B3LYP-DFT CHARACTERIZATION OF THE POTENTIAL-ENERGY SURFACE OF THE CH(X (2)PI)+C2H2 REACTION

The potential energy surface for the reaction of doubler methylidyne w ith acetylene has been investigated in detail using the B3LYP-DFT/6-31 G* quantum chemical method. Three barrierless entrance channels lead to the formation of initially highly excited C3H3 radicals, the most s table of which is the 2-propynyl radical (propargyl). Other C3H3 isome rs characterized include 1-propynyl, as well as the cyclic structures cycloprop-2-enyl and cycloprop-1-enyl which were net considered in a p revious theoretical study by Walch. All identified C3H3 isomers can in terconvert via transition states lying well below the entrance and exi t channels. The dissociation pathways of the C3H3 radicals leading to various C3H2 isomers+H have been identified. The energetically most fa vorable of these exit channels was found to be the formation of single t cyclopropenylidene+H. Other favored routes are formation of tripler prop-2-ynylidene+H and of singlet propadienylidene+H. Also identified are pathways leading to linear-C3H+H-2. The transition paths of all ba rrierless reactions were characterized by calculating a large number o f points along the reaction coordinate, allowing for a microvariationa l treatment of these reactions in later kinetic RRKM calculations. The kinetic and mechanistic aspects of the CH+C2H2 reaction are discussed qualitatively, based on the data obtained in this study and in the av ailable experimental and theoretical literature. (C) 1998 American Ins titute of Physics. [S0021-9606(98)00903-9].