Figure 1.

Categorization of hydrogen bonds based on donor–acceptor distances, from >2.5 Å (A) to ≪2.5 Å (C). The horizontal lines within the wells represent zero point energies for protium and deuterium. The low barrier hydrogen bond, LBHB (B), has the unique property of a barrier height for H-transfer that has fallen below the ground state vibrational levels of the donor and acceptor. Reprinted with permission from ref (1). Copyright 1998 American Society for Biochemistry and Molecular Biology, Inc.