Ab initio calculations on the (A)over-tilde(1)Pi and (X)over-tilde(1)Sigma(+) states of AlNC and simulation of the AlNC (A)over-tilde (1)Pi-(X)over-tilde(1)Sigma(+) emission spectra

Geometry optimization and harmonic vibrational frequencies calculations wer e carried out on the (A) over tilde (1)Pi and (X) over tilde states of AlNC , employing a variety of ab initio molecular orbital methods, including the SERHF, CIS, MP2, and QCISD methods, with basis sets up to the size of the cc-pVQZ basis set. In addition, single-point energy calculations at the CCS D(T) and CASSCF/MRCI levels were performed to determine the transition ener gy (T,) between the two electronic states. Franck-Condon calculations were carried out for the (A) over tilde (1)Pi-<(<chi>)over tilde>(1)Sigma (+) el ectronic transition, employing ab initio force constants and optimized geom etries. The (A) over tilde (1)Pi --> (X) over tilde (1)Sigma (+) dispersed fluorescence spectra arising from the (0, 0, 0), (0, 0, 1), and (0, 0, 2) s ingle vibrational levels (SVL) of the upper state were simulated using the computed Franck-Condon factors. Based on the computed T-e (36896 cm(-1) at the CASSCF/MRCI/cc-pVTZ level) and the simulated emission spectra, the band system observed at 36389 cm(-1) by Gerasimov et al. [ J Chem Phys 110, 220 (1999)] has been assigned to the AlNC (A) over tilde (1)Pi-(X) over tilde (1)Sigma (+) transition. A systematic variation of the (A) over tilde (1)Pi state geometrical parameters was carried out in an iterative Franck-Condon analysis (IFCA) treatment of the (A) over tilde (1)Pi --> (X) over tilde ( 1)Sigma +SVL emission, with the geometry of the ground state being fixed to the available experimental geometry. The best match between the simulated and observed spectra gave the first experimentally derived geometry of the (A) over tilde (1)Pi state (Al-N = 1.785 +/- 0.005 Angstrom and N-C = 1.150 +/- 0.008 Angstrom). (C) 2001 John Wiley & Sons, Inc.