Quantum effects in single hydrogen bonds Hydrogen bonds are a combination of electrostatics with a nuclear quantum contribution arising from the light mass of hydrogen nuclei. However, the quantum states of hydrogen nuclei are extremely sensitive to coupling with local environments, and these effects are broadened and averaged with conventional spectroscopic or diffraction techniques. Guo et al. show that quantum effects change the strength of individual hydrogen bonds in water layers adsorbed on a salt surface. These effects are revealed in inelastic tunneling spectra obtained with a chlorine-terminated scanning tunneling microscope tip. Science , this issue p. 321