Vibrational dynamics of strongly hydrogen-bonded acid-base complexes in solution

Hydrogen bonds are a central part of biology and chemistry. Hydrogen bonds are found in water, DNA, polymers, alcohols, and proteins. Furthermore, hydrogen bonds play a key role in proton-transfer reactions. Proton transfer reactions are involved in several chemical processes such as acid-base catalyzed reactions, enzyme-catalyzed reactions, energy storage, and energy transduction, and are one of the most fundamental chemical reactions. However, the chemical dynamics of ground-electronic-state at room-temperature systems remain elusive due to the difficulty of modeling these reactions. Establishing an experimental model system and using two-dimensional infrared (2D IR) spectroscopy as a means for detection, the chemical dynamics of a ground-electronic-state proton-transfer reaction protonation states in solution can be examined.

In this study, experimental models are established with formic acid and nitrogenous bases in a low dielectric solvent. A hydrogen bond forms between the acid and the base, which allows for the proton to transfer between the two molecules. The location of the proton determines which protonation state is favored. The carbon-deuterium (C-D) stretch and the carbonyl (C=O) stretch of the formic acid molecule are used as the reporter groups for the 2D IR measurements. The results of the C-D stretch demonstrate that there is a high sensitivity to the deprotonation, vibrational coupling, and vibrational dynamics trends that are linked to the solute-solvent interactions. The results of the C=O stretch demonstrate a sensitivity to the deprotonation and conformational disorder in which the position of the C=O changes the dynamics of the protonation state. Although, a proton-transfer is not detected, the experimental model system provides an understanding of the features that govern the chemical dynamics of proton-transfer reactions.