Can a single ammonia and water molecule enhance the formation of methanimine under tropospheric conditions? kinetics of ^•CH₂NH₂ + O₂ (+NH₃/H₂O)

The aminomethyl (•CH₂NH₂) radical is generated from the photo-oxidation of methylamine in the troposphere and is an important precursor for new particle formation. The effect of ammonia and water on the gas-phase formation of methanimine (CH₂NH) from the ^•CH₂NH₂ + O₂ reaction is not known. Therefore, in this study, the potential energy surfaces for ^•CH₂NH₂ + O₂ (+NH₃/H₂O) were constructed using ab initio//DFT, i.e., coupled-cluster theory (CCSD(T))//hybrid-density functional theory, i.e., M06-2X with the 6-311++G (3df, 3pd) basis set. The Rice−Ramsperger−Kassel−Marcus (RRKM)/master equation (ME) simulation with Eckart’s asymmetric tunneling was used to calculate the rate coefficients and branching fractions relevant to the troposphere. The results show 40% formation of CH₂NH at the low-pressure (<1 bar) and 100% formation of CH₂NH₂OO^• at the high-pressure limit (HPL) condition. When an ammonia molecule is introduced into the reaction, there is a slight increase in the formation of CH₂NH; however, when a water molecule is introduced into the reaction, the increase in the formation of CH₂NH was from 40% to ∼80%. The calculated rate coefficient for ^•CH₂NH₂ + O₂ (+NH₃) [1.9 × 10⁻²³ cm³ molecule⁻¹ s⁻¹] and for CH₂NH₂ + O₂ (+H₂O) [3.3 × 10^-17 cm³ molecule^-1 s^-1] is at least twelve and six order magnitudes smaller than those for free ^•CH₂NH₂ + O₂ (2 × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ at 298 K) reactions, respectively. Our result is consistent with that of previous experimental and theoretical analysis and in good agreement with its isoelectronic analogous reaction. The work also provides a clear understanding of the formation of tropospheric carcinogenic compounds, i.e., hydrogen cyanide (HCN).