Experimental quantification of nuclear quantum effects on the hydrogen bond of liquid water

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Abstract
Nuclear quantum effects (NQEs) significantly influence material properties upon isotopic substitution, particularly with light atoms such as hydrogen. While water is rich in hydrogen, its hydrogen-bonded structure exhibits only moderate NQEs. Simulations ascribe this to competing zero-point energies (ZPEs): Intermolecular ZPEs stabilize hydrogen-bonds, while intramolecular ZPEs destabilize them. However, experimental validation has been lacking due to the difficulty in quantifying NQEs. The air/water interface provides an ideal platform to quantify NQEs in liquid water using surface-specific vibrational spectroscopy. By analyzing the excess/depletion of interfacial HOD, H 2 O, and D 2 O molecules with one free OH/OD group and the other H-bonded OH/OD group, we found that the intermolecular ZPE destabilizes the hydrogen-bonds by 0.74 ± 0.20 kilojoule per mole upon isotope substitution from H to D, while the intramolecular ZPE stabilizes them by 0.78 ± 0.33 kilojoule per mole. This near-complete cancellation explains the overall moderate NQE in liquid water. The interface thus allows for quantifying NQEs in water.

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