Proton dynamics and the tautomerization potential in benzoic acid crystals

Abstract

Two‐proton exchange along the two hydrogen bonds mediates the tautomerization in benzoic acid (BA) dimers. Optical spectroscopy and quasi‐elastic neutron scattering (QENS) have been employed to characterize the proton dynamics in doped and pure BA crystals. The proton motion in BA is governed by a multidimensional potential energy surface (PES), and recent theoretical methods, based on a perturbative instanton approach, to describe tunneling in such PES are presented. This PES is also modulated by the interaction with the solid state environment as manifest by the energy difference between the otherwise equivalent tautomers. The value of this energy difference in pure crystals is an important parameter in the data analysis of NMR and QENS. Both methods give mutually consistent values that differ significantly from earlier determinations via infrared and 13C NMR as well as a recent evaluation using neutron diffraction data. The energy difference between tautomers is altered for dimers in the vicinity of impurity molecules. This is the basis for the optical spectroscopic methods, which enable a direct and accurate determination of the level structure and tautomerization dynamics of these coupled dimers in the limit of very low temperatures, where coherent tunneling is also observed in some cases. Measurements with new impurity molecules make it possible to monitor simultaneously at least 10 different tautomer configurations and prove that the influence of the probe molecules on the proton dynamics is small. The transition to thermally activated barrier crossing at higher temperatures is accessed via the width of the QENS line that is determined by the inverse of the proton correlation time. The quantitative data analysis of the scattered intensity as a function of temperature and scattering angle yields the energy difference, A, between the two wells (A/kB=90+20 K), the length (0.686 Å), and direction of the proton jump vector. These measurements complement NMR investigations presented in the preceding paper.

Environment-induced dephasing versus von Neumann measurements in proton tunneling

“…A well-studied example of proton tunneling in chemistry has been the double-H-bonded benzoic acid dimer [3][4][5][6][7][8][9][10][11][12][13], * a.godbeer@surrey.ac.uk † j.al-khalili@surrey.ac.uk ‡ p.stevenson@surrey.ac.uk making this simple structure useful for modeling more complex chemical and biological systems that may involve proton tunneling. The first experimental evidence for proton tunneling in biological systems came in fact from the study of enzyme catalysis in 1989 (for the enzyme alcohol dehydrogenase, which transfers a proton from an alcohol molecule to a molecule of nicotinamide adenine dinucleotide), where the effects of atomic mass on reaction rates through isotopic substitution revealed clear evidence of quantum tunneling even at relatively high temperatures [14].…”

S−H…O and O−H…O Hydrogen Bonds‐Comparison of Dimers of Thiocarboxylic and Carboxylic Acids

“…There are numerous experimental and theoretical studies on the OÀ H…O hydrogen bonds in structures of carboxylic acids. [1][2][3][4] The particular attention is often paid to centrosymmetric dimers of carboxylic acids [5][6][7][8] that are usually characterized by two equivalent and strong OÀ H…O hydrogen bonds possessing numerous features of covalent bonds. [9] Such hydrogen bonds are often considered as a preliminary stage of a double proton transfer process.…”

Absorption spectra of benzoic acid in the 5–15 THz range

“…enzoic acid (BA) has become a model molecule for studying the hydrogen transfer between two molecules [1]. BA molecules form dimers, with BA molecules connected by hydrogen bonds between O and H in their carboxyl groups [2].…”

“…These dimers are bonded through weak Van der Waals interaction to form crystals. Hydrogen transfer occurs between the carboxylic oxygens of the two BA in dimers and is affected by the multidimensional potential energy surface in the crystal, which governs the motion of the atoms [1]. Consequently, infrared spectroscopy has been a common method in studying relevant properties of BA.…”

“…Consequently, infrared spectroscopy has been a common method in studying relevant properties of BA. There are reports on the BA spectra above ~15 THz, probing intramolecular modes of BA [1,2]. Terahertz time-domain spectroscopy has been used to study inter-molecular modes of BA, typically below 6 THz [2][3][4].…”