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

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, H₂O, and D₂O 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.