Hydrogen Bonding 



Jerry Donohue 



University of Southern Calijornia, Los Angeles, Calif. 



MUCH OF THE DISCUSSION SO far has been concerned with macromole- 

 cules, but we shall now bring the scale down a bit and discuss smaller 

 molecules that have only a dozen or so atoms, because I think that 

 unless we can understand what goes on in these simple substances there isn't 

 much point in trying to worry about the much more complicated systems. 



Before discussing the properties of hydrogen bonds in various kinds of crys- 

 tals and molecules, it will be useful to set forth certain general principles which 

 govern the formation of such bonds. In most biological systems, hydrogen bond 



\-^ \ 



donors are the groupings — ^N — H, N — ^H, or — O — H, while hydrogen bond 



^ ^. 



acceptors are generally oxygen or nitrogen atoms, although chloride ion ac- 

 ceptors occur also. One sort of evidence which was formerly invoked to demon- 

 strate the existence of a hydrogen bond was the occurrence of an abnormally 

 short intermolecular distance, such as is found in the dimers of carboxylic 

 0-H— O 



/- \ 



acids R — C C — R. Here the distance between oxygen atoms is 



\ /- 



O— H-0 



several tenths of an Angstrom shorter than for a normal van der Waals contact. 

 Since the hydrogen atoms in a crystal are not in general located directly by 

 X-ray crystallographic methods, their positions can be inferred from the posi- 

 tions found for the heavier atoms in the following way. If the reasonable as- 

 sumption is made that the geometrical properties of the hydrogen bond donor 

 group are not appreciably altered by hydrogen bond formation, then the re- 

 sultant groupings, O — H • • • 0, N — H • • • O, and the like, are expected to be 

 very nearly linear. Variation of more than about 25° from linearity is rare. It 

 follows that the directly observed angles, such as C — 0- • -O in the carboxylic 

 acid dimers, will be close to those expected for the covalent bond angle, 



H 



C — O — H. In the case of the donor group C — N — H all six angles (three 



\ 

 H 



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