Abstract
Geometry optimization and harmonic vibrational frequencies calculations were carried out on the \documentclass{article}\pagestyle{empty}\begin{document}$\tilde{\hbox{A}},{^{1}}\Pi$\end{document} and \documentclass{article}\pagestyle{empty}\begin{document}$\tilde{\hbox{X}},{^{1}}\Sigma^{+}$\end{document} 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 CCSD(T) and CASSCF/MRCI levels were performed to determine the transition energy (T~e~) between the two electronic states. Franck–Condon calculations were carried out for the \documentclass{article}\pagestyle{empty}\begin{document}$\tilde{\hbox{A}},{^{1}}\Pi$\end{document}–\documentclass{article}\pagestyle{empty}\begin{document}$\tilde{\hbox{X}},{^{1}}\Sigma^{+}$\end{document} electronic transition, employing ab initio force constants and optimized geometries. The \documentclass{article}\pagestyle{empty}\begin{document}$\tilde{\hbox{A}},{^{1}}\Pi\rightarrow \tilde{\hbox{X}},{^{1}}\Sigma^{+}$\end{document} dispersed fluorescence spectra arising from the (0,0,0), (0,0,1), and (0,0,2) single 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 \documentclass{article}\pagestyle{empty}\begin{document}$\tilde{\hbox{A}},{^{1}}\Pi$\end{document}–\documentclass{article}\pagestyle{empty}\begin{document}$\tilde{\hbox{X}},{^{1}}\Sigma^{+}$\end{document} transition. A systematic variation of the \documentclass{article}\pagestyle{empty}\begin{document}$\tilde{\hbox{A}},{^{1}}\Pi$\end{document} state geometrical parameters was carried out in an iterative Franck–Condon analysis (IFCA) treatment of the \documentclass{article}\pagestyle{empty}\begin{document}$\tilde{\hbox{A}},{^{1}}\Pi\rightarrow\tilde{\hbox{X}},{^{1}}\Sigma^{+}$\end{document} 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 \documentclass{article}\pagestyle{empty}\begin{document}$\tilde{\hbox{A}},{^{1}}\Pi$\end{document} state (Al–N=1.785±0.005 Å and N–C=1.150±0.008 Å). © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1896–1906, 2001