Kinetics of CN(ν= 0) and CN(ν= 1) with HCl, HBr and HI between 295 and 764 K

Abstract

Time-resolved laser-induced fluorescence (LIF) measurements have been carried out on the kinetics of CN(ν= 0) and CN(ν= 1) radicals with HCl, HBr and HI at temperatures between 295 and 764 K. The radicals were generated by pulsed laser photolysis of NCNO at 532 nm, and the decays of concentration were monitored using a dye laser tuned to lines in either the (0, 0) or (0, 1) band of the CN(B ²Σ⁺–X ²Σ⁺) system. The rate constants for reaction of CN(ν= 0) at 295 K are: (k/ cm³ molecule^–1 s^–1)=(5.9 ± 0.3)× 10^–15 for HCl; (9.1 ± 0.6)× 10^–13 for HBr; and (7.0 ± 0.4)× 10^–11 for HI. The temperature dependences of the three reaction rates are also markedly different: they yield approximate activation energies (the Arrhenius plots show some curvature) of 18.0 ± 0.8, 7.7 ± 0.7 and –0.3 ± 0.3 kJ mol^–11, respectively. The rate constants for removal (reaction plus relaxation) of CN(ν= 1) only exceeds that for reaction of CN(ν= 0) in the case of HCl in the lower part of the temperature range. This is attributed to the occurrence of vibrational relaxation. The lack of any significant enhancement of the reaction rates on vibrational excitation of CN is consistent with the predictions of vibrationally adiabatic transition-state theory.