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 good agreement with experiment. Vinyl, ethynyl, and aryl groups all decrease the transition 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 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 ?r* orbital on the substituent) which is larger than the stabilization of the corresponding ground state.

Derivatization of /3-CD blocks the formation of -bonded dimer; presumably this occurs because two derivatized /3-CDs with TCNQ" included cannot approach each other closely enough for a -bond to form. By use of the Van't Hoff equation and the data from Figure 4, an approximate value for the AH of reaction (3) was determined to be -100 kJ.Acknowledgment. M.P.E. acknowledges the support of the Office of Organized Research at Northern Arizona University, NSF through the REU program, NIH through its MBRS grant to Northern Arizona University, and Mr.Rick Strattan of Pharmatec, Inc. for a gift of Molecusol.

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