Model studies of the optical rotation, and theoretical determination of its sign for β‐pinene and trans‐pinane

Abstract

We have carried out extensive studies on the basis set dependence of the calculated specific optical rotation (OR) in molecules at the level of the time-dependent Hartree-Fock and density functional approximations. To reach the limits of the basis set saturation, we have devised an artificial model, the asymmetrically deformed (chiral) methane (CM) molecule. This small system permits to use basis sets which are prohibitively large for real chiral molecules and yet shows all the important features of the basis set dependence of the OR values. The convergence of the OR has been studied with n-aug-cc-pVXZ basis sets of Dunning up to the 6-zeta. In a parallel series of calculations, we have used the recently developed large polarized (LPolX) basis sets. The relatively small LPolX sets have been shown to be competitive to very large n-aug-cc-pVXZ basis sets. The conclusions reached in calculations of OR in CM concerning the usefulness of LPolX basis sets have been further tested on (S)-methyloxirane and (S)-fluoro-oxirane. The smallest set of the LPolX family (LPol-ds) has been found to yield OR values of similar quality as those obtained with much larger Dunning's aug-cc-pVQZ basis set. These results have encouraged us to carry out the OR calculations with LPol-ds basis sets for systems as large as beta-pinene and trans-pinane. In both cases, our calculations have lead to the correct sign of the OR value in these molecules. This makes the relatively small LPol-ds basis sets likely to be useful in OR calculations for large molecules.