Competitive reaction pathways of C2Cl3 + NO via four-membered ring and bicyclic ring intermediates

Abstract

The products and mechanisms of the atmospherically and environmentally important reaction, C₂Cl₃ + NO, are investigated comprehensively by step-scan time-resolved Fourier transform infrared emission spectroscopy and the CCSD(T)/6-311+G(d)//B3LYP/6-311G(d) level of electronic structure calculations. Vibrationally excited products of Cl₂CO, ClNCO, CCl₃NCO and NCO have been observed in the IR emission spectra. Cyclic intermediates are found to play important roles leading to the rich variety of the chemical transformations of the reaction. Mainly two competitive reaction pathways are revealed: the four-membered ring intermediate pathway leading to the products Cl₂CO + ClCN which is essentially barrierless and the bicyclic ring intermediate pathway leading to the product channels of ClNCO + CCl_2,CCl₃NCO and CCl₃ + NCO which is rate-limited by a barrier of 42.9 kJ mol⁻¹ higher than the reactants. By photolyzing the precursor at 248 and 193 nm, respectively, C₂Cl₃ radicals with different internal energy are produced to observe the product branching ratios as a function of reactant energy. The Cl₂CO channelvia the four-membered ring intermediate pathway is shown to be overwhelmingly dominant at low energy (temperature) but become less important at high energy while the ClNCO and CCl₃NCO channels via the bicyclic ring intermediate pathway are greatly enhanced and compete effectively. The experimental observation of the products and their branching ratios varying with reactant energy is well consistent with the calculated potential energy profiles.