Infrared spectra of dibromosilane species and quantum mechanical calculations of geometry, scaled force fields and electrical properties

Infrared data in the gas phase are reported for SiH2Br2, SiD2Br2 and SiHDBr 2. Quantum mechanical (QM) calculations of geometry, force field and electr ical properties have been carried out for dibromosilane and silane using th e 6-31G** basis set at SCF, MP2 and B3LYP levels. In the gas phase, the A, torsional mode is calculated to lie at 690 +/- 1 cm(-1). RMS amplitudes and centrifugal distortion constants are given for the three species studied. Atomic charges and dipole derivative vectors with respect to SIH and SiBr s tretching are reported. The latter lie off the bond directions by about 8 d egrees, either towards the bond of the same type (SiH) or away from it (SiB r). The effect of bromine substitution on the SiH bond is to shorten it and to diminish its IR stretching intensity. However, the SiH bending moment i s increased over its Value in silane, particularly for the motion in the Br -Br direction. As a result, the changes in the King effective and Mulliken atomic charges are in opposite directions. (C) 1999 Elsevier Science B.V. A ll rights reserved.