A theoretical study of (A)over-tilde-2Π MgCN

We use the RENNER program system (see, for example, P. Jensen, G. Osmann, and P. R. Bunker, in "Computational Molecular Spectroscopy" (P. Jensen and P. R. Bunker, Eds.), Wiley, Chichester, 2000, and references therein) to make a detailed calculation of the rovibronic energies in the first excited electronic state, (A) over tilde (2)Pi, of the MgCN radical. This calculation is based on ab initio data computed at the MR-SDCI(+Q)/[TZ3P + f (Mg), aug-cc-pVQZ (N and C)] level of theory. We have also obtained the standard spectroscopic constants and have used these to calculate vibronic energies from perturbation expressions (J. T. Hougen and J. P. Jesson, J. Chem. Phys. 38, 1524-1525 (1963)); these results are compared to those computed with RENNER. It is shown that for MgCN, the perturbation theory is dilapidated due to Fermi resonances. No spectra involving the (A) over tilde (2)Pi electronic state of MgCN have been observed. To assist such observations, we have calculated Franck-Condon factors for some of the vibronic bands in the (A) over tilde (2)Pi<--(X) over tilde (2)Sigma(+) electronic transition. (C) 2002 Elsevier Science (USA).