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Introduction

Need introduction.

Hydrogen bond

The Gold Book defines hydrogen bond as "a form of association between an electronegative atom and a hydrogen atom attached to a second, relatively electronegative atom."

The "classical" hydrogen bond is considered as an electrostatic interaction between polar group A^δ-—H^δ+ and B^δ-:

A^δ-—H^δ+···B^δ-

Bhat and Gudihal note that there is no single convention to represent hydrogen bond. In text, the hydrogen bond is often represented by three middle dots:

A—H···B

but also dashes are in use, as in

A—H--B

In drawings, the situation becomes more complicated. Variable numbers of dots, dashes and even hashes (short parallel lines) have all used in the literature.

IUPAC Recommendations 1996 on stereochemistry:

"A broken line - - - - has been used instead of parallel lines but this is better reserved for a partial bond, delocalisation, or a hydrogen bond."

Thus, the use of dashes is not recommended for covalent bond; however it not the same as endorsing the use of dashes for representation of hydrogen bond. Moreover, the use of the same symbolism for the very different concepts as stated above should be rather discouraged.

Since the dotted line is widely used to represent the hydrogen bond and is the least confusing, it should be recommended. The dotted line should include at least three dots.

<Now I run into a problem: which chemical drawing software can show dotted line? ChemSketch and ISIS/Draw can’t.>

The bonds should be long enough to be clearly visible between two atom labels; bonds less than twice as long as the height of an atom label should be avoided.

RECOMMENDED	RECOMMENDED	RECOMMENDED

The label of hydrogen donor atom A should be present, even if it is a carbon.

Greenwood and Earnshaw [1, p. 53] provide at least five patterns of hydrogen bonding in 2D:

1.	Linear
2.	Non-linear
3.	"Chelating"
4.	Bifurcated
5.	Trifurcated

Hydrogen bonds may be intermolecular or intramolecular. The case of trifurcated intramolecular hydrogen bond (as in triethanolammonium [2]) is especially interesting because the hydrogen atom is bound to four atoms tetrahedrally and thus, at least in theory, may be chiral.

A—H groups with reverse polarity (A^δ+—H^δ-) can form interactions

A^δ+—H^δ-···B^δ+

analogous to hydrogen bond ("inverse hydrogen bonds") [3].