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

Matrix-isolation IR and UV spectra of Si 3 H 8, i -Si 4 H 10, and the two conformers of n -Si 4 H 10 have been recorded. A quantitative separation of the IR spectrum of n -Si 4 H 10 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 Si 3 H 8, i -Si 4 H 10, and the two conformers of n -Si 4 H 10 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 -Si 4 H 10, 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 -Si 4 H 10 are not those anticipated from simple models of the Sandorfy or ladder C type, in that it is primarily not the energy but the intensity of the low-energy excited singlet states that depends strongly on the SiSiSiSi dihedral angle ω. This result is interpreted in terms of data from 6-in-8 CASSCF 6-31G* calculations, which predict an avoided crossing between a strongly allowed σσ* B state and a very weakly allowed σπ* B state as ω changes, with the former lower in energy at 180° and the latter lower at 0°. Consequences for attempts to understand the effects of conformation on optical spectra of polysilanes are noted.