Pushing measurements and interpretation of VCD spectra in the IR, NIR and visible ranges to the detectability and computational complexity limits

Abstract

(R)-Limonene VCD and IR absorption spectra for neat liquid samples are considered from 900 to 16,000 cm^-1, using mostly data by Nafie et al. up to 10,000 cm^-1 and from previous investigations of the Brescia group. New VCD data are recorded in the merely overtone and combination region between 1800 and 2400 cm^-1 and for the Δn= 6 overtone CH-stretching region above 15,000 cm^-1. The GVPT2 anharmonic DFT calculations permit satisfactory interpretation of the fundamental + overtone/combination of deformation modes in the mid-IR up to 3500 cm^-1. The GVPT2 approach is also used for the first CH-stretching overtone regions together with their combination with deformation modes up to 9000 cm^-1. Then the local-mode approach developed within the DFT protocol is employed in all the CH-stretching regions (fundamental + overtones) and is found to satisfactorily account for the observed spectra, justifying the constant VCD pattern observed for all overtones. On the basis of the local-mode model the components of the bisignate VCD spectrum are attributed to the stretchings of the axial and equatorial CH bonds in α-position with respect to the ring CC double bond.

Keywords: Anharmonicity; Generalized Vibrational Perturbative Theory at the Second Order (GVPT2); Limonene; Local-mode approach; Near Infrared (NIR); Vibrational Circular Dichroism (VCD).