Ab initio study of magnetochiral birefringence

Abstract

The magnetically induced axial birefringence of six closed-shell chiral molecules (methyloxirane, C3H6O, fluoro- and methylcyclopropanone, C3H3OF and C4H6O, carvone, C10H14O, limonene, C10H16, and proline, C5H9NO2) is determined at the Hartree–Fock wave-function level by evaluating the frequency dependent quadratic response functions entering the molecular property expression, according to Barron and Vrbancich [Mol. Phys. 51, 715 (1984)]. Both the magnetic dipole and the electric quadrupole contributions are taken into account and their relative importance is discussed. A proof of the origin independence of the magnetochiral birefringence is presented for the exact wave function and the dependence on the origin is investigated in finite basis set calculations at the Hartree–Fock level. For carvone, limonene, and proline the results are compared with recent experimental data obtained by two different experimental groups, which are in disagreement with respect to the magnitude of the magnetochiral effect. A parallel study of the natural optical rotation shows that in the three larger molecules the optical rotatory strengths are strongly affected by changes in conformations. Nonetheless the magnetochiral birefringence computed for various different conformers—although varying remarkably—is much smaller in absolute value than experimentally observed. The disagreement—of more than three orders of magnitude—between some experimental data and theory appears to be hard to reconcile and to attribute entirely to limitations in the computational approach.