Chiroptical Properties of (R)-3-Chloro-l-butene and (R)-2-Chlorobutane

Coupled cluster and density functional models of specific rotation and vacuum UV (VUV) absorption and circular dichroism spectra are reported for the conformationally flexible molecules (R)-3-chloro-1-butene and (R)-2-chlorobutane. Coupled cluster length-and modified-velocity-gauge representations of the Rosenfeld optical activity tensor yield significantly different specific rotations for (R)-3-chloro-1-butene, with the latter providing much closer comparison (within 3%) to the available gas-phase experimental data at 355 and 633 nm. Density functional theory overestimates the experimental rotations for (R)-3-chloro-1-butene by approximately 80%. For (R)-2-chlorobutane, on the other hand, all three models give reasonable comparison to experiment. The theoretical specific rotations of the individual conformers of (R)-3-chloro-1-butene are much larger than those of (R)-2-chlorobutane, in disagreement with previous studies of the temperature dependence of the experimental rotations in solution. Simulations of VUV absorption and circular dichroism spectra reveal large differences between the coupled cluster and density functional excitation energies and the rotational strengths. However, while these differences lead to very different specific rotations for (R)-3chloro-l-butene, they have much less impact on the computed specific rotations for (R)-2-chlorobutane. In addition, the coupled cluster VUV absorption spectrum of (R)-2-chlorobutane compares well to experiment.