Quenching kinetics of I(2P½) by cyanides. Isotope effect in the quenching by CH3(D)CN

Abstract

The quenching of electronically excited iodine atoms I(²P_½ I*) by CH₃CN, CD₃CN, HCN, C₂H₅CN, and (CN)₂ was studied. Good agreement was found for I* deactivation by acetonitrile and perdeuteroacetonitrile using two widely different techniques, namely time-resolved atomic absorption (TRAA) and chemical laser threshold gain (CLTG). The kinetic isotope effect determined, k_H/k_D, was 2.2 ± 0.5 for both methods. This low value would support the existence of a noticeable reactive path and a possible channel assisted by a curve-crossing mechnism. Values here reported for the second-order rate constants for I* deactivation are, respectively, (1.2 ± 0.1)× 10^–13, (5.3 ± 0.5)× 10^–14, (3.3 ± 0.2)× 10^–14, (1.3 ± 0.1)× 10^–13, (2.2 ± 0.2)× 10^–15 cm³ molecule^–1 s^–1 for CH₃CN, CD₃CN, HCN, C₂H₅CN and (CN)₂, according to the TRAA method. Results from CLTG measurements are (1.6 ± 0.1)× 10^–13, (7.3 ± 0.2)× 10^–14 and (8.3 ± 0.1)× 10^–14 cm³ molecule^–1 s^–1 for CH₃CN, CD₃CN and HCN, respectively.