| Summary: | The title compound, C5D6ClN2O+·Cl−, crystallizes in the orthorhombic space group, Pbcm, and consists of a 4-chloro-2-methyl-6-oxo-3,6-dihydropyrimidin-1-ium cation and a chloride anion where both moieties lie on a crystallographic mirror. The cation is disordered and was refined as two equivalent forms with occupancies of 0.750 (4)/0.250 (4), while the chloride anion is triply disordered with occupancies of 0.774 (12), 0.12 (2), and 0.11 (2). Unusually, the bond angles around the C=O unit range from 127.2 (6) to 115.2 (3)° and similar angles have been found in other structures containing a 6-oxo-3,6-dihydropyrimidin-1-ium cation, including the monclinic polymorph of the title compound, which crystallizes in the monoclinic space group P21/c [Kawai et al. (1973). Cryst. Struct. Comm. 2, 663–666]. The cations and anions pack into sheets in the ab plane linked by N—H...Cl hydrogen bonds as well as C—H...O and Cl...O interactions. In graph-set notation, these form R33(11) and R32(9) rings. Theoretical calculations seem to indicate that the reason for the unusual angles at the sp2 C is the electrostatic interaction between the oxygen atom and the adjacent N—H hydrogen.
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