Hydrogen abstraction from dimethyl ether (DME) and dimethyl sulfide (DMS) by OH radical: a computational study

The electronic structures and energetics of the reactants, products, transition states, and association complexes involved in the reaction of dimethyl ether (DME) and dimethyl sulfide (DMS) with OH radical have been investigated at the MP2(FC) and DFT (BH&HLYP and B3LYP) levels of theory using the 6-311G(d,p), 6-311++G(d,p), and 6-311++G(2df,2pd) basis sets. Two channels have been discussed and two transition structures have been located for each reaction; one represents a direct hydrogen-abstraction and the other one for the abstraction through prereactive complex. Among the computational levels employed in the present work, the PMP2/6-311++G(2df,2pd)//M2/6-311++G(d,p) level was found to reproduce best the experimentally measured rate constants. Our best estimates of the rate constants at 298 K for the reactions of OH radical with DME and DMS are 5.9×10−13 and 1.1×10−12 cm3 molecule−1 s−1, respectively, which are underestimated by 4 times as compared to the experimentally derived values. At low temperatures, the direct H-abstraction predominates over the H-abstraction through the prereactive complex, while both channels compete at elevated temperatures.