Vibrational circular dichroism intensities: Ab initio vibronic coupling theory using the distributed origin gauge

Abstract

The distributed origin (DO) gauge of Stephens has been implemented in the ab initio vibronic coupling theory (VCT) of Nafie and Freedman for the calculation of vibrational circular dichroism (VCD) intensities. With this implementation, direct comparison is possible between results obtained with the DO gauge and the common origin (CO) gauge. In the VCT implementation, the VCT–DO and VCT–CO results are different even in the Hartree–Fock limit. The origin of the nonequivalence and the accuracy of VCD intensities by VCT–DO and VCT–CO when smaller basis sets are used are discussed. Detailed comparison is carried out for 13 chiral molecules. Experimental data are available for eight chiral oxiranes and thiiranes. A range of basis sets, of both the ‘‘conventional’’ and ‘‘polarized’’ varieties, is used for NHDT, H2O2, D2O2, DOOH, and oxaziridine. The results are compared with the results of Stephens’s magnetic-field perturbation theory wherever the data are available. Calculated VCD intensities are basis-set and gauge dependent. Improvement of calculated rotational strengths for most molecules studied here is observed by introducing the DO gauge to the VCT formulation.