72 JERRY DONOHUE 



tained. The numerical prefixes for each atom refer to the several different equiv- 

 alent sulfamic acid molecules in the unit cell; molecule 3 was the one chosen 

 for examination. Fig. 10 shows, accordingly, the atoms 30i and SOo and 

 3O3 (dotted, since they are below the reference plane) and the five close oxygen 

 neighbors from other molecules, of the nitrogen atom, 3N. The — NH3+ group 

 has been rotated so that the N — H bonds point as nearly as possible toward 

 the oxygen atoms. The contacts 3N • • • 6O1 , and 3N • • • 50i are possible hy- 

 drogen bonds, but this leaves the third hydrogen atom in an unfavorable po- 

 sition; moreover, the contact 3N • • • 4O3 obviously cannot possibly correspond 

 to a hydrogen bond. Note also that the best orientation of the — NH3+ is 

 eclipsed with respect to the — SO3 group. 



Strangely enough, there has not been too much work on whether sulfamic 

 acid is indeed a zwitter ion. The only evidence cited is its high melting point of 

 about 200°C, but this is not really very conclusive, as there are other amides 

 which melt even higher — oxamide, for example, melts at well over 400°C. 



Since assumption of the zwitter ion formula does not lead to a very satis- 

 factory hydrogen bonding system, the possibility should be explored that atoms 

 bonded to the central sulfur atom have been incorrectly identified. (The X-ray 

 method is capable of distinguishing between atomic numbers 7 and 8 only if 

 the intensities have been carefully estimated and if the refinement is carried out 



HO 



to a sufficient degree.) We shall then have the formulation O — S — NH2 , 



/ 

 



and should expect the S — OH bond distance to be quite a bit longer than both 



the S — NH2 and the two S — O bonds, all of which should all be approximately 



the same length. The one long bond to the sulfur atom (see Fig. 9) then fixes 



the atom labelled "N" as the hydroxyl oxygen atom, and the problem is now 



reduced to deciding which of the atoms labelled "O" is the amide nitrogen. It 



is found that one of the two close neighbors of atom "O2", namely, atom lOi , 



is in a position such that the angle S — O2 • • • O, is 175°; atom O2 , therefore, 



cannot be the amide nitrogen. This situation is not found with regard to either 



atoms Oi or O3 . The stereographic projection of the environment of atom Oi 



is shown in Fig. 11. If atom Oi is really the amide nitrogen, then we should 



expect a planar system of bonds about it, and in projection this atom and those 



with which it is forming hydrogen bonds should be on a straight line. This is 



almost achieved, and the corresponding hydrogen bonding is shown in Fig. 12. 



There is, on the other hand, the possibility that atom "O3" is the amide ni- 

 trogen, since the projection of its environment, shown in Fig. 13, is not un- 

 satisfactory. The corresponding hydrogen bonding is shown in Fig. 14. 



Since the accuracy of the original investigation is of the order of ±0.1 A 

 it is not possible to decide which of the above possibilities is the right one, but 



