The various isomers including stable structures, carbenes, and diradicals on the C,H, surface have been investigated. The two carbenes propenylidene and cyclopropylidene have been found to have singlet ground states. Vinylmethylene is predicted to have a triplet ground state with a planar diradical type of structure. The syn and anti forms of this state are degenerate. This is in agreement with the observation of two triplet states in the electron spin resonance (ESR) spectra. The r electrons are found to be delocalized over the three carbons. The singlet diradical structures are found to be more stable than the carbene structures, which retain the CH,=CH allylic structures. The orbital compositions of the frontier orbitals of all systems have been determined to examine the nature of these orbitals. 0 1996 John Wiley & Sons, Inc. * To whom correspondence should be addressed.
Various observations [3-61 suggest that a carbene or 1,3 diradical may be involved as an intermediate. The reverse reaction, i.e., ring closure of vinylcarbene to cyclopropene, has also been observed [7,8]. Many experiments and theoretical calculations have been performed to investigate the nature of the intermediate [9-171. In the present work a systematic study of the C,H, surface has been undertaken. In this article, the structures, relative energies, and other properties are reported.
Method of Calculation
The Modified Neglect of Diatomic Overlap (MNDO) method [18] with standard parameters has been used for all calculations. Open-shell systems were treated by the half-electron method.