Thermal Rate Constant and Branching Ratio for CN + HD → HCN/DCN + D/H from T = 293 to 375 K

Abstract

The reaction rate constant for the cyano (CN) radical with deuterium hydride (HD) has been determined over the temperature range from 293 to 375 K. As well, the branching ratio for the partitioning into HCN + D or DCN + H has also been measured. The CN radical was detected by time-resolved near-infrared absorption spectroscopy using the CN (A{sup 2}{Pi} {l_arrow} X{sup 2}{Sigma}) (2,0) band near 789.5 nm while the HCN molecule was monitored by time-resolved infrared absorption spectroscopy using the HCN(001) {l_arrow} (000) {upsilon}{sub 3} fundamental band near 3.0 {micro}m. Both species were monitored following the same photolysis laser pulse used to generate the CN radical. The measurements were carried out in low pressures of Ar or He as carrier gas. The frequencies and structure of the transition states for all four isotopomers for CN + H{sub 2} were calculated using ab initio quantum chemistry methods and a normal-mode analysis. Reasonable agreement was found between the experimental results and the rate constants predicted by conventional transition state theory using the theoretical transition state properties. These measurements should provide for an interesting test for theoretical predictions of thermal rate constants for this prototypical four-atom reaction system.