How to search for and reveal a hidden intermediate? The ELF topological description of non-synchronicity in double proton transfer reactions under oriented external electric field

Abstract

Context: The nature of double intermolecular proton transfer was studied with the ELF topological approach in two model dimers (the formic acid homodimer and the 1,2,3-triazole-guanidine heterodimer) under an oriented external electric field. It has been shown that each of the two dimers can have either a one-step (one transition state structure) or two-step (two transition state structures) reaction path, depending on the intensity and orientation of the external electric field. The presence of a singularly broad shoulder (plateau in the case of homodimer and plateau-like for heterodimer) around the formal transition state structure results from the strong asynchronicity of the reaction. A careful ELF topological analysis of the nature of protons, hydride (localized) or roaming (delocalized) proton, along the reaction path allowed us to unambiguously classify the one-step mechanisms governing the double-proton transfer reactions into three distinct classes: (1) concerted-synchronous, when two events (roaming proton regions) completely overlap, (2) concerted-asynchronous, when two events (roaming proton regions) partially overlap, and (3) two-stage one-step non-concerted, when two roaming proton regions are separated by a "hidden intermediate region". All the structures belonging to this separatrix region are of the zwitterion form.

Methods: Geometry optimization of the stationary points on the potential energy surface was performed using density functional theory-wB97XD functional-in combination with the 6-311+ +G(2d, 2p) basis set for all the atoms. All first-principles calculations were performed using the Gaussian 09 quantum chemical packages. We also used the electron localization function (ELF) to reveal the nature of the proton along the reaction path: a bound proton (hydride) becomes a roaming proton (carrying a tiny negative charge ≈ 0.3 e) exchanging with two adjacent atoms via two attractors (topological critical points with (3, - 3) signature). The ELF analyses were performed using the TopMod package.

Keywords: Asynchronicity; Concerted vs. stepwise; Double proton transfer; ELF topology; Electric field; Hidden intermediate.