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J. Am. Chem. SOC.1994, 116, 9719-9729 9719 Electronic Spectra of Ethynyl-and Vinylsilylenes: Experiment

Summary: J. Am. Chem. SOC.1994, 116, 9719-9729 9719
Electronic Spectra of Ethynyl- and Vinylsilylenes: Experiment
and Theory8
Yitzhak Apeloig,*vt Miriam Karni,? Robert West,'9# and Kevin Welsh*
Contributionfrom the Departments of Chemistry, Technion-Israel Institute of Technology,
32000 Haifa, Israel, and University of Wisconsin,Madison, Wisconsin53706
Received May 9, 1994"
Abstract: Electronic absorption spectra are reported for four ethynylsilylenes (A,,,, nm), MesSiC=CH (524),
MesSiC=CSiMe3 (545), MesSiC=CPh (500),and CH3SiC=CSiMe3 (473),generated in 3-methylpentane matrices
by photolysis of the corresponding trisilanes, RR'Si(SiMe3)2. Calculations of the `A' -lA" (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 goodagreement with experiment. Vinyl, ethynyl, and arylgroups alldecreasethetransition
energy of silylenes,compared with MezSi: (Le., 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
energiesof the twoelectronicstatessuggeststhat thebathochromic shift isdueto an excited-state stabilization (resulting
from interaction of the singly-occupied 3p(Si) orbital with the ?r* orbital on the substituent) which is larger than the
stabilization of the corresponding ground state.
Following the first direct observation of an organosilylene in
1979,' several silylenes have been studied in either argon or


Source: Apeloig, Yitzhak - Department of Chemistry, Technion, Israel Institute of Technology


Collections: Chemistry