An IUPAC task group chaired by PCCP Advisory Board member Professor Arunan (Indian Institute of Science) has recommended a novel definition for the hydrogen bond:
“The hydrogen bond is an attractive interaction between a hydrogen atom from a molecule or a molecular fragment X–H in which X is more electronegative than H, and an atom or a group of atoms in the same or a different molecule, in which there is evidence of bond formation.”
For more information on the task group and to view the provisional recommendation see the IUPAC report.
You can also read Professor Arunan’s recent PCCP Perspective review article:
The hydrogen bond: a molecular beam microwave spectroscopist’s view with a universal appeal
Mausumi Goswami and E. Arunan
Phys. Chem. Chem. Phys., 2009, 11, 8974-8983
