Crossed beam reaction of the cyano radical, CN(X (2)Sigma(+)), with methylacetylene, CH3CCH (X(1)A(1)): Observation of cyanopropyne, CH3CCCN (X(1)A(1)), and cyanoallene, H2CCCHCN (X(1)A')

The chemical dynamics to form cyanopropyne, CH3CCCN (X (1)A(1)), and cyanoa llene, 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 i nvestigated under single collision conditions in a crossed molecular beam e xperiment 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. Fo rward-convolution fitting of the laboratory data reveal that the reaction d ynamics 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 mol ecule to form a long lived CH3CCHCN collision complex. The latter decompose s via two channels, i.e., H atom loss from the CH3 group to yield cyanoalle ne, and H atom loss from the acetylenic carbon atom to form cyanopropyne. T he explicit identification of the CN vs H exchange channel and two distinct product isomers cyanoallene and cyanopropyne strongly suggests the title r eaction 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 at mosphere of hydrocarbon rich planets and satellites such as the Saturnian m oon Titan. (C) 1999 American Institute of Physics. [S0021-9606(99)01831-0].