Crossed beam reaction of cyano radicals with hydrocarbon molecules.: I.: Chemical dynamics of cyanobenzene (C6H5CN; X 1A1) and perdeutero cyanobenzene (C6D5CN; X 1A1) formation from reaction of CN(X 2Σ+) with benzene C6H6(X 1A1g), and d6-benzene C6D6(X 1A1g)

The chemical reaction dynamics to form cyanobenzene C6H5CN(X (1)A(1)), and perdeutero cyanobenzene C6D5CN(X (1)A(1)) via the neutral-neutral reaction of the cyano radical CN(X (2)Sigma(+)), with benzene C6H6(X (1)A(1g)) and perdeutero benzene C6D6(X (1)A(1g)), were investigated in crossed molecular beam experiments at collision energies between 19.5 and 34.4 kJ mol(-1). The laboratory angular distributions and time-of-flight spectra of the products were recorded at mass to charge ratios m/e = 103-98 and 108-98, respectively. Forward-convolution fitting of our experimental data together with electronic structure calculations (B3LYP/6-311+G**) indicate that the reaction is without entrance barrier and governed by an initial attack of the CN radical on the carbon side to the aromatic pi electron density of the benzene molecule to form a C-s symmetric C6H6CN(C6D6CN) complex. At all collision energies, the center-of-mass angular distributions are forward-backward symmetric and peak at pi/2. This shape documents that the decomposing intermediate has a lifetime longer than its rotational period. The H/D atom is emitted almost perpendicular to the C6H5CN plane, giving preferentially sideways scattering. This experimental finding can be rationalized in light of the electronic structure calculations depicting a H-C-C angle of 101.2 degrees in the exit transition state. The latter is found to be tight and located about 32.8 kJ mol(-1) above the products. Our experimentally determined reaction exothermicity of 80-95 kJ mol(-1) is in good agreement with the theoretically calculated one of 94.6 kJ mol(-1). Neither the C6H6CN adduct nor the stable iso cyanobenzene isomer C6H5NC were found to contribute to the scattering signal. The experimental identification of cyanobenzene gives a strong background for the title reaction to be included with more confidence in reaction networks modeling the chemistry in dark, molecular clouds, outflow of dying carbon stars, hot molecular cores, as well as the atmosphere of hydrocarbon rich planets and satellites such as Saturn's moon Titan. This reaction might further present a barrierless route to the formation of heteropolycyclic aromatic hydrocarbons via cyanobenzene in these extraterrestrial environments as well as hydrocarbon rich flames. (C) 1999 American Institute of Physics. [S0021-9606(99)00940-X].