Crossed beam reaction of cyano radicals with hydrocarbon molecules. II. Chemical dynamics of 1-cyano-1-methylallene (CNCH3CCCH2; X (1)A(')) formationfrom reaction of CN(X (2)Sigma(+)) with dimethylacetylene CH3CCCH3 (X (1)A(1)('))

The reaction dynamics to form the 1-cyano-1-methylallene isomer CNCH3CCCH2 in its (1)A' ground state via the radical-closed shell reaction of the cyan o radical CN(X (2)Sigma(+)) with dimethylacetylene CH3CCCH3 (X (1)A(1)') ar e unraveled in a crossed molecular beam experiment at a collision energy of 20.8 kJ mol(-1) together with state-of-the-art electronic structure and Ri ce-Ramsperger-Kassel-Marcus (RRKM) calculations. Forward convolution fittin g of the laboratory angular distribution together with the time-of-flight s pectra verify that the reaction is indirect and proceeds by addition of the CN radical to the pi orbital to form a cis/trans CH3CNC=CCH3 radical inter mediate. This decomposes via a rather lose exit transition state located on ly 6-7 kJ mol(-1) above the products to CNCH3CCCH2 and atomic hydrogen. The best fit of the center-of-mass angular distribution is forward-backward sy mmetric and peaks at pi/2 documenting that the fragmenting intermediate hol ds a lifetime longer than its rotational period. Further, the hydrogen atom leaves almost perpendicular to the C5H5N plane resulting in sideways scatt ering. This finding, together with low frequency bending and wagging modes, strongly support our electronic structure calculations showing a H-C-C ang le of about 106.5 degrees in the exit transition state. The experimentally determined reaction exothermicity of 90 +/- 20 kJ mol(-1) is consistent wit h the theoretical value, 80.4 kJ mol(-1). Unfavorable kinematics prevent us from observing the CN versus CH3 exchange channel, even though our RRKM ca lculations suggest that this pathway is more important. Since the title rea ction is barrierless and exothermic, and the exit transition state is well below the energy of the reactants, this process might be involved in the fo rmation of unsaturated nitriles even in the coldest interstellar environmen ts such as dark, molecular clouds and the saturnian satellite Titan. (C) 19 99 American Institute of Physics. [S0021-9606(99)01040-5].