Temperature dependences of singlet methylene removal rates

Abstract

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). © 1996 American Chemical Society.