Fragmentation path for hydrogen atom dissociation from methoxy radical

Abstract

Salient features of the potential surface for hydrogen atom dissociation from the methoxy radical (CH3O) have been investigated via high-level coupled-cluster methods using a TZ2P(f,d) basis set for geometry optimization and harmonic vibrational analyses and the correlation-consistent cc-pVXZ (X=2–6) series for final energetic determinations and extrapolations. Of central concern for continuing photofragmentation dynamics experiments is the Cs-symmetry A′2 transition state for dissociation, which TZ2P(f,d) RCCSD(T) theory locates at a critical C–H distance of 1.79 Å with a barrier frequency of 947i cm−1. Our zero-point-corrected focal-point extrapolations place this transition state 4.7 kcal mol−1 above the CH2O+H products and yield a dissociation energy of 20.1 kcal mol−1; the latter differs from the most reliable experimental values by only 0.2–0.3 kcal mol−1. A revised enthalpy of formation, ΔHf,0°(CH3O)=6.5 kcal mol−1, is proposed. Disappointingly, TZ2P(f,d) UB3LYP theory underestimates the CH2O+H association barrier by 2.3 kcal mol−1, missing about half the barrier height. The complete set of TZ2P(f,d) RCCSD(T) data for structures and frequencies coupled with final focal-point energetics provides definitive values for parameters essential to the analysis of experimental photofragmentation rate profiles.