# AN AB INITIO STUDY OF THE $\tilde{A} ^{2}\Pi$ STATE AND THE $\tilde{A} ^{2}\Pi\leftarrow\tilde{X} ^{2}\Sigma^{+}$ ELECTRONIC TRANSITION OF MgNC

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## Abstract

We have used the RENNER program $system^{b}$ to make a detailed calculation of the rovibronic energies in the first excited electronic state, $\tilde{A} ^{2}\Pi$, of the MgNC radical. This calculation was based on recent ab initio surfaces $(MRSDCI(+Q)/[TZ3P+f(Mg)$, aug-cc-$pVQZ(N and C)])^{c}$ for the Renner-degenerate electronic states. Previous calculations of vibronic $energies^{c}$ employing the same ab initio data in conjunction with perturbation $expressions^{d}$ suggested that an observed band belonging to the $\tilde{A} ^{2}\Pi\leftarrow\tilde{X} ^{2}\Sigma^{+}$ electronic $transition^{e}$ should be reassigned. The present work confirms this conclusion which is further substantiated by the rotational structures calculated in the vibronic states, and by Franck-Condon theory predicting relative intensities. $\tilde{A} ^{2}\Pi$ MgNC affords an example of the ``classic'' Renner $effect^{f}$ involving component electronic states with linear equilibrium geometries. We present detailed analyses of rovibronic wavefunctions aimed at providing further insight into the nature of the Renner interaction.

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