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Theoretical expressions for two types of discrepancy factors have been obtained for crystals containing both heavy and light atoms in the unit cell. One of the discrepancy factors is defined in terms of the structure amplitude and is called the Booth's reliability index (RD. The other discrepancy factor (denoted by R2) is based on intensities. While the expressions for R2 can be used for crystals of any space group and for crystals containing any number and type of atoms in the unit cell, those for Rn can be used only for crystals in the triclinic and a few monoclinic space groups.

The synthesis of aryl‐bis(6‐amino‐1,3‐dimethyluracil‐5‐yl)‐methanes 3a‐m by condensation of 6‐amino‐1,3‐dimethyluracil (1) with aromatic aldehydes 2a‐m at room temperature is reported. The structures of the compounds were established using various spectroscopic analyses and X‐ray crystallography. The crystal structures of two aryl‐bis (6‐amino‐1,3‐dimethyluracil‐5‐yl) methanes are presented.

The probability distributions of the normalized Bijvoet differences x and A are derived for non-centrosymmetric crystals with type-I and type-II degree of centrosymmetry assuming that there is a single anomalous scatterer in the unit cell besides a sufficiently large number of similar normal scatterers. The theoretical results are used to study the influence of degree of centrosymmetry on the measurability of Bijvoet differences. It is found that, for given values of k and a 22, the measurability is in general poorer in the type-I case than in the type-II case.

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