A Comparison of the ab Initio Calculated and Experimental Conformational Energies of Alkylcyclohexanes

Fillmore Freeman and Zufan M. Tsegai

Department of Chemistry, University of California, Irvine, Irvine, CA 92697-2025

Marc L. Kasner

Department of Chemistry and Biochemistry, Montclair State Univerity, Upper Montclair, NJ 07043

Warren J. Hehre

Department of Chemistry, University of California, Irvine, Irvine, CA 92697, and Wavefunction, Inc., Irvine CA 92612

Abstract

Ab initio 6-31G(d) and MP2/6-31G(d)//6-31G(d) methods were used to calculate the energies of the rotamers of the chair conformers of alkylcyclohexanes and trimethylsilylcyclohexane. The MP2/6-31G(d)//6-31G(d) calculated conformational energies ( Gibb's free energy changed or A values, in kcal/mol) of the alkylcyclohexanes (Me = 1.96; Et = 1.80; Pr = 1.73 iso-Pr = 1.60; t-Bu = 5.45; neo-pent = 1.32) and trimethylsilylcyclohexane ( SiMe3 = 2.69) are similar to the experimental values. Plots of the calculated conformational energies for the alkylcyclohexanes and trimethylsilylcyclohexane versus their experimental values are linear (slope = 1.253 and r = .993 for 6-31G(d) and slope = 1.114 and r = .982 for MP2/6-31G(d)//6-31G(d)). The conformational energies are determined primarily by steric effects which include gauche (synclinal) interactions and repulsive nonbonded interactions in both the axial and equatorial conformers.

Journal of Chemical Education, 2000, 77, 661                       Full text (PDF)

-- SiripornJu - 08 Mar 2007