Coherent Proton Tunneling in Hydrogen Bonds of Isolated Molecules: Carboxylic Dimers

Abstract

IntroductionDue to the exceptional importance of hydrogen bonds in biology and chemistry the detailed investigation of their structure and dynamics has attracted the attention of several experimental and theoretical groups. Laser spectroscopic measurements and quantum mechanical calculations have led, in recent years, to remarkable progress towards the detailed understanding of important prototype systems such as (NH 3 ) 2 [1], (H 2 O) 2 [2], and DNA base pairs [3].Double hydrogen bonded systems play a crucial role in that they serve as model systems for the understanding of DNA base pairs. Proton transfer is of fundamental interest for biology as well as being a fundamental reaction in chemistry [4,5]. Moreover, multiple-proton transfer in hydrogen bonded systems is one of the most fundamental processes in biology and chemistry. It governs oxidationreduction reactions in many chemical and biological reactions [6]. In proton pumping mechanisms of trans membrane proteins protons are transported across the membrane by subsequent proton transfer. These reactions may incorporate strong quantum effects due to the low mass of the proton. The observation of pronounced isotopic effects is taken as an indication of a strong tunneling contribution. In contrast, the classical transmission probability is zero as long as the energy of the particle is lower than the barrier height (V ‡ ) and is equal to 1 if the energy (E) exceeds this. For an ensemble at temperature T this leads to the well known Arrhenius equation for the reaction rate which is proportional to exp (-(E -V ‡ )/ kT). The transmission will, therefore, depend solely on the barrier height and not on the width or on the exact shape of the transition barrier. Isotopic substitution will shift the zero point energies relative to the transition barrier and will, therefore, lead to a change in the reaction constant.Quantum mechanical tunneling is a result of the wavelike nature of particles which allows transmission through a reaction barrier. The quantum mechanical transmission probability for energies below V ‡ is governed by tunneling and reflection at the barrier. The transmission is larger than zero even well below the barrier and will depend crucially on the barrier width. In Hydrogen-Transfer Reactions. Edited by

Insights into the Mechanism of Ground and Excited State Double Proton Transfer Reaction in Formic Acid Dimer

“…In particular, double proton transfer in formic acid dimer (FAD) has been studied extensively as a typical example of multiple proton transfer reaction. Formic acid dimer (FAD) which is an eight-membered ring with double hydrogen bonds is often chosen as model complex system for studying the DPT not only due to its double H-bonded structure but also because of the consequence of the proton transfer for the biological systems,e.g., enzymatic catalyst . In summary, proton transfer reactions play a major role in many phenomena in chemistry, physics, and biology. − …”

“…Formic acid dimer (FAD) which is an eight-membered ring with double hydrogen bonds is often chosen as model complex system for studying the DPT not only due to its double H-bonded structure but also because of the consequence of the proton transfer for the biological systems, 7 e.g., enzymatic catalyst. 8 In summary, proton transfer reactions play a major role in many phenomena in chemistry, physics, and biology. 9−12 Nowadays with the help of high level ab initio and density functional theory (DFT) calculations one can fully characterize the reaction mechanism in terms of energetic and structural parameters.…”