Mechanism and branching ratios of hydroxy ethers + ^·oH gas phase reactions: Relevance of H bond interactions

A theoretical study on the mechanism and branching ratios of the gas phase reactions of hydroxyl radicals with a series of hydroxy ethers is presented. This is the first report on branching ratios for these reactions. The studied hydroxy ethers are: methoxy-methanol (MM), ethoxy-methanol (EM), 1-methoxy-ethanol (1ME), 2-methoxy-ethanol (2ME), and 2-ethoxy-ethanol (2EE). All the possible H abstraction channels have been modeled, involving the rupture of C-H and O-H bonds. The H abstractions from the alcohol group were found to be almost negligible for all the studied systems. The role of H bond interactions in the transition states (TS) is discussed, as well as the importance of the location of the reaction site with respect to the alcohol and the ether functional groups. TSs with seven-member ring-like structures were found to lead to stronger H bond interactions than TSs with six- and five-member ring-like structures, with the latter leading to the weakest interactions. Kinetic calculations have been performed within the 250-440 K temperature range. Rate coefficients for the reactions of ^·OH with MM, EM, and 1ME are reported here for the first time. Nonlinear Arrhenius plots were found for all the overall reactions. Negative activation energies at room temperature are proposed for the ^·OH reactions with EM, 2ME, and 2EE. The excellent agreement with the scarce experimental data available supports the reliability of the data reported here for the first time.