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

Area of Science:

• Physical Chemistry
• Spectroscopy
• Computational Chemistry

Background:

• Proton transfer is fundamental to many chemical and biological processes.
• Monitoring proton transfer kinetics is crucial for understanding these phenomena.
• Malonaldehyde serves as a model compound for studying proton transfer dynamics.

Purpose of the Study:

• To computationally demonstrate the utility of 2D IR spectroscopy with a C-D reporter for monitoring proton transfer kinetics.
• To investigate the sensitivity of the C-D stretch vibrational frequency to malonaldehyde tautomers.
• To validate computational methods for simulating 2D IR spectra and proton transfer rates.

Main Methods:

• Mixed quantum mechanics/molecular mechanics (QM/MM) simulations were employed.
• The self-consistent-charge density functional tight binding (SCC-DFTB) method was used for electronic structure calculations.
• 2D IR line shapes for the C-D stretch in labeled malonaldehyde were computed in aqueous solution.

Main Results:

• The C-D stretch vibrational frequency showed approximately 150 cm(-1) sensitivity to the two tautomers of malonaldehyde.
• 2D IR spectra revealed cross peaks indicative of proton chemical exchange.
• The kinetics derived from the growth of cross-peaks and decay of diagonal peaks accurately matched the simulated proton transfer rate.

Conclusions:

• 2D IR spectroscopy with a C-D reporter is a viable computational tool for studying proton transfer kinetics.
• The C-D reporter group provides sufficient spectral sensitivity for distinguishing tautomers.
• The SCC-DFTB method accurately simulates 2D IR spectra and proton transfer dynamics.