The product branching and dynamics of the reaction of chlorine atoms with methylamine

Abstract

Nascent rotational level population distributions have been measured for the HCl and DCl products of photo-initiated reaction of Cl atoms with CH₃NH₂, CD₃NH₂, CH₃ND₂ and CD₃ND₂. The use of the different isotopomers of methylamine enables the dynamics of abstraction of H/D atoms from carbon or nitrogen atoms to be distinguished, and the former channel is found to produce HCl or DCl with greater rotational excitation than the latter channel. Mean HCl rotational energies for the two reactions Cl + CH₃ND₂ → HCl + CH₂ND₂ and Cl + CD₃NH₂ → HCl + CD₃NH are, respectively, 501 ± 84 and 122 ± 12 cm⁻¹. Fits of the HCl and DCl rotational distributions for reaction of Cl atoms with the CH₃NH₂ and CD₃ND₂ molecules with distributions obtained from reactions of the two partially deuterated isotopomers yield branching ratios for C–H ∶ N–H and C–D ∶ N–D abstraction of 0.48 ∶ 0.52 and 0.58 ∶ 0.42, respectively. Ab initio electronic structure theory calculations of the energetics and geometries of transition states and molecular complexes along the reaction pathways for the two competing channels provide key insights to aid the interpretation of the experimental data. The computed bond angle of the N–H–Cl moiety (142.0°) in the transition state for the CH₃NH forming channel deviates more from linearity than the C–H–Cl bond angle (171.1°) in the competing transition state, but leads to lower rotational excitation of the nascent HCl products. This counter-intuitive outcome is explained in terms of dipole–dipole interactions between the HCl and organic radical products as they separate.