Structural and theoretical studies of 4-chloro-2-methyl-6-oxo-3,6-dideuteropyrimidin-1-ium chloride ( ).

The title compound, CDClNO·Cl, crystallizes in the ortho-rhom-bic space group, , and consists of a 4-chloro-2-methyl-6-oxo-3,6-di-hydro-pyrimidin-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-di-hydro-pyrimidin-1-ium cation, including the monclinic polymorph of the title compound, which crystallizes in the monoclinic space group 2/ [Kawai (1973 ▸). , 663-666]. The cations and anions pack into sheets in the plane linked by N-H⋯Cl hydrogen bonds as well as C-H⋯O and Cl⋯O inter-actions. In graph-set notation, these form (11) and (9) rings. Theoretical calculations seem to indicate that the reason for the unusual angles at the C is the electrostatic inter-action between the oxygen atom and the adjacent N-H hydrogen.