Kinetics of Gas-Phase Reaction of OH with Morpholine: An Experimental and Theoretical Study

Abstract

Kinetics of reaction of OH radical with morpholine, a heterocyclic molecule with both oxygen and nitrogen atoms, has been investigated using laser photolysis-laser-induced fluorescence technique, in the temperature range of 298-363 K. The rate constant at room temperature (k(298)) is (8.0 +/- 0.1) x 10(-11) molecule(-1) cm(3) s(-1). The rate constant decreases with temperature in the range studied, with the approximate dependence given by k(T) = (1.1 +/- 0.1) x 10(-11) exp[(590 +/- 20)/T] cm(3) molecule(-1) s(-1). The rate constants are high compared with those of similar heterocyclic molecules with oxygen atom but comparable to those reported for aliphatic amines. Ab initio molecular orbital calculations show that prereactive complexes, 5-7 kcal mol(-1) lower in energy as compared with the reactants, are formed because of hydrogen bond interaction between OH and the N/O atom of morpholine. The stability of the complex involving the nitrogen atom is found to be more than that involving the oxygen atom. The optimized transition-state structures and energies for the different pathways of hydrogen abstraction from these prereactive complexes explain the observation of negative activation energy.

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

“…The potential profile is further refined with interpolated single-point energy (ISPE) method [24], which uses higher-level single-point energies at the lower-level saddle point plus a few selected points along the lower-level reaction path. In order to compare with the barrier heights reported by SenGupta et al [6], the low-level optimization and barrier heights calculations are also done by using Møller-Plesset second-order perturbation theory [25][26][27] with 6-311+G(d, p) basis set (MP2/6-311+G(d, p)).…”

“…Recently, both the experimental and theoretical studies have been performed to investigate the degradation of morpholine in the atmosphere. For the reaction of OH radical with C 4 H 9 NO, two experimental measurements [5,6] have been performed to estimate the rate constants. Rashidi et al [5] obtained the relative rate constants with the value of (14.0 ± 1.9) × 10 −11 cm 3 molecule −1 s −1 at 295 K by the ultraviolet spectra (UV), and they also reported that the Abstract A dual-level direct dynamic study has been employed to investigate the H-abstraction reaction of morpholine (C 4 H 9 NO) with OH.…”

“…SenGupta et al [6] reported the Arrhenius expression of k(T) = (1.1 ± 0.1) × 10 −11 exp [(590 ± 20)/T] cm 3 molecule −1 s −1 over the temperature range of 298 to 363 K, with a value of (8.0 ± 0.1) × 10 −11 cm 3 molecule −1 s −1 at 298 K by the laser photolysis-laser-induced fluorescence technique (PLP-LIF). It should be noted that the negative temperature dependence of the rate constants was found in the temperature range of 298 to 363 K. The rate constants determined by Rashidi et al [5] are almost two times those proposed by SenGupta et al [6]. Therefore, accurate data for the rate constants and its temperature dependences of the reaction with OH radical are required to evaluate the atmospheric lifetime of morpholine.…”

“…Furthermore, SenGupta et al [6] calculated the barrier heights and the reaction enthalpies for the H-abstraction channels from the methylene group (-CH 2 ) of morpholine, in which the carbon atom is adjacent to oxygen atom at MP2 (PMP2)/6-311+G(d, p) level. However, the H-abstraction channels from -CH 2 group linked to N atom and the imine (-NH) group are short of a clear and definite theoretical study in SenGupta's paper [6].…”

“…However, the H-abstraction channels from -CH 2 group linked to N atom and the imine (-NH) group are short of a clear and definite theoretical study in SenGupta's paper [6]. As far as I know, the details about the mechanism and dynamics of the total reaction for C 4 H 9 NO with OH are also unavailable now, such as the major abstraction channel, the contribution of each channel to the overall reaction, as well as the temperature dependence of the rate constant over a wide temperature range.…”