Gas-Phase Reactions of OH with Methyl Amines in the Presence or Absence of Molecular Oxygen. An Experimental and Theoretical Study

Abstract

The rate coefficients for the reaction of OH with the alkyl amines: methylamine (MA), dimethylamine (DMA), trimethylamine (TMA), and ethylamine (EA) have been determined using the technique of pulsed laser photolysis with detection of OH by laser-induced fluorescence as a function of temperature from 298 K to ∼600 K. The rate coefficients (10(11) × k/cm(3) molecule(-1) s(-1)) at 298 K in nitrogen bath gas (typically 5-25 Torr) are: k(OH+MA) = 1.97 ± 0.11, k(OH+DMA) = 6.27 ± 0.63, k(OH+TMA) = 5.78 ± 0.48, k(OH+EA) = 2.50 ± 0.13. The reactions all show a negative temperature dependence which can be characterized as: k(OH+MA) = (1.889 ± 0.053) × 10(-11)(T/298 K)(-(0.56±0.10)), k(OH+DMA) = (6.39 ± 0.35) × 10(-11)(T/298 K)(-(0.75±0.18)), k(OH+TMA) = (5.73 ± 0.15) × 10(-11)(T/298 K)(-(0.71±0.10)), and k(OH+EA) = (2.54 ± 0.08) × 10(-11)(T/298 K)(-(0.68±0.10)). OH and OD reactions have very similar kinetics. Potential energy surfaces (PES) for the reactions have been characterized at the MP2/aug-cc-pVTZ level and improved single point energies of stationary points obtained in CCSD(T) and CCSD(T*)-F12a calculations. The PES for all reactions are characterized by the formation of pre- and post-reaction complexes and submerged barriers. The calculated rate coefficients are in good agreement with experiment; the overall rate coefficients are relatively insensitive to variations of the barrier heights within typical chemical accuracy, but the branching ratios vary significantly. The rate coefficients for the reactions of OH/OD with MA, DMA, and EA do not vary with added oxygen, but for TMA a significant reduction in the rate coefficient is observed consistent with OH recycling from a chemically activated peroxy radical. OH regeneration is pressure-dependent and is not significant at 298 K and atmospheric pressure, but the efficiency of recycling increases strongly with temperature. The PES for OH recycling have been calculated. There is evidence that the primary process in TMA photolysis at 248 nm is the loss of H atoms.