Vibrational spectra of disilenes. Normal coordinate analysis and identification of the nu(Si=Si) stretching vibration

Experimental vibrational spectra are reported for three carbon-substituted disilenes along with data on normal coordinate analysis of several simplified disilene models. Calculations show that there is no normal mode in the disilene spectrum which is well-localized in the Si=Si bond. The Si=Si and Si-C stretching coordinates are heavily mixed. Their in-phase combination results in a normal mode with frequency in the region 450-550cm(-1) which manifests itself as a Raman line of variable intensity (the so-called nu(Si=Si)). Their out-of-phase combination gives a mode in the region of about 700 cm(-1) which shows itself as an intense Raman line (the so-called nu(s)(Si-C)). Vibrations of an aromatic ring, if present in a disilene molecule, can also couple with the nu(Si=Si). Particular eigenvector elements of the two modes in question and thus their potential energy distributions strongly depend on the structure and symmetry of a given disilene molecule. However, the contribution of the Si=Si stretching coordinate to the normal coordinate with frequency in the region 450-550 cm(-1) is always significant, justifying the designation of this mode as the nu(Si=Si). (C) 1997 Elsevier Science S.A.