State-selective rate constants of the self-reaction NH₃⁺ + NH₃: a combined experimental and theoretical study

The self-reaction NH₃⁺ + NH₃ has been studied by combined experimental and theoretical efforts. Experimental rate constants have been measured in a guided ion beam apparatus employing state-selected NH₃⁺ ions formed via resonance enhanced multiphoton ionization (REMPI). The rate constant for the combined proton transfer/hydrogen abstraction reaction forming NH₄⁺ + NH₂ decreases monotonically as a function of the collision energy k(E_c.m.) in the range from 0.22 eV up to 10 eV. The decrease of the rate constant is in line with predictions from classical approximate dipole orientation (ADO) model. For the lowest collision energy of 0.22 eV the measured rate constant matches well the ADO rate constant of k = 2.1 × 10^-9 cm³ s^-1. For higher collision energies the rate constant decreases steeper than predicted and falls below the Langevin limit. The state-selected ions are created in one vibrational state while the rotational quantum state is varied. Due to propensity rules, higher transitions lead to higher rotational quantum states. The rate constant is not significantly influenced by this variation. Complementary potential energy surface calculations and ab initio molecular dynamics methods quantify the relative rates of various competing reactions and provide mechanistic insight into the dominant formation of NH₄⁺ + NH₂. The reaction proceeds via a direct-stripping proton transfer with strong orientation selectivity.