198 Topics in Current Chemistry Editorial Board: A. de Meijere KN ...

16 J.P. Glusker
hydrogen bond H◊◊◊B [26]. Moderate hydrogen bonds are formed to neutral
atoms, and weak hydrogen bonds are formed when the hydrogen is attached to
a neutral atom such as carbon (rather than oxygen or nitrogen as in the strong
hydrogen bonds), or when the acceptor group B has a p electron system rather
than a lone pair of electrons.
It has been usual to take the sum of the van der Waals radii as a criterion for
hydrogen bonding; if the distance between A and B is less than the sum of the
van der Waals radii, the interaction is presumed to be a hydrogen bond, provided
a hydrogen atom is available to form such an interaction. The A-H◊◊◊B angle
defines the directionality of the hydrogen bond, lying near 180° for high directionality;
it may, however, be bent to as low a value as 120°. The hydrogen bond is,
however, primarily electrostatic and far-ranging in effect.As a result, sometimes
the hydrogen atom may be attracted by two electronegative atoms and a threecenter
hydrogen bond may be formed, even though distances from the hydrogen
atom may be longer than the sum of their van der Waals radii [3]. In such a case,
all four atoms involved are in approximately the same plane, and the distances
between the hydrogen atom and the two acceptor atoms are often not the same.
This arrangement of atoms occurs about 25% of all O-H◊◊◊O and N-H◊◊◊O
hydrogen bonds [3]. On the other hand, it can be argued that the directionality
of hydrogen bonds excludes its description as a purely electrostatic interaction.
The strongest hydrogen bond is believed to be the [F◊◊◊H◊◊◊F] – bond in the
hydrogen bifluoride ion (Fig. 7). It has an energy near 50 kcal mol –1 [2] and an
F◊◊◊F distance of 2.27(6) Å in potassium hydrogen bifluoride (KHF 2). The hydrogen
atom appears to be symmetrically located between the two fluorines [32].
Another very strong hydrogen bond is the O-H◊◊◊O hydrogen bond in certain
carboxylates and organic hydrogen anions. The distances are also very short for
O◊◊◊H but the O-H bond is lengthened so that both O-H and O◊◊◊H distances
lie in the range 1.1–1.3 Å, with O◊◊◊O distances in the range 2.35–2.5 Å. The
O-H◊◊◊O angle is in the range 170–180°. Normal O-H–O and N-H◊◊◊O hydrogen
bonds, with angles of 165(5)° are powerful aligners of molecules, and are well
known for their great contributions to macromolecular folding and function [3,
25]. H◊◊◊O distances are generally in the range 1.8–2.0 Å. Such hydrogen bonds
account for the formation of a helices and b sheets in proteins, base pairing in
nucleic acids, and many protein-nucleic acid interactions. Examples of such
hydrogen bonds in sulfuric acid and its hydrate [33] and in citric acid monohydrate
[34] are shown in Fig. 8 and 9. These illustrate the cooperativity of
hydrogen bonding so that a given oxygen atom both gives and receives a hydrogen
bond, so that such interactions continue throughout the crystal.
Fig. 7. The bifluoride ion in the crystal structure of its potassium salt [32]