VIBRATIONAL-SPECTRA OF DISILENES - NORMAL-COORDINATE ANALYSIS AND IDENTIFICATION OF THE NU(SI=SI) STRETCHING VIBRATION

Experimental vibrational spectra are reported for three carbon-substit uted disilenes along with data on normal coordinate analysis of severa l simplified disilene models. Calculations show that there is no norma l mode in the disilene spectrum which is well-localized in the Si=Si b ond. The Si=Si and Si-C stretching coordinates are heavily mixed. Thei r in-phase combination results in a normal mode with frequency in the region 450-550cm(-1) which manifests itself as a Raman line of variabl e 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 a n intense Raman line (the so-called nu(s)(Si-C)). Vibrations of an aro matic ring, if present in a disilene molecule, can also couple with th e nu(Si=Si). Particular eigenvector elements of the two modes in quest ion and thus their potential energy distributions strongly depend on t he structure and symmetry of a given disilene molecule. However, the c ontribution of the Si=Si stretching coordinate to the normal coordinat e with frequency in the region 450-550 cm(-1) is always significant, j ustifying the designation of this mode as the nu(Si=Si). (C) 1997 Else vier Science S.A.