Dynamics of proton transfer in imidazole hydrogen-bond chains

Abstract

The dynamics and mechanism of proton transfer in the imidazole (Im) hydrogen-bond (H-bond) chain were studied using a unit cell of the Im crystal structure (H⁺(Im)_n, n = 2–4) as a model system and B3LYP/TZVP calculations and Born–Oppenheimer molecular dynamics (BOMD) simulations as model calculations. The B3LYP/TZVP results suggested that only linear H-bond structures are involved in proton transfer and H⁺(Im)₂ is the smallest, most active Zundel-like intermediate complex, which is preferentially formed in a low local dielectric environment. The potential energy curves for proton displacement confirmed the Eigen–Zundel–Eigen scenario that consists of breaking and forming H-bonds in the Im H-bond chain, and because the energy barrier for the reorientation of Im molecule is high, the ring-flip process is ruled out from the proton-transfer mechanism. The BOMD results over the temperature range of 298 to 500 K confirmed that proton transfer in the Im H-bond chain is a local (short-range) process by showing that the activation energies for proton displacement in H⁺(Im)₂ and H⁺(Im)₄ are nearly the same and comparable to experimental and theoretical values. The proton transfer profiles and vibrational spectra suggested that at low temperatures, the N–N vibration, transferring proton and librational motion in the protonated H-bond are synchronized (coherent), resulting in effective structural diffusion process. The ¹H NMR results confirmed these findings and further revealed that the dynamics of proton transfer at low and high temperatures are different due to the interferences of vibrational and librational motions and increases in the oscillatory shuttling motion at elevated temperatures. These theoretical results lead to the conclusion that the rate-determining process of proton transfer in the Im system is the oscillatory shuttling motion in the Zundel-like intermediate complex and does not necessarily involve reorientation of Im molecule as a key process.