Hydrogen bond has been extensively investigated largely due to its importance in the structure of biological systems, water phase equilibrium and catalytic activity. It also plays a paramount role in proton coupled electron transfer in enzymes and photosynthesis. Conductance single-molecule experiments are especially relevant because their interpretation can be tied directly to changes in the electronic and transport properties of a bridge molecule attached to two nano-electrodes under bias. Low voltage electric conductance can be translated into electron transfer rates by using a fundamental relationship between the two properties when tunneling is the dominant transport mechanism. As a test case, we have selected a relatively simple system for which experimental I-V characteristics exist obtained using a break-junction technique. One of the main results of this experiment, although only qualitatively understood, is that the effect of hydrogen bonding depends strongly on the length of the chain. We have modeled two carboxylic acid groups attached via a hydrogen bond and lengthened the chain of carbons both symmetrically and asymmetrically in order to investigate the electronic properties of the bond and the influence of the hydrogen bond location on the conductance. The specific dyad used was of the form, CH3(CH2)nCOOH-HOOC(CH2)nCH3, and for this system we have reproduced the experimental trend and also provided a physical rationale for the observed dependence. In a more general context, our results are of importance in understanding how single-molecule electron transport and electron transfer can be affected by the chemical nature of the intervening units.

Modeling electron transfer through hydrogen bonding

249th National Meeting (2015)

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