Complementary IR/NMR approach for the determination of IR extinction coefficients and thermodynamic parameters for conformers in rapid equilibrium. The Halocyclohexanes

Abstract

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.

Ab initio calculations and RRKM/Master Equation modeling of chloroalkanes → alkenes + HCl reactions for use in comparative rate studies

“…From the temperature dependence of the intensity of vibrational bands, Durig et al [28] determined the axial conformer of chlorocyclohexane to be less stable than the equatorial conformer by 1.58 ± 0.16 kJ mol −1 . These measurements improved earlier determinations of 1.1-2.1 kJ mol −1 by Stokr et al [29], Gardiner et al [30], Caminati et al [31], Bugay et al [32], and Diky et al [33]. Both Stokr et al and Bugay et al also determined the barrier-to-conversion to be about 45 kJ mol −1 .…”

Conformational stability from variable temperature infrared spectra of xenon solution, r0 structural parameters, and vibrational assignment of fluorocyclohexane

“…These results indicate the value should be near 100 cm À1 (0.29 kcal/mol) but theoretical predictions from molecular mechanics (MM) calculations [8] give a value of 67 cm À1 (0.19 kcal/mol) whereas from MP2 ab initio calculations [9] a value of 33 cm À1 (0.09 kcal/mol) was obtained. Therefore these results do not provide a value which can be easily compared with the enthalpy values recently obtained for chloro- [2] and bromocyclohexane [3].…”

Conformational equilibria of trans ‐3‐X‐cyclohexanols (X = Cl, Br, CH3 and OCH3). A low temperature NMR study and theoretical calculations

“…The following values of G X (kcal mol −1 ) were chosen for Cl (0.64 ± 0.15), [16] Br (0.67 ± 0.09), [16] CH 3 (1.80 ± 0.02), [5] OCH 3 (0.55 ± 0.02), [17] and G OH for OH (−1.01 ± 0.04). [16] Error limits were taken from the data in cited references.…”