Curent definition of Hydrogen Bond according to IUPAC Compendium of Chemical Terminology 2nd Edition (1997)
‘’A form of association between an electronegative atom and a hydrogen atom attached to a second, relatively electronegative atom.[…]Both electronegative atoms are usually (but not necessarily) from the first row of the Periodic Table, i.e. N, O or F. Hydrogen bonds may be intermolecular or intramolecular. With a few exceptions, usually involving fluorine, the associated energies are less than 20 - 25 kJ mol −1 (5 - 6 kcal mol −1).’’ (ref.1)
On the other hand, several observations made by chemists confirm that the definition needs to be revised to incorporate a wider range of hydrogen bonds. It should be known that since 2005, recommendations have been made to IUPAC to describe ''unconventional hydrogen bonds'' often weaker (or sometimes conversely stronger) than the conventional ones.
The proposed new definition of Hydrogen Bond (IUPAC Recommendations 2011)
‘’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.‘’ (ref.1)
In this new definition, the atom X is not restricted to the atoms O, N or F, because, for example, an atom of C is also a more electronegative atom than an H. It is thus possible to have Hydrogen Bond like C-H...O (weaker). The evidence of the presence of the HB can be described for example by the formation of a contact which is smaller than the sum of the vDW interactions of the atoms involved and also by the directional character (example: an angle that tends to 180o
) . By studying the crystallographic databases, we can see these trends (ref.1)
. This new definition of the HB broadens the interaction domain of the latter at energies ranging from 0.5 to 40 kcal/mol. (ref.2)