| dc.description | The integration of I H and C-13 NMR spectra, at -90 degrees C in CS2/CD2Cl2 (9: 1), for the trans-3-chlorocyclohexanol (1), trans-3-bromocyclohexanol (2), and trans-3-methoxycyclohexanol (4) showed that the equatorial-axial (ea) conformer occurs as ca 63, 63, and 69% in the conformational equilibrium, respectively. This corresponds to the following Delta G(ea-ae) values (from H-1 spectrum): -0.32 +/- 0.01, -0.32 +/- 0.04, -0.48 +/- 0.05 kcal mol(-1); and to (from 13C spectrum): -0.31 +/- 0.04, -0.35 +/- 0.05, and -0.44 +/- 0.01 kcal mol(-1), respectively, in very good agreement within both series. Thus, although bromine is bulkier than chlorine, the 1,3-diaxial steric effects are similar in these equilibria. However, the integration of I H NMR spectrum for the trans-3-methylcyclohexanol (3) gave 90% of the 3ae conformer in the equilibrium, at -90 degrees C on CS2/CD2Cl2 (9: 1), corresponding to a Delta G(ea-ae) value of 1.31 +/- 0.02 kcal mol(-1). The values obtained through the additivity rule, with data from monosubstituted cyclohexanes (Delta G(Ad) = Delta G(x) + Delta G(OH)), for compounds 1, 2, and 4 (-0.37 +/- 0.15, -0.34 +/- 0.09, and -0.46 +/- 0.04 kcal mol(-1), respectively) are in very good agreement with the experimental values, but it is significantly smaller for compound 3 (0.79 0.02 kcal mol-1). Theoretical calculations through different levels of theory (HF/6-311+g**, B3LYP/6-311+g**, MP2/6-31+g**, and CBS-4M) showed that CBS-4M is the best method for the study of conformational equilibria for these systems, since it provides Delta G(ea-ae) values similar to the experimental values. Copyright (c) 2008 John Wiley & Sons, Ltd. | |
