CALCULATION OF INFRARED AND VIBRATIONAL CIRCULAR-DICHROISM INTENSITIES VIA NUCLEAR ELECTROMAGNETIC SHIELDINGS

Infrared and vibrational circular dichroism intensities have been evaluated theoretically for trans-(1S,2S)-dideuteriooxirane, trans-(1S,2S)-dideuteriocyclopropane, (S)-propylene oxide, and trans-(1R,2R)-dimethyl-cyclopropane molecules, via electric and electromagnetic nuclear shielding tensors determined by random-phase approximation within the framework of 6-31g, 6-31g**, 6-31gext, and 6-31gpol Gaussian basis sets. Differentiation of matrix elements is avoided by the procedure adopted in this work. By comparing the results provided by several numerical tests it is found that reliable force fields of Hartree-Fock quality can be obtained at the 6-31g** and 6-31gex level; 6-31gpol basis provides the most accurate polar and axial atomic tensors. The nuclear shieldings are to be calculated with great accuracy in order to achieve a good prediction of the intensity patterns, as they bias the overall quality of theoretical oscillator strengths and rotational strengths. Comparison with available experimental data and previous ab initio calculations demonstrates the practicality of the present method for small and medium size molecules. The Hartree-Fock accuracy of the theoretical estimates of vibrational intensities is also proven by analyzing constraints for translational and rotational invariance which should be exactly fulfilled in the limit of complete basis sets. These results document the competitivity of the nuclear shielding method with the analytic derivative procedure.