Kinetics of CN radical reactions with selected cycloalkanes: CN reactivity towards secondary CH bonds

The kinetics of the reaction of CN radicals with four molecules containing only secondary CH bonds, cyclopropane (c-C₃H₆), cyclopentane (c-C₅H₁₀), cyclohexane (c-C₆H₁₂) and cyclooctane (c-C₈H₁₆), have been studied with the two-laser pump-probe laser photolysis/laser induced fluorescence technique in the temperature range 219 to 740 K. The rate constant for CN + c-C₃H₆ was found to increase monotonically with temperature. However, a small negative temperature dependene of the rate was observed for CN + c-C₅H₁₀, c-C₆H₁₂, and c-C₈H₁₆. The rate constants for these reactions can be effectively represented, in units of cm³/s, by k_c-C3H6 = 1.26 × 10^-15 T^1.50 exp(174/T), k_c-C5H10 = 10^-9.76±0.06 exp(40±19/T), k_c-C6H12 = 10^-9.67±0.08 exp(69±12/T), and k_c-C8H16 = 10^-9.56±0.05 exp(73±13/T). The reactivity per secondary CH bond for c-C₃H₆ is significantly smaller than that for c-C₅H₁₀, c-C₆H₁₂, and c-C₈H₁₆, the latter three of which have essentially the same reactivity. This is primarily due to the significant differences between the CH bond strength in c-C₃H₆ and the other cyclocompounds. The small negative temperature dependences in the larger cycloalkanes observed here are attributed to long-range dipole-induced-dipole attractive interactions.