ELECTRONIC-SPECTRA OF ETHYNYLSILYLENES AND VINYLSILYLENES - EXPERIMENT AND THEORY

Electronic absorption spectra are reported for four ethynylsilylenes ( X(max), nm), MesSiC drop CH (524), MesSiC drop CSiMe(3) (545), MesSiC drop CPh (500), and CH3SiC drop CSiMe(3) (473), generated in 3-methylp entane matrices by photolysis of the corresponding trisilanes, RR'Si(S iMe(3))(2). Calculations of the (1)A' --> (1)A'' (n --> 3p) transition energies for these and for some vinyl- and phenylsilylenes, by ab ini tio methods including the use of spin-projection methods, provided res ults in good agreement with experiment. Vinyl, ethynyl, and aryl group s all decrease the transition energy of silylenes,compared with Me(2)S i: (i.e., they cause a red shift). Also when H in HSiMe is replaced by vinyl or phenyl groups, a bathochromic (red) shift results, but ethyn yl induces a small blue shift. The calculated stabilization energies o f the two electronic states suggests that the bathochromic shift is du e to an excited-state stabilization (resulting from interaction of the singly-occupied 3p(Si) orbital with the pi orbital on the substituen t) which is larger than the stabilization of the corresponding ground state.