The case of infrared carbonyl stretching intensities of 2-bromocyclohexanone: conformational and intermolecular interaction insights

Abstract

The infrared spectrum of 2-bromocyclohexanone in the vapor phase was obtained for the first time, and the νCO intensity for the more polar equatorial conformer was surprisingly found to be higher than for the axial form, suggesting its larger population, opposite to calculations. Theoretical data for dimeric models showed that attractive intermolecular interactions in the equatorial–equatorial dimer are dominant, but this is not enough to explain the spectroscopic behavior, since the axial–axial dimer remains more stable. The higher CO molar absorptivity in the equatorial form is therefore invoked to explain its more intense CO band, as confirmed by charge calculations.