The conformers of bromomethyl dimethyl chlorosilane studied by vibrational spectroscopy and ab initio methods  A. Nilsen a , P. Klaeboe a, * , C.J. Nielsen a , G.A. Guirgis b , V. Aleksa 1,a a Department of Chemistry, University of Oslo, P. O. Box 1033, 0315 Oslo, Norway b Bayer Corporation, Bushy Park Plant, Research and Development, P. O. Box 118088, Charleston, SC 29423-8088, USA Received 9 August 1999; accepted 21 September 1999 Abstract Bromomethyl dimethyl chlorosilane (CH 2 Br(CH 3 ) 2 SiCl) was synthesised and the infrared spectra of the vapour and of the amorphous and crystalline states at liquid nitrogen temperature were obtained. Additional spectra of the compound, isolated in argon, nitrogen and xenon matrices were recorded at 5 and 15 K. Raman spectra of the liquid were obtained at various temperatures between 295 and 173 K, and spectra of the amorphous and crystalline solids were recorded. The compound is present as anti and gauche conformers in the vapour and in the liquid states. Various infrared and Raman bands present in these phases vanished upon crystallisation. Raman temperature studies in the liquid gave a gauche – anti value of 1:0 ^ 0:3 kJ mol -1 ; anti was the low energy conformer and was also present in the crystal. The infrared bands diminishing in the argon, nitrogen and xenon matrix spectra after annealing to 28–60 K suggested that the anti conformer also had a slightly lower energy than gauche in all the matrices. The conformational barrier was estimated to be 8–10 kJ mol -1 . Ab initio calculations on different levels of approximation gave optimised geometries, infrared and Raman intensities and vibrational frequencies for the anti and gauche conformers. All calculations predicted anti as the low energy conformer. After scaling, a reasonably good agreement between the experimental and calculated wavenumbers for the two rotamers was obtained.  2000 Elsevier Science B.V. All rights reserved. Keywords: Silanes; Ab initio calculations; Conformations; Infrared; Raman, Matrix isolation spectroscopy 1. Introduction Bromomethyl dimethyl chlorosilane, CH 2 Br(CH 3 ) 2 SiCl, to be abbreviated as BDCS, was synthesised for the first time. The molecule can exists in two conformations, anti and gauche as shown in Fig. 1. The infrared and Raman spectra of BDCS were invest- igated among a series of five halomethyl dimethyl halo- silanes, CH 2 X(CH 3 ) 2 SiY (X  Cl, Br; Y  H, F, Cl) and these results have recently been reported. Various silanes with conformational equilibria containing one silicon atom have been studied [1–6] and molecules with two [7–9] and three [10–12] silicon atoms have recently been investigated by Hassler and coworkers and their vibrational spectra reported. The vapour, amorphous and crystalline samples of BDCS were recorded in the middle (MIR) and far infrared (FIR) regions. Infrared matrix isolation technique was employed to obtain spectra of the Journal of Molecular Structure 550–551 (2000) 199–215 0022-2860/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0022-2860(00)00387-2 www.elsevier.nl/locate/molstruc  Dedicated to Professor James R. Durig on the occasion of his 65th birthday. * Corresponding author. Tel.: + 47-22-855678; fax: + 47-22- 855441. E-mail address: peter.klaboe@kjemi.uio.no (P. Klaeboe). 1 Permanent address: Department of General Physics and Spec- troscopy, Vilnius University, Vilnius 2734, Lithuania.