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

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.