Crossed beam reaction of the cyano radical, CN(X 2Σ+), with methylacetylene, CH3CCH (X1A1):: Observation of cyanopropyne, CH3CCCN (X1A1), and cyanoallene, H2CCCHCN (X1A')

The chemical dynamics to form cyanopropyne, CH3CCCN (X (1)A(1)), and cyanoallene, H2CCCHCN (X (1)A(')), via the neutral-neutral reaction of the cyano radical, CN (X (2)Sigma(+)), with methylacetylene, CH3CCH (X (1)A(1)), is investigated under single collision conditions in a crossed molecular beam experiment at a collision energy of 24.7 kJ mol(-1). The laboratory angular distribution and time-of-flight spectra of the C4H3N products are recorded at m/e=65, 64, 63, and 62. The reaction of d(3)-methylacetylene, CD3CCH (X (1)A(1)), with CN radicals yields reactive scattering signal at m/e=68 and m/e=67 demonstrating that two distinct H(D) atom loss channels are open. Forward-convolution fitting of the laboratory data reveal that the reaction dynamics are indirect and governed by an initial attack of the CN radical to the pi electron density of the beta carbon atom of the methylacetylene molecule to form a long lived CH3CCHCN collision complex. The latter decomposes via two channels, i.e., H atom loss from the CH3 group to yield cyanoallene, and H atom loss from the acetylenic carbon atom to form cyanopropyne. The explicit identification of the CN vs H exchange channel and two distinct product isomers cyanoallene and cyanopropyne strongly suggests the title reaction as a potential route to form these isomers in dark molecular clouds, the outflow of dying carbon stars, hot molecular cores, as well as the atmosphere of hydrocarbon rich planets and satellites such as the Saturnian moon Titan. (C) 1999 American Institute of Physics. [S0021-9606(99)01831-0].