Experimental and computational studies of Criegee intermediate reactions with NH3 and CH3NH2. Issue 26 (17th January 2019) - Chhantyal-Pun, Rabi
Abstract : The significance of removal of atmospheric ammonia and amines by reaction with Criegee intermediates is assessed by kinetic studies. Abstract : Ammonia and amines are emitted into the troposphere by various natural and anthropogenic sources, where they have a significant role in aerosol formation. Here, we explore the significance of their removal by reaction with Criegee intermediates, which are produced in the troposphere by ozonolysis of alkenes. Rate coefficients for the reactions of two representative Criegee intermediates, formaldehyde oxide (CH2 OO) and acetone oxide ((CH3 )2 COO) with NH3 and CH3 NH2 were measured using cavity ring-down spectroscopy. Temperature-dependent rate coefficients, k (CH2 OO + NH3 ) = (3.1 ± 0.5) × 10 −20 T 2 exp(1011 ± 48/ T ) cm 3 s −1 and k (CH2 OO + CH3 NH2 ) = (5 ± 2) × 10 −19 T 2 exp(1384 ± 96/ T ) cm 3 s −1 were obtained in the 240 to 320 K range. Both the reactions of CH2 OO were found to be independent of pressure in the 10 to 100 Torr (N2 ) range, and average rate coefficients k (CH2 OO + NH3 ) = (8.4 ± 1.2) × 10 −14 cm 3 s −1 and k (CH2 OO + CH3 NH2 ) = (5.6 ± 0.4) × 10 −12 cm 3 s −1 were deduced at 293 K. An upper limit of ≤2.7 × 10 −15 cm 3 s −1 was estimated for the rate coefficient of the (CH3 )2 COO + NH3 reaction. Complementary measurements were performed with mass spectrometry using synchrotron radiation photoionization giving k (CH2 OO + CH3 NH2 ) = (4.3 ± 0.5) × 10 −12 cm 3 s −1 at 298 K and 4 Torr (He). Photoionization mass spectra indicated production of NH2 CH2 OOH and CH3 N(H)CH2 OOH functionalized organic hydroperoxide adducts from CH2 OO + NH3 and CH2 OO + CH3 NH2 reactions, respectively. Ab initio calculations performed at the CCSD(T)(F12*)/cc-pVQZ-F12//CCSD(T)(F12*)/cc-pVDZ-F12 level of theory predicted pre-reactive complex formation, consistent with previous studies. Master equation simulations of the experimental data using the ab initio computed structures identified submerged barrier heights of −2.1 ± 0.1 kJ mol −1 and −22.4 ± 0.2 kJ mol −1 for the CH2 OO + NH3 and CH2 OO + CH3 NH2 reactions, respectively. The reactions of NH3 and CH3 NH2 with CH2 OO are not expected to compete with its removal by reaction with (H2 O)2 in the troposphere. Similarly, losses of NH3 and CH3 NH2 by reaction with Criegee intermediates will be insignificant compared with reactions with OH radicals.