Hydrogen Bonding in Charge Transport

(a) Proton-Coupled Electron Transfer

Doubly hydrogen-bonded interfaces between carboxylic acids and amidines are a general structural motif in biological systems involving proton-transfer. We are using these systems to investigate how proton transfer in this system depends on changes in electronic structure across the hydrogen bonding interface. The hydrogen bonded OH and NH vibrations in such hydrogen-bonded interfaces directly couple into the proton transfer coordinate and exhibit broad structured IR spectral features that depend on the hydrogen bonding environment. We study the symmetric doubly hydrogen-bonded dimers of 7-azaindole (7-AI) and 1H-pyrrolo[3,2-h]quinoline (PQ) and their asymmetric counterparts with acetic acid using ultrafast 2D-IR spectroscopy, IR transient grating and echo peak-shift measurements. These measurements are strongly modulated by the inter-dimer hydrogen-bond stretch and twist motions, and resolve the spectral substructure observed in the linear FTIR spectra. These equilibrium studies will pave the way for photo-initiated excited state proton transfer experiments.

Selected Reference:

Ultrafast N-H Vibrational Dynamics of Cyclic Doubly Hydrogen-Bonded Homo- and Heterodimers P.B. Petersen, S.T. Roberts, K. Ramasesha, D.G. Nocera, and A. Tokmakoff, J. Phys. Chem. B (2008).

(b) Charge Transport in Aqueous Hydroxide

Compared to ions of similar size and charge density, the aqueous hydroxide ion possesses an anomalously fast diffusion constant due to its ability to accept a proton from a neighboring water molecule, leading to the translocation of the ion. Ultrafast infrared spectroscopy can shed light on the proton transport mechanism since the OH stretching frequency of water shifts depending on whether it is hydrogen bonded to another water molecule or to an OH- ion. Fig. 1 shows FT-IR spectra of the OH stretch of HOD dissolved in different concentrations of NaOD solution. A small peak can be seen on the high frequency side of the spectrum due to the OH- stretch, but the most striking feature is the large broadening of the transition to low frequency. This low frequency band results from HOD molecules hydrogen bonded to OD- ions. To some extent, these complexes can be thought of as highly polarizable H3O2- ions, and the large broadening of the absorption spectrum is due to coupling between the motion of the bridging hydrogen and solvent fluctuations that raise and lower the barrier for the transport of the proton. Currently, we are using 2DIR spectroscopy to track the dynamics of water and hydroxide to form a concrete picture of how charge transport occurs in these solutions.