The infrared spectra of primary amides, Part 2. Deuteration of benzamide and hydrogen bonding effects of ortho alkoxybenzamides

Abstract

The wavenumber values in the carbonyl and NH₂ stretching regions of a further 21 amides, all ortho substituted benzamides or heterocyclic carboxamides, are presented. The previous limits on the spectral ranges are thereby widened marginally, but in predictable ways. The NH stretching vibrations of ortho alkoxybenzamides are related to benzamide itself by a curve dependent on intramolecular hydrogen bonding, quantified by further elaboration of the Linnett equations to account for non-equivalence of the two NH bonds. Deuteration experiments enabled uncoupling of the NH bonds and assignment of bond strengths. Although it is widely believed that there is no coupling between XH and XD stretching frequencies, we show that the true NH frequency is raised by 2 cm⁻¹ as a result of deuterium coupling in NHD groups. The calculation of ND₂ wavenumbers from those of the fundamental and first overtone of the NH₂ values was carried out by 4 procedures. Better concordance with the antisymmetric vibration than the symmetric vibration was observed. The calculated shortening of the NH bonds on deuteration is 0.0035±0.0009 Å.