HYDROGEN BONDING 



65 



C — N • • • O and three O ■ • • N • • • O) should be close to the expected tetra- 

 hedral value. ]\Ioreover, it has been found that all available donor groups are 

 usually involved in hydrogen bonds — the presence in a crystal of a possible 

 donor hydrogen atom which does not enter into hydrogen bonding is very 

 unusual. 



One of the simplest substances in which a hydrogen bond occurs is hydra- 

 zine dihydrochloride (Donohue and Lipscomb, 1947), the structure of which 

 is shown in Fig. 1. The chloride ions are arranged about the positive ions so 

 that the angles N — N- • -CI are very nearly tetrahedral. The NoHe"'"'" ions lie 

 on three-fold axes, so each — NH3+ group forms hydrogen bonds to three equiv- 

 alent close Cl~ neighbors. There is a fourth chloride ion at very nearly the 

 same distance from the nitrogen atom, but reference to Fig. 1 shows that it lies 

 on the extension of the N — N axis, so that, if the angle N — N — H is near the 

 expected tetrahedral value, hydrogen bond formation is geometrically impos- 

 sible, and the criterion of an abnormally short distance obviously cannot be 

 applied in this case to show the presence of a hydrogen bond. 



The geometry of the hydrogen bond system in hydrazine hydrochloride is 

 easy to grasp because of the high (cubic) symmetry of the crystals. For more 

 complicated crystals, those in which the picture is not quite so clear, I have 



Fig. 1. The structure of hydrazine dihydrochloride, N2H6CI2. The dumbbell shaped 

 groups represent the N2H6"'"'" groups, the larger spheres represent the chloride ions. 

 Hydrogen atoms are not shown. Note the staggered (ethane-like) configuration of the 

 chloride ions about the hydrazinium ions. 



