摘要：

The barrier to rotation about the C2-C3 bond in n-butane has been calculate using several basis sets, complete geometry optimization, and correction for electron correlation. Neither the basis set size nor inclusion of electron correlation has a large effect on the magnitude of the rotational barrier or the trans/gauche energy difference. After correction for zero-point energy differences, the former is found to be 6.34 kcal/mol, while the latter is 0.86, in very good agreement with the experimental value. The trans/gauche energy difference in n-pentane is the same as that for butane, and similar values are found for the two gauche forms of n-hexane and the symmetrical gauche form of octane. The structures and energies of several conformers of pentane and hexane with two and three gauche fragments also have been obtained. It is found that the pentane rotamer with two consecutive gauche kinks has roughly twice the gauche energy, but the hexane conformer with three consecutive gauche kinks has considerably less than 3 times that value. The energy differences for the rotamers of 2-methylbutane and 2,3-dimethylbutane are well reproduced by the calculations. Vibrational frequencies are estimated at the 3-21G level for all species, and the zero-point energies and enthalpy changes (H-H) are calculated. The difference in enthalpy between an axial and equatorial methyl substituent on cyclohexane is calculated to be 2.17 kcal/mol after correcting for vibrational energy differences. The relationship between the energy of a methylene group in cyclohexane and in trans-n-alkanes is examined. 49 refs., 9 tabs.

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