hydrogen-bond considerations

Descriptions and discussion of weak and non-conventional hydrogen bonds (C-H...X, X-H...C and C-H...C) need not always parallel what is accepted for the stronger and more conventional types of hydrogen-bond interactions (X-H...X). The following may be noted:

(1) The van der Waals criterion for a weak hydrogen bond can sometimes be unnecessarily restrictive. This criterion is without real scientific basis because hydrogen bonds and van der Waals interactions have different distance dependence characteristics. In practical terms this means that a short hydrogen bond formed by an activated C-H donor (chloroform, alkyne or more acidic C-H group) would usually be correctly identified. The problem arises for longer contacts formed by donors of moderate activity. Here, a more detailed look is often necessary and useful. In general, statements like `the contact is shorter than the van der Waals limit and is therefore a hydrogen bond' should be avoided.

(2) Unlike strong hydrogen bonds, the weaker varieties are easily distorted by the crystal environment. Therefore, it is useful to discuss hydrogen-bond lengths and angles jointly. A longer C-H...O contact (D > 3.6 Å) may be acceptable as a hydrogen bond if the angle tends towards linearity (θ > 150°). Very short C-H...O contacts (D < 3.2 Å) with very bent angles (90-110°) should be viewed with some scepticism. In cases of doubt, angular considerations should take precedence over length considerations.

(3) Unactivated methyl groups could pose special problems. These are known to sometimes form very short hydrogen-bond-like contacts (perhaps even tending to linearity) but these contacts are most likely repulsive and possibly even destabilising. This situation is, however, uncommon. Methyl groups also form genuine hydrogen bonds.

In summary, the results of commonly used geometry programs should not be transferred directly and routinely into tables without adequate inspection of the molecular and crystal structure. Unqualified statements like `the structure is stabilized by C-H...O hydrogen bonds' should be avoided. The study of weak hydrogen bonds is still evolving. Factors like donor acidity, acceptor basicity, hydrogen-bond length, hydrogen-bond angle and possible co-operativity effects should be assessed (even if they are not commented upon explicitly in the paper). For further information, authors could refer to some of the following:

(a) Desiraju, G. R. (1991). Acc. Chem. Res. 24, 290.
(b) Desiraju, G. R. (1996). Acc. Chem. Res. 29, 441.
(c) Steiner, T. (1996). Crystallogr. Rev. 6, 1.
(d) Steiner, T. (1997). Chem. Commun. 727.
(e) Desiraju, G. R. & Steiner, T. (1999). In The Weak Hydrogen Bond in Structural Chemistry and Biology, Oxford University Press.

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