The x-ray crystal structure analysis of the monohydrate of the potassium salt of squaric acid is reported. Squaric acid is the name given to 3,4-diketocyclobutenediol. The main feature of the structure lies in the packing of the squarate anions. These are arranged in stacks with the planes of the anions almost parallel. Two of the carbon atoms, on neighboring anions, approach one another very closely, their separation being 3.237 Å (σ=0.012 Å). It is concluded that the structure represents a charge-transfer self-complex. The anion was found to have the expected D4h symmetry with mean CO bond length of 1.259 Å (σ=0.007 Å) and mean CC bond length of 1.457 Å (σ=0.008 Å).

The x-ray analysis of the crystal structure of 8-azaguanine monohydrate is reported. Bond lengths are given with standard deviations of 0.02 A. Bond angles are given with standard deviations of 1 to 2 degrees . All the hydrogen atoms in the structure were found. The hydrogen atom in the imidazo ring is on N9. N8 was found to take no part in the hydrogen bonding pattern in the crystal, but all other possible hydrogen bonds are formed. A very short separation (3.25 A) was observed between the planes of successive 8-azaguanine molecules. This distance is characteristic of charge-transfer complexes. A mechanism for the cell poisoning action of 8-azaguanine is suggested. The mechanism is based upon the assumption that there is an interaction of the charge-transfer type between molecules of 8-azaguanine.

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An automatic system is described for the measurement of intensities of X-ray reflections from single crystals. It is controlled by a programmable plugboard and punched paper tape output from an IBM 1620 computer. The goniometer used is the G.E. single crystal orienter. A circuit is provided which maximizes the counting rate before the intensity is measured.

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