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The first photoelectron band of difluorocarbene CF2, has been studied by threshold photoelectron (TPE) spectroscopy. CF2was prepared by microwave discharge of a flowing mixture of hexafluoropropene, C3F6, and argon. A vibrationally resolved band was observed in which at least twenty‐two components were observed. In the first PE band of CF2, the adiabatic ionization energy differs significantly from the vertical ionization energy because, for the ionization CF2+(X̃2A1)+e−← CF2(X̃1A1), there is an increase in the FCF bond angle (by ≈20°) and a decrease in the CF bond length (by ≈0.7 Å). The adiabatic component was not observed in the experimental TPE spectrum. However, on comparing this spectrum with an ab initio/Franck–Condon simulation of this band, using results from high‐level ab initio calculations, the structure associated with the vibrational components could be assigned. This led to alignment of the experimental TPE spectrum and the computed Franck–Condon envelope, and a determination of the first adiabatic ionization energy of CF2as (11.362±0.005) eV. From the assignment of the vibrational structure, values were obtained for the harmonic and fundamental frequencies of the symmetric stretching mode (ν1′) and symmetric bending mode (ν2′) in CF2+(X̃2A1).

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 (Te) 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 Te (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 &Angs; and N–C=1.150±0.008 &Angs;). © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1896–1906, 2001

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 initiomolecular 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 (Te) 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 initioforce 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 Te(36896 cm−1at the CASSCF/MRCI/cc‐pVTZ level) and the simulated emission spectra, the band system observed at 36389 cm−1by 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