Experimental and computational studies of Criegee intermediate reactions with NH3 and CH3NH2

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 (CH₂OO) and acetone oxide ((CH₃)₂COO) with NH₃ and CH₃NH₂ were measured using cavity ring-down spectroscopy. Temperature-dependent rate coefficients, k(CH₂OO + NH₃) = (3.1 ± 0.5) × 10⁻²⁰T² exp(1011 ± 48/T) cm³ s⁻¹ and k(CH₂OO + CH₃NH₂) = (5 ± 2) × 10⁻¹⁹T² exp(1384 ± 96/T) cm³ s⁻¹ were obtained in the 240 to 320 K range. Both the reactions of CH₂OO were found to be independent of pressure in the 10 to 100 Torr (N₂) range, and average rate coefficients k(CH₂OO + NH₃) = (8.4 ± 1.2) × 10⁻¹⁴ cm³ s⁻¹ and k(CH₂OO + CH₃NH₂) = (5.6 ± 0.4) × 10⁻¹² cm³ s⁻¹ were deduced at 293 K. An upper limit of ≤2.7 × 10⁻¹⁵ cm³ s⁻¹ was estimated for the rate coefficient of the (CH₃)₂COO + NH₃ reaction. Complementary measurements were performed with mass spectrometry using synchrotron radiation photoionization giving k(CH₂OO + CH₃NH₂) = (4.3 ± 0.5) × 10⁻¹² cm³ s⁻¹ at 298 K and 4 Torr (He). Photoionization mass spectra indicated production of NH₂CH₂OOH and CH₃N(H)CH₂OOH functionalized organic hydroperoxide adducts from CH₂OO + NH₃ and CH₂OO + CH₃NH₂ 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 CH₂OO + NH₃ and CH₂OO + CH₃NH₂ reactions, respectively. The reactions of NH₃ and CH₃NH₂ with CH₂OO are not expected to compete with its removal by reaction with (H₂O)₂ in the troposphere. Similarly, losses of NH₃ and CH₃NH₂ by reaction with Criegee intermediates will be insignificant compared with reactions with OH radicals.