Silacyclopropenylidene and Its Most Important SiC 2 H 2 Isomers † Qunyan Wu, ‡,§ Andrew C. Simmonett, § Yukio Yamaguchi, § Qianshu Li, ‡,⊥ and Henry F. Schaefer III* ,§ Institute of Chemical Physics, Beijing Institute of Technology, Beijing, People’s Republic of China 100081, Center for Computational Quantum Chemistry, UniVersity of Georgia, Athens, Georgia 30602, and Center for Computational Quantum Chemistry, South China Normal UniVersity, Guangzhou, People’s Republic of China 510631 ReceiVed: June 23, 2009; ReVised Manuscript ReceiVed: August 18, 2009 In pioneering matrix isolation experiments, Maier and Reisenauer reported and identified infrared spectra for 1-silacyclopropenylidene and three of its isomers. These experiments were followed by microwave spectroscopy identification of two of the structures. Seven low-lying isomers of SiC 2 H 2 are systematically investigated here using ab initio restricted Hartree-Fock (RHF), coupled cluster with single and double excitations (CCSD), and CCSD with perturbative triple excitations [CCSD(T)] methods and the correlation-consistent polarized valence cc-pVXZ/cc-pV(X+d)Z (X ) D, T, and Q) basis sets. Full valence complete active space self- consistent field (CASSCF) wave functions are used for interpretation. All levels of theory employed in this study confirm the global minimum of the SiC 2 H 2 potential energy surface (PES) to be 1-silacyclopropenylidene (1S). Among the seven singlet stationary points, six isomers are found to be minima, whereas one structure (6S) is a second-order saddle point. For the lowest-lying six singlet minima, with our highest level of theory, cc-pVQZ(C,H)/cc-pV(Q+d)Z(Si) CCSD(T), the relative energies (in kcal mol -1 , with the zero-point vibrational- energy-corrected values in parentheses) are predicted to be 1-silacyclopropenylidene (1S) [0.0 (0.0)] < vinylidenesilanediyl (3S) [18.2 (17.6)] < ethynylsilanediyl (2S) [21.9 (18.6)] < ethynylsilylene (7S) [42.1 (38.7)] < silacyclopropyne (4S) [49.9 (46.1)] < silavinylidenecarbene (5S) [56.7 (52.1)]. Theoretically determined harmonic vibrational frequencies and associated infrared (IR) intensities for four isomers (1S-4S) are in good agreement with available experimental observations. 1. Introduction The singlet state of cyclopropenylidene (1S) is known to be the global minimum on the C 3 H 2 potential energy surfaces (PESs). Isomer 1S is abundant in molecular clouds of interstellar space and plays an important role in the chemistry of the interstellar medium. The first laboratory detection of 1S was achieved by Reisenauer, Maier, Riemann, and Hoffmann in 1984. 1 This was enabled by the ab initio prediction of its vibrational frequencies and infrared (IR) intensities provided by Lee, Bunge, and Schaefer. 2 Shortly thereafter, Reisenauer and Maier were able to show that upon irradiation, 1S is photoisomerized into propynylidene (propargylene, 2T) and in a second photostep into propadienylidene (vinylidenecarbene, 3S), as presented in Scheme 1. 3,4 1S and 3S have also been detected in interstellar space, 5,6 where 1S appears to be the most abundant of all hydrocarbons. Maier and Reisenauer achieved another breakthrough in their research on the corresponding silicon-containing derivatives. In 1994 and 1995, they reported matrix spectroscopic identification of four SiC 2 H 2 molecules. 7-9 The global minimum of the SiC 2 H 2 molecules was shown to be 1-silacyclopropenylidene (1S). 10 1S has been generated by pulsed flash pyrolysis of 2-ethynyl-1,1,1- trimethyldisilane. By subsequent photolysis, 1S can be isomer- ized into ethynylsilanediyl (2S), vinylidenesilanediyl (3S), and silacyclopropyne (4S), as shown in Scheme 2. The identification of the SiC 2 H 2 isomers and their 13 C and D isotopologues was based on the comparison of their experimental and theoretical infrared (IR) spectra. 7-12 Maier et al. also reported that isomer † Part of the “Barbara J. Garrison Festschrift”. * To whom correspondence should be addressed. ‡ Beijing Institute of Technology. § University of Georgia. ⊥ South China Normal University. SCHEME 1: For C 3 H 2 SCHEME 2: For SiC 2 H 2 J. Phys. Chem. C 2010, 114, 5447–5457 5447 10.1021/jp905898j  2010 American Chemical Society Published on Web 12/11/2009