Matrix-Isolation IR and UV Spectra of Si3H8 and Si4H10: Isomers and Conformers of Oligosilanes

Abstract

Matrix-isolation IR and UV spectra of Si3H8, i-Si4H10, and the two conformers of n-Si4H10 have been recorded. A quantitative separation of the IR spectrum of n-Si4H10 into contributions from the anti and gauche forms was accomplished by a combination of matrix annealing and selective monochromatic photodestruction experiments. A qualitative separation of their UV spectra was achieved as well. The IR spectra of Si3H8, i-Si4H10, and the two conformers of n-Si4H10 have been assigned by comparison with results of ab initio calculations, which reproduce the frequencies and even the relative intensities quite well. The calculations predict dihedral angles ω of 180° and 57° for the anti and the gauche conformer of n-Si4H10, respectively, and confirm earlier predictions of nearly equal stability for an isolated molecule. In the matrix, the anti conformer is more stable. The conformational effects on the UV spectrum of n-Si4H10 are not those anticipated from simple models of the Sandorfy or ladder C type, in that ...