H+ClCN→HCl+CN: Product excitations and reaction mechanism at Ec.m. ≂21.6 kcal mol−1

Abstract

Nascent CN(X 2Σ) rovibrational and kinetic energy distributions have been obtained for the title reaction using laser induced fluorescence, and regions of the HClCN potential energy surface appropriate to the observed HCl+CN channel, as well as the HCN+Cl and HNC+Cl channels, have been calculated ab initio at the MP4/3-21G* level. The CN spectator has low internal excitation; the average fractions of the available energy deposited in CN vibration and rotation are 〈 fV(CN)〉 ∼0 and 〈 fR(CN)〉 =0.06, respectively, with the rotational distribution corresponding to a temperature of ∼950 K. Sub-Doppler resolution spectroscopy on several CN B 2Σ←X 2Σ transitions provides an average value for the fraction of the available energy appearing as center-of-mass kinetic energy, 〈 fT〉 =0.33, as well as approximate kinetic energy distributions for specific CN V,R levels. Since the kinetic energy distributions are for specific CN levels, the corresponding HCl internal energy distributions are obtained by energy conservation, and the experimental results indicate a vibrational population inversion in HCl. Product energy disposal is similar to that of many ‘‘light+heavy–heavy’’ systems which exhibit repulsive energy release and which prefer end-on rather than broadside entrance channels. The dominant reaction pathways are probably HCN+Cl and HNC+Cl, with HCl+CN being minor. Because of the competitiveness between the chemically distinct product channels, the entrance channel associated with the observed products is more stereospecifically constrained than for a similar reaction which has a single set of products. The ab initio calculations suggest that end-on encounters can be reactive, thus accounting for the observed channel. A local minimum exists for a three-center structure involving hydrogen, chlorine, and carbon, and the barrier from here to HCN+Cl is small. Broadside approaches give rise to HCN+Cl, and we therefore surmise that the observed products derive predominantly from encounters in which the H atom approaches the chlorine over a modest range of angles and impact parameters. Linear approaches at the nitrogen end of the molecule are unreactive at the energies of the present experiments. However, there is a cis transition state leading to HNC+Cl which is low enough to participate in the overall chemistry. Thus, the ab initio calculations are consistent with the experimental observations, and suggest that the title reaction is a minor channel, and that CN+HCl is produced by direct reaction via attack at the chlorine.