70 JERRY DONOHUE 



lies between the other two close oxygen neighbors, giving a bifurcated hydrogen 

 bond. It was later pointed out that the normal coordination number to be ex- 

 pected for hydrogen is two, and that bifurcated hydrogen bonds should occur 

 rarely (Pauling, 1940). It is possible that a refinement of glycine structure would 

 remove this rather unusual kind of hydrogen bond, since the original study was 

 made many years before the present refinement techniques had been developed. 



There have been several other compounds in which bifurcated hydrogen 

 bonds were postulated. Iodic acid (Rogers and Helmholz, 1941) is such an ex- 

 ample: an alternate interpretation of the X-ray results was later given by Wells 

 (1949), who showed that the data were consistent with an arrangement without 

 the bifurcated hydrogen bonds. The same is true in the case of dicyandiamide 

 (Hughes, 1940), where the original interpretation included a bifurcated hydro- 

 gen bond, but it has since been shown (Donohue, 1952) that this interpreta- 

 tion is probably not correct. 



Recently there appeared the results of the determination of the structure of 

 sulfamic acid (Kanda and King, 1951), in which there were said to be two bi- 

 furcated hydrogen bonds. The results were said to indicate that in the crystals 

 the molecule was in the zwitter ion form, ""O3S — NH3+. The environment of 

 one sulfamic acid molecule with the assumed hydrogen bonds is shown in Fig. 9. 

 Clearly we have here a very complex situation : each molecule has twelve close 

 neighbors; normally we would expect three close hydrogen bond acceptors and 

 three close hydrogen bond donors, leaving the other six neighbors as van der 

 Waals contacts, if there are no bifurcated hydrogen bonds. The environment 

 of the nitrogen atom, projected down the S-N axis, is shown in Fig. 10. In 

 this and the following figures, the notation of the original paper has been re- 



FiG. 8. The environment of the — NH3+ group in glycine shown in stereographic 

 projection. The oxygen atom at 3.06A is shown as an open circle, indicating that the 

 angle C— N- • -O is a little less than 90°. 



