Volume 147. number 1 CHEMICAL PHYSICSLETTERS 27 May 1988 A PRELIMINARY THEORETICAL STUDY OF THE SiC,H RADICAL: IMPLICATIONS IN ASTROPHYSICS A. LARGO-CABRERIZO ’ zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA University Chemical Laboratory, Canterbury, Kent CT2 7NH, UK and J.R. FLORES Departamento de Quimira-Fisica, Fact&ad de Ciencias, 4 7005 Valladolid, Spain Received25 January 1988;in final form 14 March 1988 An ab initio study of the low-lying states of SiC2H has been carried out at the unrestricted Hartree-Fock level. A linear SiCCH species (‘II) is found to be the ground state, with a cyclic *A’ state lying about 4.5 kcal/mol higher at correlated levels. A *A (C, symmetry) state is found to be the lowest-lying state of HSiCC. In view of the predicted rotational constants and dipole moments, the possibility of any of these SiC2H species being the new radical recently detected in space is discussed. 1. Introduction Quite recently Guelin et al. [ 1 ] reported the dis- covery of a new free radical in the envelope of the carbon star IRC+ 102 16. The three doublets they observed indicate a linear molecule with a ‘C ground state (or a slightly asymmetric molecule). The ro- tational constant (B or f (B+ C) ) was calculated to be 5966.8 MHz and the symmetry parameter, de- fined as lb,1 = (C-B)/(2A-B-C), smaller than 3x 10-4. Among the potential molecules which could be carriers of the three doublets they mentioned HSiCC, SiCCH, SiCN, and ClCC. Based on abundance cri- teria (the envelope of IRC + 102 16 is the strongest source of Sic, in the sky and sulfur and chlorine compounds are much less abundant) they proposed HSiCC as the most likely candidate. Since very little is known about the structure of these molecules, a conclusive assignment is difficult. In this paper we present an ab initio study of the low-lying states of Si&H. The aim of this work is not I Permanent address: Departamento de Quimica-Fisica, Facul- tad de Ciencias, 47005 Valladolid, Spain. only to aid in the assignment of radioastronomical lines, but also to gain some insight into the molec- ular structure of silicon-carbon compounds. 2. Theoretical approach The calculations were performed with the GAUS- SIAN 82 program package [ 2 1. Analytical gradient techniques [ 31 were employed to determine the ge- ometry of the various electronic states at the unre- stricted Hartree-Fock (UHF) level. We used the 6- 3 lG** basis set [ 4,5] in these calculations. We also performed restricted open-shell Hartree-Fock (RHF) [ 61 optimizations with the 3-2 1 G* basis set [ 71 for one state where high spin contamination was found. The optimization technique of Fletcher and Powell [ 81 was used in this case. Electron correlation effects were incorporated through fourth order in Msller-Plesset perturbation theory [ 9, lo], and harmonic vibrational frequencies were computed at the UHF/6-3 1 G** level of theory. 90 0 009-2614/88/$ 03.50 0 Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )