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Carboxylic acids are a typical example of a discrete oligomeric species that are held together by intermolecular hydrogen bonds ( [link] a). A wide range of structurally analogous compounds also form head-to-tail hydrogen bonded dimers (e.g., [link] ). In a polymeric hydrogen bonded species every molecule hydrogen bonds but in a random form. As an example, liquid primary alcohols form extended hydrogen networks ( [link] b). Such an arrangement is labile and as such it is difficult to determine definitive speciation. Liquids that form this type of hydrogen-bonded network are known as associated liquids . In the solid state the networks generally adopt a more ordered structure. For example as is seen in the structure of ice.

Structure of (a) the head-to-tail dimmer formed between two carboxylic acid molecules, and (b) a typical network of a primary alcohol in the liquid phase.
Structure of the hydrogen bonded diner of (C 6 H 5 )NNN(H)(C 6 F 5 ). Adapted from J. T. Leman, J. Braddock-Wilking, A. J. Coolong, and A. R. Barron, Inorg. Chem. , 1993, 32 , 4324.

Methods of study

The study of the structure arising from hydrogen bonding and the properties exhibited due to the presence of hydrogen bonds is very important.

Diffraction methods

X-ray diffraction of single crystals is the most common structural method employed to determine the presence, effect, and strength of a hydrogen bond. Unfortunately, in order for the location of the hydrogen to be determined with some degree of accuracy, diffraction data of a high quality is needed and/or low temperature (e.g., -196 °C) data collection is required. Neutron scattering can be used where very accurate data is required because hydrogen atoms scatter neutrons better than they do X-rays. [link] summarizes the key parameters that are obtained from X-ray (and neutron) diffraction experiments.

Structural parameters obtained from diffraction methods.

Given the electrostatic nature of a hydrogen bond between a polar X-H bond and a Lewis base it is reasonable that the X-H ... Y angle (θ) is roughly linear (i.e., 180°). However, it is not always so and non-linear interactions are known where steric or conformational restrictions limit the orientation of the X-H bond with respect to Y.

The distance between X and Y, d(X ... Y), is less than the sum of the van der Waal radii of X and Y ( [link] ). This is in line with the relative strength of these interactions. As would be expected the shorter the X ... Y distance the stronger the hydrogen bond.

Comparison of the X ... Y distance in hydrogen bonded species with the sum of the van der Waal radii.
X Y Sum of van der Waal radii (Å) Typical X ... Y distance (Å)
O O 2.8 2.50 – 2.69
O N 2.9 2.75 - 2.85
N N 3.0 2.69 – 2.98

The bond distance to hydrogen, d(X-H), is often longer in hydrogen bonded species. For example the O-H distance for an alcohol in the absence of hydrogen bonding is typically 0.97 Å. In contrast, the value typically seen for a hydrogen-bonded analog is 1.05 Å.


Spectroscopy is a simple method of comparing hydrogen-bonded systems in particular in the solution or liquid phase.

Questions & Answers

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1. x + 6 2 -------------- = _ x + 9 + 6 3 x + 6 3 ----------- x -- (cross multiply) x + 15 2 3(x + 6) = 2(x + 15) 3x + 18 = 2x + 30 (-2x from both) x + 18 = 30 (-18 from both) x = 12 Test: 12 + 6 18 2 -------------- = --- = --- 12 + 9 + 6 27 3
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