Can a single ammonia and water molecule enhance the formation of methanimine under tropospheric conditions?: kinetics of (•)CH(2)NH(2) + O(2) (+NH(3)/H(2)O)

The aminomethyl (•CH(2)NH(2)) 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 (CH(2)NH) from the (•)CH(2)NH(2) + O(2) reaction is not known. Therefore, in this study, the potential energy surfaces for (•)CH(2)NH(2) + O(2) (+NH(3)/H(2)O) 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 CH(2)NH at the low-pressure (<1 bar) and 100% formation of CH(2)NH(2)OO(•) 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 CH(2)NH; however, when a water molecule is introduced into the reaction, the increase in the formation of CH(2)NH was from 40% to ∼80%. The calculated rate coefficient for (•)CH(2)NH(2) + O(2) (+NH(3)) [1.9 × 10(−23) cm(3) molecule(−1) s(−1)] and for CH(2)NH(2) + O(2) (+H(2)O) [3.3 × 10(-17) cm(3) molecule(-1) s(-1)] is at least twelve and six order magnitudes smaller than those for free (•)CH(2)NH(2) + O(2) (2 × 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).