The Hydrogen Bonded Structures of Two 5-Bromobarbituric Acids and Analysis of Unequal C5-X and C5-X′ Bond Lengths (X = X′ = F, Cl, Br or Me) in 5,5-Disubstituted Barbituric Acids

The crystal structure of the methanol hemisolvate of 5,5-dibromobarbituric acid (1MH) displays an H-bonded layer structure which is based on N-H center dot center dot center dot O=C, N-H center dot center dot center dot O(MeOH) and (MeOH)O-H center dot center dot center dot O interactions. The barbiturate molecules form an H-bonded substructure which has the fes topology. 5,5'-Methanediylbis(5-bromobarbituric acid) 2, obtained from a solution of 5,5'-dibromobarbituric acid in nitromethane, displays a N-H center dot center dot center dot O=C bonded framework of the sxd type. The conformation of the pyridmidine ring and the lengths of the ring substituent bonds C5-X and C5-X' in crystal forms of 5,5-dibromobarbituric acid and three closely related analogues (X = X' = Br, Cl, F, Me) have been investigated. In each case, a conformation close to a C5-endo envelope is correlated with a significant lengthening of the axial C5-X in comparison to the equatorial C5-X bond. Isolated molecule geometry optimizations at different levels of theory confirm that the C5-endo envelope is the global conformational energy minimum of 5,5-dihalogenbarbituric acids. The relative lengthening of the axial bond is therefore interpreted as an inherent feature of the preferred envelope conformation of the pyrimidine ring, which minimizes repulsive interactions between the axial substituent and pyrimidine ring atoms.