Donor-acceptor interaction and the peculiar structures of dications.

vibrational spectrum has recently been questioned.22 The vibrational frequencies of 3BI and 'B, dimethylsilylene are listed in Table V. One interesting feature of these is the intensity pattern for the two modes occurring near 1400 cm-' which, by analogy with 'Al, should be observed experimentally near 1200 cm-'. In 'Al these were both observable with moderate to strong intensity; for 3Bl they are predicted to both be very weak, and for IB, only the lower mode has appreciable intensity. Another striking feature is that the out-of-plane C H 3 rocking modes vlo and v I 4 have been shifted up several hundred wavenumbers. Finally, we note that the symmetric and asymmetric Si-C stretches, v6 and v 2 ' , are now split by about 100 cm-' instead of being nearly degenerate. We hope these data will be of use to experimentalists. Finally, to determine whether the species observed by Griller and c o-~ o r k e r s ~ '. ~ ~ could possibly be the triplet state of di-methylsilylene, we have determined CISD-Q energies of various triplet states at the 3Bl optimized geometry with the DZ+d basis set. Specifically, we have examined the 3A2, 3A,, and 'B2 states which arise from the configurations 5b28a:3bl (3A2), 5b:8a19al (3A1), and 5bi8a16b2 (3Bl). These states are analogous to the lowest energy triplet states of SiH, found by Rice and Handy.I33 kcal/mol. Thus we predict that excitations from 3B, to these higher lying triplets lie approximately 15-20 kcal/mol above the highest energy transitions observed by Griller (300 nm = 95.3 kcal/mol). While this suggests that the species observed by Griller is not 3B1 dimethylsilylene, it is possible that larger basis sets and more highly correlated wave functions could reduce the splitting ~ignificant1y.l~ Of course, the minima on the excited triplet state surfaces will be below the vertical excitation energies given above. Our results, in this regard, are therefore inconclusive. Conclusions The first excited 3B, and 'B, states of dimethylsilylene are predicted to lie 25 and 54 kcal/mol above the ground state, respectively. We find a vertical excitation energy of 63 kcal/mol for the 'AI-'BI transition. This agrees very well with the spec-troscopic observations of West, Michl, and co-workers who find the absorption maximum at 450 nm (62.7 kcal/mol = 456 nm). Thus we concur that the species observed by these researchers is due to ground-state dimethylsilylene and that the recent objections of …