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Time-Resolved Studies of the Reactions of CN Radical Complexes with Alkanes, Alcohols, and Chloroalkanes

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journal contribution
posted on 23.04.2009 by Andrew C. Crowther, Stacey L. Carrier, Thomas J. Preston, F. Fleming Crim
Ultrafast transient absorption experiments monitor the reaction of CN radicals with 16 different alkane, alcohol, and chloroalkane solutes in CH2Cl2 and with a smaller number of representative solutes in CHCl3 and CH3CCl3. In these experiments, 267-nm photolysis generates CN radicals, and transient electronic absorption at 400 nm probes their time evolution. A crucial feature of the reactions of CN radicals is their rapid formation of two different types of complexes with the solvent that have different stabilities and reactivities. The signature of the formation of these complexes is the CN transient absorption disappearing more slowly than the infrared transient absorption of the HCN product appears. Studying both the growth of HCN and the decay of the CN−solvent complexes in the reaction of CN with pentane in CH2Cl2 and CHCl3 solutions provides the information needed to build a kinetic model that accounts for the reaction of both complexes. This model permits analysis of the reaction of each of the 16 different solutes using only the decay of the CN transient absorption. The reaction of CN−solvent complexes with alkanes and chloroalkanes is slower than the corresponding reactions of Cl. However, the reactions of alcohols with both CN and Cl occur at about the same rate, likely reflecting additional complexation of the CN radical or its ICN precursor by the alcohol. The rates for the reactions of CN with the chloroalkanes decrease with increasing Cl content of the solute, in keeping with previous observations for the reactions of Cl in both gases and liquids.