Mechanistic insights into electric field-dependent polarization and kinetics of the elementary reaction NH + H2 → NH2 + H.

Abstract

The elementary reaction NH + H2→ NH2 + H is a pivotal pathway in the NH3 reaction network, yet its polarization mechanism and kinetic parameters under external electric fields (EEFs) remain unexplored despite well-established equilibrium kinetics. Here, we employ CCSD(T)/CBS//M06-2X/6-311G(d,p) calculations and transition state theory to systematically investigate 23 EEF directions across 0.005-0.03 a.u., revealing cooperative control of reaction kinetics by the field direction and strength. Alignment with the reaction axis (e.g., -X, (-X, Y), and (-X, Y, -Z)) enhances rate constants by 4 orders of magnitude at 0.03 a.u., while misaligned planar fields suppress reactivity at 0.02 a.u. Crucially, field orientation governs product selectivity through charge transfer that exhibits exponential sensitivity to field strength. The molecular rearrangement induced by the EEFs ensures that the reaction proceeds along the most favorable path. As a result, three advantageous directions, (-X), (-X, Y), and (-X, Y, -Z), were selected for further analysis. By calculating the electronic structure and employing molecular orbital theory, valence bond theory and the quantum theory of atoms in molecules (QTAIM) method, it was found that the reaction responds to EEFs due to the initial regulation of molecular polarity and the influence of the electric field on the charge transfer during the reaction. The results also show that the dipole moment of the transition state is significantly reduced by EEFs in different (-X, Y, and -Z) directions, initially decreasing and then increasing with increasing field strength. The electrostatic potential distribution further illustrates the regulatory effect of different electric field directions on reaction products. Additionally, the EEFs along the reaction axis direction significantly lower the LUMO energy level of the transition state, which may reduce the probability of ionic/charge transfer state wavefunctions mixing into the transition state wavefunction. These findings establish a quantitative framework for leveraging EEFs to manipulate energy barriers and orbital interactions, offering mechanistic insights for optimizing product yields in EEF-driven ammonia reaction systems.