VINYLIDENE-ACETYLENE REARRANGEMENT - SELF-CONSISTENT ELECTRON PAIRS STUDY OF A MODEL UNIMOLECULAR REACTION

As a model for a potentially important pathway for the disappearance of carbenes and nitrenes, the unimolecular rearrangement of vinylidene into acetylene has been studied using the self-consistent electron pairs (SCEP) method. SCEP provides an accurate accounting of correlation effects by yielding a wave function equivalent to a configuration expansion including all singly and doubly substituted configurations relative to one reference determinant. A double-ς (DZ) basis set of 24 contracted Gaussian functions was used to determine the equilibrium structures of vinylidene and acetylene and the transition state for the rearrangement pathway. A larger basis set of 42 functions including polarization (DZ+P) functions was then used to determine the energetics of the reaction, and the difference in energy between vinylidene and acetylene was found to be 40 kcal/mol. Polarization functions and the inclusion of correlation effects contribute about equally to lowering the rearrangement barrier from the DZ-SCF result of 26 kcal/mol to 8.6 kcal/mol (DZ + PSCEP). Other electronic states of vinylidene were investigated, and the 3B2(5al→ 2b2) state was found to be the lowest excited state with a vertical excitation energy of 11 500 cm-1. Finally, SCEP calculations were performed on Li+-C2H2in an attempt to consider rearrangement through electrostatic interaction with a positive center. © 1978, American Chemical Society. All rights reserved.