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Observing chemical dynamics in hydrogen-bonded systems

A hydrogen-bond flip-flop through a Bjerrum-type defect

Pinacol (2,3-dimethylbutane-2,3-diol)PINACOL_CHOH_SPECTRUM_AND_DECO.JPG is a vicinal diol bearing two hydoxyl groups (OH groups) in close proximity. In nonpolar liquid solutions at room temperature, the pinacol molecule forms an intramolecular hydrogen-bridge (H-bond). This H-bond formation is readily visible in the infrared spectrum, where well separated absorption bands from the two OH-stretching vibrations of the molecule can be seen.

A frequency-upshifted band centered at 3626 cm-1 originates from the hydrogen-accepting OH-group (the "dangling" OH) while a frequency-downshifted band around 3579 cm-1 originates from the hydrogen-donating OH-group (the "bound" OH).  We used femtosecond two-dimensional infrared spectroscopy (2DIR) to observe in real time the dynamics of the hydrogen-bond flip-flop motion leading to the dynamic reversal of the direction of the intramolecular hydrogen-bridge. These studies were recently published in Angewandte Chemie (DOI: 10.1002/anie.201208625).

In combination with calculations based on density functional theoory it was shown that these dynamics are facilitated by two synchronous disrotatory conformational isomerizations about the CC-OH single bonds. The flip-flop motion guides the molecule through a intermediate structure, which can be regarded as a Bjerrum defect because it has its hydrogen bond formally occupied by two H-atoms at the same time. The 2DIR data yield a time scale of 2 ps for these elementary molecular dynamics. A movie illustrating this highly intriguiging H-bond flip-flop motion in the pinacol molecule is shown here.

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