Decomposition of anthranil. Single pulse shock-tube experiments, potential energy surfaces and multiwell transition-state calculations. The role of intersystem crossing.

The thermal decomposition of anthranil diluted in argon was studied behind reflected shock waves in a 2 in. i.d. pressurized driver single-pulse shock tube over the temperature range 825-1000 K and overall densities of approximately 3 x 10(-5) mol/cm(3). Two major products: aniline and cyclopentadiene carbonitrile (accompanied by carbon monoxide) and four minor products resulting from the decomposition were found in the postshock samples. They were, in order of decreasing abundance, pyridine, CH(2)=CHCN, HCN and CHC-CN, and comprised only a few percents of the overall product distribution. Quantum chemical calculations were carried out to determine the sequence of the unimolecular reactions that lead to the formation of cyclopentadiene carbonitrile and of phenylnitrene/phenylimine that are the precursors of aniline. They form aniline by reactions with traces of water impurities. To produce cyclopentadiene carbonitrile, two main processes must take place: CO elimination and ring contraction from a six- to a five-membered ring. It was shown that this can occur via two parallel pathways where CO elimination takes place prior to or following ring contraction. Singlet potential energy surfaces for all the elementary reactions that lead to the formation of cyclopentadiene carbonitrile and phenylnitrene/phenylimine were obtained. Their rate constants were calculated on the basis of the results of the quantum chemical calculations using transition-state theory. A kinetic scheme containing these reactions was constructed and multiwell calculations were performed to evaluate the mole percent of the products as a function of temperature. A very serious disagreement between the experimental results and the results of calculations showed that the singlet PESs could not account for the observed experimental rates. No other singlet PESs that lead to the formation of these products could be found. In view of this observation, attempts to find pathways that lead to the formation of cyclopentadiene carbonitrile and phenylnitrene/phenylimine on triplet surfaces were made. Such surfaces were found, and singlet <--> triplet intersystem crossing probabilities and crossing rate constants were calculated as well as the rate constants of all the elementary steps on the triplet surfaces. A reaction scheme was constructed and multiwell calculations were performed, including also the pathways on the singlet surfaces, to evaluate the mole percent of the products as a function of temperature. The agreement between the experimental results and these calculations was quite satisfactory.