation of the formyl group in halobenzaldehydes (halo = F, Cl, and Br) has been studied through high-level density functional theory (DFT) calculations and natural bond orbital (NBO) analysis. The isomeric and conformational energies are dictated by non-Lewis and Lewis-type interactions. The 4-substituted benzaldehydes the most stable isomer due especially to an effective electron resonance and small dipole moment, while destabilizing Lewis-type interactions slightly override the non-Lewis electron delocalization both in the syn and anti conformers of meta isomer. No significant halogen effect is observed on stabilities, excepting for the least stable ortho isomer, where the halogen interacts with the formyl group. Such interaction is more repulsive in the syn conformer (where the halogen and oxygen atoms face one another), but the nature of this interaction changes from predominantly electrostatic to steric contribution on going from F to Br. Infrared stretching vibrations, as well as NMR chemical shifts and spin-spin coupling constants, provided valuable insight into through-bond and through-space effects influencing the stabilities of these compounds.