Singlet methylene removal rate constants from the (0,1,0) vibrational level: Enhancement via complex-mediated vibrational relaxation

Abstract

The technique of laser flash photolysis/laser absorption has been used to obtain absolute removal rate constants for singlet methylene, ¹CH₂ (ã¹A₁), from the vibrationally excited (0,1,0) level. (0,1,0) removal rate constants are reported for the 13 species N₂, CH₄, C₂H4, C₂H₆, CH₂CO, CH₃Cl, CH₃OC₂H₅, CH₃OH, CH₃CN, CH₃F, CF4, C₂F₄, and C₂F₆. With the exception of the slowest three reacting species, CH₄, CH₃F and C₂F₄, for the species which are known to react from the (0,0,0) level the removal rate constants are essentially identical for the (0,0,0) and (0,1,0) levels. The removal rate constants for CH₄, CH₃F and C₂F₄ increase by factors of 1.2, 2.2 and 1.45 respectively with vibrational excitation to (0,1,0). For the species N₂, CF₄, and C₂F₆, which do not appear to react from (0,0,0), the removal rate constants are increased in the (0,1,0) state. The increase is very large for N₂, for which the factor is 4, and is substantial for CF₄ and C₂F₆, which show factors of 2.55 and 2.7 respectively. The increased removal rate constants for N₂ and for the fluorine-containing species are attributed to the formation of an association complex between ¹CH₂ (0,1,0) and the partner followed by dissociation to ¹CH₂ (0,0,0) and the partner. We refer to this as complex-mediated vibrational relaxation (CMVR). Removal rate constants for the (0,0,0) level are likely to be in error when experimental conditions allow the simultaneous production of ¹CH₂ (0,1,0) from the methylene precursor and where CMVR, which typically has comparable efficiency to collision induced intersystem crossing, provides a growth term for the (0,0,0) population. The species for which CMVR is likely to be significant are those for which reaction is either absent or a minor pathway and where there are electron-rich centres for the empty orbital of ¹CH₂ to "bind", allowing the formation of a long lived ¹CH₂-collider complex.