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-methylpentane matrices by photolysis of the corresponding trisilanes, RR'Si(SiMe(3))(2). Calculations of the (1)A' --> (1)A'' (n --> 3p) transition energies for these and for some vinyl- and phenylsilylenes, by ab initio methods including the use of spin-projection methods, provided results in good agreement with experiment. Vinyl, ethynyl, and aryl groups all decrease the transition energy of silylenes,compared with Me(2)Si: (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 ethynyl induces a small blue shift. The calculated stabilization energies of the two electronic states suggests that the bathochromic shift is due to an excited-state stabilization (resulting from interaction of the singly-occupied 3p(Si) orbital with the pi* orbital on the substituent) which is larger than the stabilization of the corresponding ground state.