Direct dynamics study on hydrogen abstraction reaction of morpholine with hydroxyl radical

A dual-level direct dynamic study has been employed to investigate the H-abstraction reaction of morpholine (C4H9NO) with OH. The chair conformer of C4H9NO molecule is performed as the reactant because of the lower energy. The potential energy surfaces are obtained at the MC-QCISD/3//M06-2X/6-311+G(d, p) level. The rate constants are estimated using the improved canonical variational transition state theory with the small-curvature tunneling correction over a wide temperature range of 200 to 1000 K, and a fitted four-parameter rate constant expression is obtained. The calculated rate constants are in good agreement with the available experimental values, and the negative temperature dependence of rate constant is found within 200-600 K. Moreover, the contribution of each reaction site to the title reaction is discussed with respect to the temperature. The result shows that the H-abstraction reaction mainly takes place at the -CH2 group attached to the N atom at low temperatures, while as the temperature increases the contribution of the H-abstraction to the title reaction from the -NH group becomes larger rapidly, and the -NH group turns into the most important H-abstraction reaction site when the temperature is higher than 800 K.