The utility of infrared spectroscopy for quantitative conformational analysis at a single temperature

The carbonyl stretching vibration of 2-bromocyclohexanone (1) has been measured in a variety of solvents. It is shown that its component intensities are not only dependent on the populations of the axial and equatorial conformers, but are also dependent on the molar absorptivities (ε) which are specific for each conformer in each solvent. In CCl4, the axial and equatorial conformers have ε values of 417 and 818 l mol−1 cm−1, respectively, while in CH3CN solution, the values were 664 and 293 l mol−1 cm−1. These results are supported by results of theoretical calculations of frequencies, which gave an intensity of 223.8 kM mol−1 (1782 cm−1) for the axial and 174.4 kM mol−1 (1802 cm−1) for the equatorial conformer, indicating that the axial conformer presents a larger molar absorptivity than the equatorial one in the vapor phase. Moreover, the results presented here clearly demonstrate that although infrared spectroscopy at a single temperature can be an important auxiliary technique for conformational analysis, it must not be used to quantify conformational preferences of a molecule if the absorption molar coefficients for each conformer are not known or not amenable to experimental determination.