4180
J . Phys. Chem. 1991,95, 4180-4182
Electronic Structure of a Stable Nucleophlllc Carbene David A. Dixon* and Anthony J. Arduengo III* DuPont Central Research and Development,?Experimental Station, Wilmington, Delaware 19880-0328 (Received: February 26, 1991)
The electronic and physical structure of a nucleophilic carbene in the imidazol-2-ylidene series was investigated by extended basis set ab initio SCF molecular orbital theory. Both singlet and triplet structures are considered. The calculated structure for the singlet ('A,) is in excellent agreement with that determined from a X-ray crystallographic study of 1,3-di-1adamantylimidazol-2-ylidene.Configuration mixing coefficients and orbital populations at the carbene centers in imidaz01-Zylidene and :CF2indicate similar carbene character for the two molecules. The proton affinity of imidazol-Zylidene is calculated and found to be one of the highest known for neutral organic molecules.
The electronic structure of carbenes has been of much interest to theoreticians and experimentalists.' One of the important original contributions of molecular orbital theory was the prediction of a bent triplet ground state for :CH2that was done at essentially the same time as the experimentalwork? Calculations then played an important role in unravelling the singlet-triplet splitting in :CH2.' The calculation of singlet-triplet splittings in substituted carbenes has been useful in providing insight into how the singlet is stabilized (or destabilized) relative to the triplet and also how the geometry of the two states is affected by substitution.c6 A general conclusion of this work is that the singlets have small bond angles near 100' at the carbene center and the triplets have significantly expanded bond angles from about 120 to 130'. The geometry of the singlet and triplet can be predicted from traditional RHF and ROHF calculations, but a reasonable prediction of the singlet-triplet splitting requires the use of a two-configuration wave function for the singlet. For :CH2, a ground-state triplet, this prescription yields 13.7 kcal/mol for the tripletsinglet splitting with a DZ Dc basis set with a standard d exponent. This can be compared to the best available value of 9.05 & 0.06 k ~ a l / m o l .Thus, ~ one can do a reasonable job of predicting the energy splitting at this level. We recently reported the X-ray crystallographic structure of the stable nucleophilic carbene 1 (1,3-di- l-adamantylimidazol2-ylidene).' The stability of carbene I is a result of both steric
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TABLE I: Molecular Ce0metrie.P l(upt) '2 lkcsCD 32(pl) 3~(.onpl) 137.0 135.3 135.2 140.9 141.9 r(C4-G) 133.8 133.9 135.0 133.2 132.7 138.2 138.8 139.7 140.1 141.8 (Nl(3)-%4)) e(N,42-N3) 102.2 101.4 101.9 106.7 112.8 O(Cqs)-N3{,)
