Ab initio kinetic mechanism of OH-initiated atmospheric oxidation of pyrrole.

Mai, Tam V-T; Nguyen, Hieu T; Huynh, Lam K

The comprehensive kinetic mechanism of the OH-initiated gas-phase oxidation of pyrrole is first theoretically reported in a broad range of conditions (T = 200-2000 K &P = 1-7600 Torr). On the potential energy surface constructed at the M06-2X/aug-cc-pVTZ level, the temperature- and pressure-dependent behaviors of the title reaction were characterized using the stochastic Rice-Ramsperger-Kassel-Marcus based Master Equation (RRKM-ME) rate model. The corrections of the hindered internal rotation and quantum tunneling treatments were included. The calculated results reveal the competition between the two distinct pathways: OH-addition and direct H-abstraction. The former channels are found favorable at low-temperature and high-pressure range (e.g., T < 900 K and P = 760 Torr) where non-Arrhenius and positive pressure-dependent behaviors of the rate constants are noticeably observed, while the latter predominate at temperatures higher than 900 K at atmospheric pressure and no pressure dependence on the rate constant is found. The predicted global rate constants are in excellent agreement with laboratory values; thus, the derived kinetic parameters are recommended for modeling/simulation of N-heterocycle-related applications in atmospheric and even in combustion conditions. Besides, pyrrole should not be considered as a persistent organic pollutant owing to its short atmospheric lifetime (∼1 h) towards OH radicals. The secondary mechanisms of the subsequent reactions of two OH-pyrrole adducts (namely, I1 and I2) with two abundant species, O2/NO, which are relevant to the atmospheric degradation process, were also investigated. It is also revealed by TD-DFT calculations that two OH-pyrrole adducts (I1 &I2), nine intermediates, Ii (i = 3-11) and four products (P1, P2, P3 and P6) can undergo photodissociation under the sunlight.