The reaction O+2+CH4 → CH3O+2+H studied from 20 to 560 K in a supersonic jet and in a SIFT

Abstract

Rate coefficients k have been determined for the reaction O+2+CH4 → CH3O+2+H at several temperatures in the uniquely wide temperature range 20–560 K using a new expanding jet (CRESU) apparatus and a selected ion flow tube (SIFT) apparatus. In the overlapping temperature range of the experiments the rate coefficients are in excellent agreement. A minimum is observed in the rate constant near 300 K but the outstanding feature is the rapid increase at low temperatures from the minimum value k (290 K)=5.4×10−12 cm3 s−1 to k (20 K)=4.7×10−10 cm3 s−1, the latter being about half of the collisional limiting value kc for the reaction (kc=1.16×10−9 cm3 s−1). Indeed the data show that k → kc as T → 0 K and the low temperature values can be fitted to a power law of the form k=1.1×10−7 T−1.8. The results strongly indicate that the CH3O+2 product ion is formed via rearrangement in a long-lived intermediate (CH4O2)+ complex, the lifetime against unimolecular decomposition of which largely controls the rate of the reaction. It is suggested that this rearrangement proceeds via H− abstraction within the complex forming an electrostatically bound (CH+3 ⋅ HO2) complex which can then undergo concerted molecular rearrangement leading to CH3O2++H products. These data and other available data on exothermic ion–molecule reactions at low temperatures suggest that complex lifetimes may be the controlling parameter in many reactions at low temperatures.