Direct measurement of rate constants for the reactions of CH and CD with HCN and DCN

Multiphoton laser photolysis of CHBr₃ (or CDBr₃) at 266 nm is utilized for the production of ground state CH(X ²Π) (or CD)radicals in the gas phase. The relative concentration of CH (or CD) is monitored by laser-induced fluorescence near 430 nm. Absolute rate constants are determined by addition of HCN or DCN to the slowly flowing gas mixture. Heating of the reaction cell permits temperature studies between 293 and 676 K. The following Arrhenius expressions are derived: k = (5.0±0.4) × 10^-11 ×exp[(500±30)/T] cm³ s^-1 for CH+HCN; k = (8.1±1.7)×10^-11 exp[(420±80)/T] cm³ s^-1 for CH+DCN; k = (9.1±1.5)×10^-11 exp[(420±70)/T] cm³ s^-1 for CD+HCN and k = (5.4±0.7)×10^-11 exp[(560±50)/T] cm³ s^-1 for CD+DCN. The measured rate constants for the CH + HCN reaction do not vary with total pressure over the range 35-300 Torr at 296 K. The present observations are consistent with CH insertion into the CH bond in HCN followed by decomposition of the adduct to H+HCCN as the dominant reactive pathway. The rate measurements on mixed isotope species (i.e. CH+DCN or CD+HCN) support the conclusion that exchange reactions do occur to some extent above 475 K as in CH+DCN→CD+HCN and CD+HCN→CH+DCN. The result of the totally deuterated reaction (i.e. CD+DCN) indicates that the inverse isotope effect is important.