In principle, IR spectroscopy can be used to measure the populations of diastereomeric conformers that are in rapid equilibrium. However, the use of IR spectroscopy suffers from the necessity to obtain independent measures of the extinction coefficients for pertinent bands in a given conformer. Common assumptions are that extinction coefficients are equal for different conformers and independent of temperature. We have examined the NMR spectra of four halocyclohexanes over a temperature range where the axial-to-equatorial equilibration is slow on the NMR chemical exchange time scale. Using complete line-shape analysis, we carefully determined the variation of the equilibrium constant with temperature (allowing calculation of thermodynamic parameters). Measurements of the IR spectra over the same temperature range enabled the calculation of the relative IR extinction coefficients for the C-X bond stretch in the axial and equatorial conformers. The vibrational extinction coefficients ratios are indeed not unity and exhibit a slight temperature dependence. Based on the IR data and on our measurements of the one-bond scalar coupling of the methine carbon to the proton, there are indications that axial carbon-halogen and carbon-hydrogen bonds are weaker than the equatorial counterparts.

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ChemInform Abstract (studied by 1H dynamic NMR of three selectively deuteriated derivatives).

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