Temperature dependences of singlet methylene removal rates

The technique of laser flash photolysis/laser absorption has been used to obtain absolute removal rate constants for singlet methylene, ¹CH₂ (ã ¹A₁), in the temperature range 298-462 K. The temperature dependences of the removal rate constants for N₂, CH₄, C₂H₄, C₂H₆, H₂O, H₂CO, CH₂CO, CH₃F, C₂F₆, and CH₃OH were measured and the Arrhenius parameters determined. For the reacting species, CH₄, C₂H₄, C₂H₆, H₂O, H₂CO, CH₂CO, CH₃F, and CH₃OH, the rate constants exhibit negative temperature dependences with activation energies covering the range -0.7 ± 0.7 kJ mol^-1 for CH₃F to -3.5 ± 1.0 kJ mol^-1 for CH₃OH. In contrast, positive temperature dependences are observed for the nonreacting species, N₂ and C₂F₆, with activation energies of 1.8 ± 0.9 and 1.6 ± 2.0 kJ mol^-1, respectively. Good agreement was obtained with previously reported measurements for the N₂, CH₄, CH₂CO, C₂H₄, and C₂H₆ temperature dependences. For the nonreacting species, comparison is made with theoretical predicted temperature dependences for collision-induced intersystem crossing from ¹CH₂ (ã ¹A₁) to ³CH₂ (X̃ ³B₁).