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

The aminomethyl (center dot 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)-C-center dot + O-2 reaction is not known. Therefore, in this study, the potential energy surfaces for (CH2NH2)-C-center dot + O-2 (+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 center dot 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 similar to 80%. The calculated rate coefficient for (CH2NH2)-C-center dot + O-2 (+NH3) [1.9 x 10(-23) cm(3) molecule(-1) s(-1)] and for CH2NH2 + O-2 (+H2O) [3.3 x 10(-17) cm(3) molecule(-1) s(-1)] is at least twelve and six order magnitudes smaller than those for free (CH2NH2)-C-center dot + O-2 (2 x 10(-11) cm(3) 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)

Automated summary

This study investigates the effect of ammonia and water on the gas-phase formation of methanimine from the reaction between aminomethyl radical and oxygen. The results show that the introduction of water significantly increases the formation of methanimine. The rate coefficients for the reactions involving ammonia and water are much smaller compared to the reaction without them.