Optical Activity of 1-Butene, Butane, and Related Hydrocarbons

All molecules except those conformations that have a plane of symmetry, a center of inversion, or an improper axis of rotation are chiral. The magnitudes of the optical rotation of the chiral rotamers of 1-butene, butane, propene, propane, and ethane have been examined. Changes in optical rotation with torsional angle are quite large. The calculated specific rotations for 1-butene are about as large as those for the substituted 1-butenes previously studied, and changes in the basis set used in the calculations have relatively small effects on the rotations. With butane, the calculated specific rotations are somewhat smaller than found with 1-butene, and here, going from 6-311++G** to aug-cc-pVDZ has a major effect on the relationship between the C−C−C−C torsional angle and the calculated rotation. With propene, a change in basis set led to a change in the calculated rotation with torsional angle, and with propane, the magnitude of the specific rotation is markedly reduced, and the sign is changed on going from 6-311++G** to aug-cc-pVDZ or aug-cc-pVTZ. The same is found with ethane. The sign change for ethane is found to originate with the p-type diffuse orbitals in the aug-cc-pVDZ basis set.