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We have used two-dimensional infrared spectroscopy and related femtosecond nonlinear infrared spectroscopies to investigate strongly hydrogen bonded proton transfer systems, including aqueous hydroxide and molecular dimers linked by O-H...:N and N-H...O. Hydroxide is a model system for proton migration along chains of hydrogen bonded water molecules. Asymmetric doubly hydrogen bonded interfaces between amidine and carboxylic acid functionalities are models for biological energy conversion dynamics that involve proton-coupled-electron-transfer (PCET) and excited state proton transfer (ESPT). ESPT occurs from a number of polyaromatic amidines, where the proton transfer is mediated by the surrounding hydrogen-bonding network either through a dimer counterpart or solvent molecules. Our experiments have been designed to directly visualize the dynamics of the proton potential between and during proton transfer events, and are modelled to understand the influence of the molecular environment on those dynamics.

Hydroxide: We have made 2D IR measurements of solutions of dilute HOD in concentrated NaOD: D2O solution. Upon the addition of NaOD to HOD:D2O solution, two new spectral features appear, a new peak at high frequency due to the OH- stretch and a broad shoulder that extends down to very low frequency due to HOD molecules hydrogen bonded to OD- ions. Interestingly a large off-diagonal feature in the 2D IR lineshape for short waiting times disappears on a ~120 fs timescale, roughly matching a OD- concentration dependent component in pump probe measurements. At longer waiting times a new peak in the 2D spectra which may indicate chemical exchange between OH- and HOD molecules grows in on a ~2ps timescale. To understand these features, we modeled our experiments using an empirical valence bond molecular dynamics simulation model of aqueous sodium hydroxide developed by Todd Martinez. In order to calculate spectra, we have developed a DFT based electrostatic frequency map which we can use to determine the “instantaneous” OH frequency of a given OH bond for a static configuration of the simulation. We find that in configurations where a proton is significantly shared between two water molecules, all of the transitions within the proton stretching potential undergo significant red shifts, and in particular the OH stretch overtone (v = 2 -> 0) tunes into our experimentally accessible bandwidth. Thus, the rapid loss of intensity in the 2D and pump probe measurements is due to direct excitation of the overtone transition for configurations corresponding to nearly shared protons. They fluctuate in the shared Zundel state for about 5-6 periods (120 fs) before localizing on one side of the complex.

Assymmetric Dimers: We have applied ultrafast IR and 2D IR spectroscopy to directly monitor the proton transfer mechanism through the interchange of hydrogen-bonding donor and acceptor. The present study is focused two symmetric and two asymmetric cyclic doubly hydrogen-bonded dimers. The four dimers are the two homo-dimers of 7-azaindole (7-AI) and 1H-pyrrolo[3,2-h]quinoline (PQ) and their hetero-dimer counterparts with acetic acid. We have performed a complete set of third order spectroscopies: transient grating, echo peak shift, pump probe and full 2D-IR surface of all four dimers in the NH region. Whereas the linear spectra might be dominated by Fermi-resonances, the transient grating and echo peak-shift measurements have shown that the vibrational dynamics are dominated by the low-frequency inter-monomer stretching and twisting vibrations that modulate the inter-dimer hydrogen-bond strength. Frequency dispersed pump-probe experiments and 2D surfaces resolve the intricate beating pattern observed. Energy is observed to flow from the NH modes into the Fermi-resonances and strong coherent beating is observed between the different cross-peaks. Ongoing experiments are extending these equilibrium measurements to non-equilibirum measurements of the proton transfer process initiated by ESPT in 7-AI.

Methods: As part of this work we have found a need for femtosecond spectroscopy across the entire mid-infrared region. To that end we have developed a broadband IR continuum source which can be used in pump-probe and 2D IR spectroscopy. A manuscript is in preparation.