Conformational analysis of 2-bromocyclohexanone. A combined NMR, IR, solvation and theoretical approach

An improved method of conformational analysis using H-1 and C-13 NMR, IR, theoretical calculations and solvation theory is reported for 2-bromocyclohexanone, used here as a model compound. The solvent dependence of the (3)J(HH), (1)J(CH) and (1)J(CD) NMR coupling constants and the associated changes in the IR first overtone carbonyl band intensities together with theoretical calculations allow the direct determination of the conformational equilibria without recourse to model compounds. Calculations with the Gaussian 98 program at the HF/6-31 g(d,p) and B3LYP/6-31 + g(d,p) levels together with solvation theory gave the conformer free energy difference (E-eq - E-ax) in different solvents. The observed couplings, when analyzed by solvation theory and utilizing DFT geometries, gave a value of E-eq - E-ax of 1.15 kcal mol(-1) in the vapor phase, decreasing to 0.6 kcal mol(-1) in CCl4 and to -0.5 kcal mol(-1) in DMSO solution (1 kcal = 4.184 kJ). The axial percentage changes from 74% (in CCl4) to 30% (in DMSO), and these are in good agreement with infrared data (nu (C=O), first overtone), despite the uncertainties of the latter method. The results illustrate the advantages of the joint application of these techniques, which represents an improved approach to the study of the conformational equilibria of substituted cyclohexanones. Copyright (C) 2001 John Wiley & Sons, Ltd.