Direct Trace Fitting of Experimental Data Using the Master Equation: Testing Theory and Experiments on the OH + C2H4 Reaction.

Laser flash photolysis coupled with laser-induced fluorescence observation of OH has been used to observe the equilibration of OH + C2H4 ↔ HOC2H4 over the temperature range 563-723 K and pressures of bath gas (N2) from 58 to 250 Torr. The time-resolved OH traces have been directly and globally fitted with a master equation in order to extract ΔRH00, the binding energy of the HOC2H4 adduct, with respect to reagents. The global approach allows the role that OH abstraction plays at higher temperatures to be identified. The resultant value ofΔRH00, 111.8 kJ mol-1, is determined to be better than 2 kJ mol-1 and is in agreement with our ab initio calculations (carried out at the CCSD(T)/CBS//M06-2X/aug-cc-pVTZ level), 111.4 kJ mol-1, and other state of the art calculations. Parameters for the abstraction channel are also in good agreement with previous experimental studies. To effect this analysis, the MESMER master equation code was extended to directly incorporate secondary chemistry: diffusional loss from the observation region and reaction with the photolytic precursor. These extensions, which, among other things, resolve issues related to the merging of chemically significant and internal energy relaxation eigenvalues, are presented.