JOURNAL OF RAMAN SPECTROSCOPY J. Raman Spectrosc. 2004; 35: 975–990 Published online 7 September 2004 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jrs.1245 Vibrational spectroscopic studies, conformations and ab initio calculations of n-propyltrichlorosilane V. Aleksa, 1,2 P. Klaeboe, 1∗ C. J. Nielsen, 1 G. A. Guirgis 3 and J. R. Durig 4 1 Department of Chemistry, University of Oslo, P.O. Box 1033, 0315 Oslo, Norway 2 Department of General Physics and Spectroscopy, Vilnius University, Vilnius 2734, Lithuania 3 Department of Chemistry and Biochemistry, College of Charleston, 66 George Street, Charleston, South Carolina 29424, USA 4 Department of Chemistry, University of Missouri–Kansas City, 5100 Rockhill Road, Kansas City, Missouri 64110-2499, USA Received 24 February 2004; Accepted 29 May 2004 Infrared spectra of n-propyltrichlorosilane (CH 3 CH 2 CH 2 SiCl 3 ) were obtained in the vapour, amorphous and crystalline solid phases in the range 4000 – 50 cm −1 . Additional spectra in argon and nitrogen matrices at 5 K were recorded before and after annealing to 20–36 K. Raman spectra of the compound as a liquid were recorded at various temperatures between 300 and 153 K and spectra of the amorphous and crystalline solids were obtained. The spectra revealed the existence of two conformers (anti and gauche) in the fluid phases and in the matrices. When the vapour was shock-frozen on a cold-finger at 78 K and subsequently annealed to 120–130 K, 10 Raman bands and eight IR bands of the liquid were reduced in intensity in the amorphous phase and vanished in the crystal. Frequent intensity variations were observed in the infrared spectra of argon and nitrogen matrices before and after annealing. These spectra revealed the existence of one conformer (anti) in the crystal. From intensity variations of four independent pairs of anti and gauche bands in the Raman spectra between 300 and 150 K, an average value 1H ◦ (gauche–anti) =4.6 ± 0.3 kJ mol −1 was obtained in the liquid. Annealing experiments on n-propyltrichlorosilane in the matrices demonstrate that the gauche bands vanish after annealing, revealing that the anti conformer has the lower energy. The spectra of both conformers were interpreted in detail. Ab initio and DFT calculations gave optimized geometries, infrared and Raman intensities and scaled vibrational wavenumbers for the anti and gauche conformers. The conformational energy difference derived was 7.6 and 6.4 kJ mol −1 for the HF/6–311G ∗ and B3LYP/6–311G ∗ basis sets, respectively, with anti being the low-energy conformer. Copyright  2004 John Wiley & Sons, Ltd. KEYWORDS: vibrational spectra; ab initio calculations; silanes; conformational analysis INTRODUCTION n-Propyltrichlorosilane (CH 3 CH 2 CH 2 SiCl 3 ) (PTCS) was pur- chased and an infrared and Raman spectroscopic study of this compound was carried out. As is apparent from Fig. 1, PTCS can form anti and gauche conformers owing to restricted rotation around the central C—C bond. Although named a silane, the CH 3 and SiCl 3 groups form substituents in an ethane. The potential functions in ethanes with a central C—C bond are different from those of the silanes with Si — C or Si — Si central bonds. The compound is related to n- butane 1,2 and to n-propyl fluoride, chloride and bromide. 3,4 In particular, PTCS is closely related to the symmetrically equivalent molecules n-propylsilane including the Si-d 3 iso- topomer ⊲CH 3 CH 2 CH 2 SiH 3 ⊳ 5 and to n-propyltrifluorosilane L Correspondence to: P. Klaeboe, Department of Chemistry, University of Oslo, P.O. Box 1033, 0315 Oslo, Norway. E-mail: peter.klaboe@kjemi.uio.no ⊲CH 3 CH 2 CH 2 SiF 3 ⊳; 6 both of these were recently studied. In n- propyl fluoride, chloride and bromide the gauche conformer had slightly lower enthalpy than the anti form, 3,4 but in n- butane, 1,2 n-propylsilane 5 and n-propyltrifluorosilane 6 anti was the more stable conformer and was also present in the crystals. It was decided to extend these conformational stud- ies to include PTCS, having bulky chlorine atoms at the end of the chain, which is expected to increase the 1,4 repulsion of the gauche conformer relative to n-propyltrifluorosilane. Various molecules containing a central C—Si bond have been reported, including: bromomethyldimethylsilane, 7 bromomethylmethylchlorosilane, 8 chloromethylmethyl difl- uorosilane, 9 chloromethylmethyldichlorosilane 10 and dic- hloromethylmethyldichlorosilane. 11 A number of disila- nes 12 – 14 with Si—Si and trisilanes 15,16 with Si—Si—Si link- ages have been synthesized by Hassler and co-workers and the vibrational spectra reported. In the latter silanes there Copyright  2004 John Wiley & Sons, Ltd.