Formation of Nitrosamines and Alkyldiazohydroxides in the Gas Phase: The CH₃NH + NO Reaction Revisited

Aminyl free radicals of the form RN(•)H are formed in the photochemical oxidation of primary amines, and their reaction with (•)NO is an important tropospheric sink. Reaction of the parent methylamidogen radical (CH3N(•)H) with (•)NO in the gas phase has been studied using quantum chemical techniques and RRKM theory/master equation based kinetic modeling. Calculations with the G3X-K composite theoretical method indicate that reaction proceeds via exothermic formation of a primary nitrosamine intermediate, CH3NHNO, which can isomerize to an alkyldiazohydroxide, CH3NNOH, and further eliminate water to form diazomethane, CH2NN. Master equation simulations conducted at tropospheric conditions identify that the collisionally stabilized CH3NHNO and CH3NNOH isomers are the major reaction products, with smaller yields of CH2NN + H2O. A previously proposed mechanism in which the primary nitrosamine is destroyed via isomerization to CH2NHNOH, followed by reaction with O2 to produce CH2NH + HO2(•) + (•)NO, is disproved. In the atmosphere, CH2NN may be formed with sufficient vibrational energy to directly dissociate to singlet methylene ((1)CH2) and N2, whereas under combustion conditions this is expected to be the dominant pathway. This study suggests that stabilized primary nitrosamines can indeed form in the photochemical oxidation of amines, along with alkyldiazohydroxides and diazoalkanes. Both classes of compound are potent alkylating agents that may need to be considered in future atmospheric studies.