Kinetics of free radicals generated by IR laser photolysis. IV. Intersystem crossings and reactions of C2(X 1Σ+g) and C2(a 3Πu) in the gaseous phase

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Abstract
Rate coefficients (300 K) for the removal of C2(X 1Σ+g) and C2(a 3Πu), hereafter referred to as 1C2 and 3C2 respectively, by H2, NO, and a number of hydrocarbons are reported as well as rate coefficients for intersystem crossing between 1C2 and 3C2 induced by collisions with N2, CO2, CF4, Ar, Kr, and Xe. C2 molecules are produced via ir photolysis of C2H3CN or C2HCl3, and their concentrations are monitored by laser induced fluorescence. We find that collisionally induced intersystem crossing is significant only when it is spin allowed or involves heavy collision partners (e.g., Kr, Xe). 1C2 reacts more rapidly with NO than does 3C2, and excited CN molecules in the A and B states are formed predominantly in reactions of 3C2. 1C2 reactions result (mainly) in ground state CN, as expected from adiabatic state correlations. Radiationless transitions between the X and B states of CN, induced by collisions with Ar, are observed. It is suggested that both 1C2 and 3C2 are removed by hydrocarbons mainly via chemical reactions, and 1C2 reacts more rapidly than 3C2 for every case that we have measured. With the 3C2+H2 reaction, molecular orbital correlation diagrams show that there are barriers in the entrance channel, thereby rationalizing the small rate coefficient for this reaction, compared to the allowed 1C2+H2 reaction. Similar correlations may explain the smaller reactivity of 3C2, compared to 1C2, toward hydrocarbons in general.