IUPAC – CORE GROUP MEETING
The hydrogen bond is an
attractive interaction between a group X–H and an atom or a group of atoms Y,
in the same or different molecule(s), when there is evidence of bond formation.
The evidence that may be used in this context should include the
criteria below. To improve the
reliability of the definition of a particular interaction as a hydrogen bond,
as many of the following criteria as possible should be satisfied.
It is understood that there will be borderline cases for which the
interpretation of the evidence might be subjective. In any case, the exceptions to these
conditions should not be gross violations, for an interaction which is termed a
hydrogen bond.
The following criteria are recommended as evidence for
a hydrogen bond X–H•••Y–(Z):
(1) The physical forces involved in hydrogen bonding must
include electrostatic and inductive forces in addition to
(2) The atoms X and H are covalently bonded to one
another, and the bond is polarized in X–H•••Y–Z, such
that the H becomes more electropositive.
(3) The lengths of the X–H bond and, to a lesser extent,
of the Y–Z bond deviate from the X–H and Y–Z equilibrium values.*2
(4) The X–H and Y–Z bond vibrational frequencies and intensities
show changes on hydrogen bond formation and new vibrational modes associated
with the formation of the H…Y bond are generated.
(5) The X–H and Y–Z NMR nuclear chemical shielding tensors
(chemical shifts) undergo changes, associated with spin-spin coupling and NOE
(nuclear Overhauser enhancement) transmitted through X–H•••Y–Z.
(6) The stronger the hydrogen bond, the more linear the
X–H•••Y angle, θ, and the shorter the H…Y distance.*3
(7) The interaction energy per hydrogen bond is greater
than at least a few kT ( 1 kT = 600
cal mol–1 = 2.4 kJ mol–1 at 300 K) in order to ensure its
stability.*4
The following characteristics are typical of hydrogen
bonds:
(1) The pKs of X–H and Y–Z and
the strength of any hydrogen bond formed are correlated.
(2) A hydrogen bond may be involved in a proton transfer
reaction.
(3) Networks of hydrogen bonds show the phenomenon of cooperativity,
manifest by more than additive changes in the hydrogen bond properties with an
increase in network size.*5
(4) Hydrogen bonds affect crystal packing and other supramolecular properties.*6
(5) Theoretical estimates of charge transfer in hydrogen
bonding shows that the interaction energy is highly correlated with the amount
of electronic charge transferred.
* A more-detailed explanation to be added.