Pi Underwater

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Science  16 Dec 2011:
Vol. 334, Issue 6062, pp. 1473
DOI: 10.1126/science.334.6062.1473-b

Hydrogen bonding among hydroxyl, carbonyl, and amine groups is a critical organizing force in the structure of proteins and nucleic acids. Over the past several decades, an analogous attraction of O-H groups to the pi-electron cloud of aromatic rings has drawn increasing study. Gierszal et al. have now probed the strength of such pi-hydrogen bonds in dilute aqueous solutions of benzene. Their experiment relied on an unusually sensitive combination of Raman scattering measurements and spectral deconvolution to tease out the vibrational signature of this motif from amid the thousandfold more-intense set of vibrations spanning water-water interactions. Selective deuteration of both solvent and solute bolstered the assignment. A temperature-dependent study of the relative energetics supported a picture of pi-H bonds enthalpically weaker than the OH-H variety, but entropically favored on account of the greater conformational freedom accorded the water molecule by a benzene partner.

J. Phys. Chem. Lett. 2, 2930 (2011).

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