Singlet Methylene Removal Rate Constants from the (0,1,0) Vibrational Level: Enhancement via Complex-Mediated Vibrational Relaxation

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.