'66 JERRY DONOHUE 



Fig. 2. The environment of the — NH3+ group in geranylamine hydrochloride shown 

 in stereographic projection. The numbers give the distances, in A, of each chloride 

 ion from the nitrogen atom. The assumed configurations of the amino hydrogen atoms 

 are also shown as filled circles connected to the nitrogen atom by solid lines. Note 

 that in projection the angles H — N — H are all 120°. The angles C — N- • -CI are all 

 seen to be a little less than tetrahedral, but the — NH3+ can be rotated so that each 

 N — H direction very nearly coincides with an N- • -CI direction. 



used the method of stereographic projection^ to show the environment of the 

 hydrogen-bond-forming groups. 



The close chloride ion neighbors of the — NH3+ groups in crystals of geranyl- 

 amine hydrochloride (Jeffrey, 1945) are shown in stereographic projection in 

 Fig. 2. The directions of the hydrogen atoms, assuming that the angles 

 C — N — H and H — N — H are all tetrahedral, are shown as small filled circles. 



* The properties and method of construction of stereographic projections may be 

 found in almost any standard work on morphological crystallography. The projec- 

 tion, as used in this discussion on amino acids, is made in the following way: The 

 a-carbon to amino-nitrogen atom covalent bond is chosen as the polar axis of a sphere, 

 with the NH3+ group in the center of that sphere. Lines to the bonded hydrogen atoms 

 and near oxygen neighbors of the nitrogen atom are then drawn from the center of 

 the sphere to its surface. This point of intersection is then connected by a straight line 

 with the opposite pole of sphere. The stereographic projection consists of the inter- 

 sections of these lines with the equatorial plane. The directions, relative to the C — N 

 bond, of the neighbors close enough to form hydrogen bonds to the nitrogen atom, 

 are then shown in the projection. If the angle C — N- • -O is 90°, the projected oxygen 

 atom lies on the circumference of the projection. Some feeling for angle values can 

 be obtained from some of the following figures by noting that the angles C — N — H 

 were all set equal to 109°28'. Oxygen atoms located so that the angle C — N- • -O is 

 greater than that value will project closer to the center of the projection. Since the 

 projection shows angular relations only, the distances of close neighbors are indicated. 



