Matrix-isolation IR and UV dpectra of SiaHg and SUHio: Isomers and conformers of oligosilanes

Matrix-isolation IR and UV spectra of Si3Hs, /-Si.iHio, and the two conformers of rt-Si.iHio have been recorded. A quantitative separation of the IR spectrum of n-Si-iHm 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 SijHg, j-Si.(Hio, and the two conformers of a-SJ4Hio have been assigned by comparison with results of ab initia calculations, which reproduce the frequencies and even the relative intensities quite well. The calculations predict dihedral angles <a of 180° and 57° for the anti and the gauche conformer of w-Si4Hio, 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-SLHio 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 (a. This result is interpreted in terms of data from 6-in-8 CASSCF 6-3IG calculations, which predict an avoided crossing between a strongly allowed ao B state and a very weakly allowed on B state as w 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.