How the two-center three-electron hemibond affect the inversion barrier of NH\(_3\) in X-NH\(_3\) complex (X = F, Cl and Br)

Abstract

In the present work, we have investigated the two-center three-electron (2c–3e) bond in X–NH\(_3\) (\(X=\mathrm{F,\ Cl\ and\ Br}\)) complex using quantum chemical calculations. It was found that ammonia can form two types of the complex with halogen radical through the 2c–3e bond, one when halogen interacts with ammonia from the opposite side of hydrogen (RC1), and the other from the adjacent side of hydrogen (RC2). Further, it was found that RC1 and RC2 are connected to each other via ammonia inversion mode. However, compared to ammonia inversion, which is a symmetric double-well potential, here one stationary point (RC2) is close to TS, whereas the other (RC2) is far from the TS. Further, the energy barrier for the conversion of RC1 to RC2 in X–NH\(_3\) complexes are found to be lower than the barrier associated with ammonia inversion in bare NH\(_3\) molecule. Interestingly, the inter-conversion dynamics of RC1 and RC2 investigated via Born–Oppenheimer molecular dynamics (BOMD) simulation suggests that it can be an example of Polanyi’s mode selective rules for an extreme late and early barrier reaction system, and can play a crucial role in explaining various peculiar dynamical effects found in \(X+\mathrm{NH}_3\rightarrow \mathrm{HX}+\mathrm{NH}_2\) chemical systems.

In the present work, the effect of hemibond formed between ammonia and halogen (F, Cl and Br) atom on the inversion barrier of ammonia is studied. It was found that in the presence of halogen atom the inversion barrier of ammonia decreases substantially. It was also observed that higher the electronegative halogen atom, greater the effect on the inversion barrier of ammonia.