The Shapes of Protons in Hydrogen Bonds Depend on the Bond Length

Disklike, cigarlike, and spherical protons are characteristic of long, normal hydrogen bonds (see picture, top), the tunneling regime (bottom), and the limiting case of ultrashort, centered hydrogen bonds, respectively, according to fully quantum mechanical ab initio path-integral calculations on ice. This dependence on the donor–acceptor distance (here dOO) is expected to be generic for many hydrogen-bonded systems.