Monitoring Intramolecular Proton Transfer with Two-Dimensional Infrared Spectroscopy: A Computational Prediction

Proton transfer processes are ubiquitous and play a vital role in a broad range of chemical and biochemical phenomena. The ability of two-dimensional infrared (2D IR) spectroscopy with a carbon-deuterium (C-D) reporter to monitor the kinetics of proton transfer in the model compound malonaldehyde was demonstrated computationally. One of the two carbonyl/enol carbon atoms in malonaldehyde was labeled with a C-D bond. The C-D stretch vibrational frequency provides ∼150 cm(-1) of sensitivity to the two tautomers of malonaldehyde. Mixed quantum mechanics/molecular mechanics simulations employing the self-consistent-charge density functional tight binding (SCC-DFTB) method were used to compute 2D IR line shapes for the C-D stretch of labeled malonaldehyde in aqueous solution. The 2D IR spectra reveal cross peaks from the chemical exchange of the proton. The kinetics for the growth of the cross-peaks (and the decay of the diagonal peaks) precisely match the proton transfer rate observed in the SCC-DFTB simulations.