Crossed beam reaction of cyano radicals with hydrocarbon molecules. I. Chemical dynamics of cyanobenzene (C6H5CN; X (1)A(1)) and perdeutero cyanobenzene (C6D5CN; X (1)A(1)) formation from reaction of CN(X (2)Sigma(+)) with benzene C6H6(X (1)A(1g)), and d(6)-benzene C6D6(X (1)A(1g))

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 p erdeutero 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). Th e laboratory angular distributions and time-of-flight spectra of the produc ts were recorded at mass to charge ratios m/e = 103-98 and 108-98, respecti vely. Forward-convolution fitting of our experimental data together with el ectronic structure calculations (B3LYP/6-311+G**) indicate that the reactio n is without entrance barrier and governed by an initial attack of the CN r adical on the carbon side to the aromatic pi electron density of the benzen e molecule to form a C-s symmetric C6H6CN(C6D6CN) complex. At all collision energies, the center-of-mass angular distributions are forward-backward sy mmetric and peak at pi/2. This shape documents that the decomposing interme diate has a lifetime longer than its rotational period. The H/D atom is emi tted almost perpendicular to the C6H5CN plane, giving preferentially sidewa ys scattering. This experimental finding can be rationalized in light of th e electronic structure calculations depicting a H-C-C angle of 101.2 degree s 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 re action exothermicity of 80-95 kJ mol(-1) is in good agreement with the theo retically 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 s cattering signal. The experimental identification of cyanobenzene gives a s trong background for the title reaction to be included with more confidence in reaction networks modeling the chemistry in dark, molecular clouds, out flow of dying carbon stars, hot molecular cores, as well as the atmosphere of hydrocarbon rich planets and satellites such as Saturn's moon Titan. Thi s reaction might further present a barrierless route to the formation of he teropolycyclic aromatic hydrocarbons via cyanobenzene in these extraterrest rial environments as well as hydrocarbon rich flames. (C) 1999 American Ins titute of Physics. [S0021-9606(99)00940-X].