The solvent effect on the conformational equilibrium of 1,3-dichloro-5-methylcylclohexane

Abstract

The conformational equilibrium has been measured for r-1,c-3-dichloro-t-5-methylcyclohexane(2) in six solvents from the 500 MHz and 400 MHz 1H NMR spectra. The room-temperature and low-temperature C1H couplings are used as well as the vicinal proton-proton coupling constants based on the MM2-calculated dihedral angles and the empirically generalized Karplus equation. The equilibrium exhibits a very weak solvent dependence; ΔGaa-ee varies from 1.22 kcal mol-1 in CDCl3 to 0.87 kcal mol-1 in DMSO-d6. The free energy difference, Δaa-ee II 0.90 kcal mol-1, in CD2Cl2 is determined by peak-area NMR measurements at -100°C. The reaction field theory is used to calculate the variation of Δaa-ee with solvent polarity. The calculated solvent shift δΔGaa-ee (CDCl3 to DMSO-d6) is -0.21 kcal mol-1, compared with -0.35 kcal mol-1 determined experimentally.