Conformational equilibria of trans -3-X-cyclohexanols (X = Cl, Br, CH3 and OCH3). A low temperature NMR study and theoretical calculations.
The integration of 1H and 13C 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 DeltaG(ea-ae) values (from (1)H 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 (1)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 DeltaG(ea-ae) value of 1.31 +/- 0.02 kcal mol(-1). The values obtained through the additivity rule, with data from monosubstituted cyclohexanes (DeltaG(Ad) = DeltaG(X) + DeltaG(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 DeltaG(ea-ae) values similar to the experimental values.
de Oliveira
Paulo R
PR
Departamento de Química e Biologia, Universidade Tecnológica Federal do Paraná, Av. Sete de Setembro, 3165, Rebouças, Curitiba, PR 80230-901, Brazil.
Rittner
Roberto
R
D000079:Acetaldehyde / Q000737:chemistry*; D001966:Bromine / Q000737:chemistry; D002713:Chlorine / Q000737:chemistry; D003511:Cyclohexanols / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods / Q000592:standards*; D008956:Models, Chemical; D008968:Molecular Conformation; D011789:Quantum Theory; D013696:Temperature; D013816:Thermodynamics
D003511:Cyclohexanols; D002713:Chlorine; D000079:Acetaldehyde; D001966:Bromine