A b i n i t i o study of the electronic magnetic circular dichroism spectrum in acetylene: The B̃←X̃ and 1 1B2←X̃ transitions

Abstract

This study utilizes a newly implemented method based on first-order perturbation theory for calculating the electronic magnetic circular dichroism (MCD) spectra in molecules. As an initial application, the MCD band maxima have been calculated for the B̃(1 1Bu)←X̃(1 1Ag) and 1 1B2←X̃(1 1A1) electronic transitions in the trans and cis bent conformations (respectively) of acetylene. The band intensity is assumed to come entirely from the ℬ0 term in the MCD equations of Stephens, which explicitly includes a first-order perturbation correction to the two electronic states involved in the transition. The wave functions are determined using ab initio quantum chemical techniques including state averaged CASSCF and multireference CI. There has been speculation that the 1 1B2←X̃ band system might overlap the B̃←X̃, and be part of the reason for the diffuse nature of the spectrum in the 185–170 nm region. This study considers this claim. The current calculations predict MCD band maxima for the 0–0 and 1–0 vibrational bands in the B̃←X̃ to be Δεmax0=−3.48 and −5.82, respectively, while experiment gives −0.8 and −1.6. This is 103 times larger than the largest band maximum [Δεmax0(3–0)=−0.0037] calculated for the 1 1B2←X̃ transition. This study also finds the absorption oscillator strengths for the B̃←X̃ to be a factor of 60 larger than that found in the 1 1B2←X̃. Thus while these results do predict both the B̃←X̃ and 1 1B2←X̃ transitions to lie in the same spectral region, they do not support the hypothesis that the 1 1B2←X̃ is a major contributor to either the absorption or MCD intensity in the 185–170 nm region.