Branching Ratios in Reactions of OH Radicals with Methylamine, Dimethylamine, and Ethylamine

Abstract

The branching ratios for the reaction of the OH radical with the primary and secondary alkylamines: methylamine (MA), dimethylamine (DMA), and ethylamine (EA), have been determined using the technique of pulsed laser photolysis–laser-induced fluorescence. Titration of the carbon-centered radical, formed following the initial OH abstraction, with oxygen to give HO₂ and an imine, followed by conversion of HO₂ to OH by reaction with NO, resulted in biexponential OH decay traces on a millisecond time scale. Analysis of the biexponential curves gave the HO₂ yield, which equaled the branching ratio for abstraction at αC–H position, <i>r</i>_αC–H. The technique was validated by reproducing known branching ratios for OH abstraction for methanol and ethanol. For the amines studied in this work (all at 298 K): <i>r</i>_αC–H,MA = 0.76 ± 0.08, <i>r</i>_αC–H,DMA = 0.59 ± 0.07, and <i>r</i>_αC–H,EA = 0.49 ± 0.06 where the errors are a combination in quadrature of statistical errors at the 2σ level and an estimated 10% systematic error. The branching ratios <i>r</i>_αC–H for OH reacting with (CH₃)₂NH and CH₃CH₂NH₂ are in agreement with those obtained for the OD reaction with (CH₃)₂ND (<i>d</i>-DMA) and CH₃CH₂ND₂ (<i>d</i>-EA): <i>r</i>_{αC–H,d‑DMA} = 0.71 ± 0.12 and <i>r</i>_{αC–H,d‑EA} = 0.54 ± 0.07. A master equation analysis (using the MESMER package) based on potential energy surfaces from G4 theory was used to demonstrate that the experimental determinations are unaffected by formation of stabilized peroxy radicals and to estimate atmospheric pressure yields. The branching ratio for imine formation through the reaction of O₂ with α carbon-centered radicals at 1 atm of N₂ are estimated as <i>r</i>_CH2NH2 = 0.79 ± 0.15, <i>r</i>_CH2NHCH3 = 0.72 ± 0.19, and <i>r</i>_CH3CHNH2 = 0.50 ± 0.18. The implications of this work on the potential formation of nitrosamines and nitramines are briefly discussed