MATRIX-ISOLATION IR AND UV SPECTRA OF SI3H8 AND SI4H10 - ISOMERS AND CONFORMERS OF OLIGOSILANES

Matrix-isolation IR and UV spectra of Si3H8, i-Si4H10, and the two con formers of n-Si4H10 have been recorded. A quantitative separation of t he IR spectrum of n-Si4H10 into contributions from the anti and gauche forms was accomplished by a combination of matrix annealing and selec tive monochromatic photodestruction experiments. A qualitative separat ion 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 co mparison with results of ab initio calculations, which reproduce the f requencies and even the relative intensities quite well. The calculati ons predict dihedral angles omega of 180 degrees and 57 degrees for th e anti and the gauche conformer of n-Si4H10, respectively, and confirm earlier predictions of nearly equal stability for an isolated molecul e. In the matrix, the anti conformer is more stable, The conformationa l effects on the UV spectrum of n-Si4H10 are not those anticipated fro m simple models of the Sandorfy or ladder C type, in that it is primar ily not the energy but the intensity of the low-energy excited singlet states that depends strongly on the SiSiSiSi dihedral angle omega. Th is result is interpreted in terms of data from 6-in-8 CASSCF 6-31G ca lculations, which predict an avoided crossing between a strongly allow ed sigma sigma B state and a very weakly allowed sigma pi* B state as omega changes, with the former lower in energy at 180 degrees and the latter lower at 0 degrees. Consequences for attempts to understand th e effects of conformation on optical spectra of polysilanes are noted.