The progressive intercalation of graphite by potassium has been studied by x-ray
diffraction methods. It has been established that potassium graphites of stages 4, 3,
2 and 1 (with intercalated potassium layers separated by 4, 3, 2 and 1 graphite
layers respectively) can be formed successively within a single graphite sample
encapsulated with excess potassium. Each stage was prepared in turn by adjusting the
temperature difference between the graphite and the potassium in a specially designed
double furnace. The graphite layer stacking sequences and repeat distances for stages
2, 3 and 4 are
stage 2
AB| BC| CA| A
c=26·25 Å
stage 3
ABA| ACA| A
24·20 Å
stage 4
ABAB| BCBC| CACA| A
46·35 Å
Within each intercalate layer the potassium atoms are distributed irregularly over
the centres of carbon hexagons in adjacent graphite layers.
The fully ordered structure of the stage 1 compound, A α A β A γ A δ A, has been
confirmed.
Uracil crystallizes in the monoelinic system with cell dimensions a = 11.82, b = 12.35, c = 3.62 A, fl = 120 ° and space group P2,/a. The structure was solved by trial and refined by successive threedimensional differential syntheses, a correction being applied to allow for series termination effects. The molecules are held together in layers by NH • • • O hydrogen bonds of length 2-81 and 2.86 A. The planar molecule is found to contain two different C-N distances, two bonds having a probable length of 1-379-t-0-006 A and the other two, of 1.342±0.008 A. The two C-C bonds are not significantly different and have a probable length of 1.409±0.009 A. The probable length of the C-O bonds is 1.236±0-007 A. Attention is drawn to the close similarity between the dimensions of the pyrimidine ring in adenine hydrochloride and in uracil.
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