Abstract
Electron population parameters for the atoms in four 1 : 1 hydrogen-bonded crystal complexes of barbital (with urea, acetamide, N-methyl-2-pyridone and imidazole) have been determined by the L-shell projection method. 1 1 R. F. Stewart, J. Chem. Phys., 53 (1970) 205. The X-ray intensity data (Cu Kα radiation, room temperature) were those used in the previously reported crystal structure determinations. Some significant differences (0.2 electrons or more) were found in the charges on chemically similar atoms. In the barbital molecule, CH hydrogen atoms are negatively charged with respect to NH hydrogen atoms, and the tetrahedral C(5) carbon atom is more negative than the carbon atoms to which it is bonded. The barbital oxygen atom charges are not significantly different from each other and are unchanged by participation in hydrogen bonding. However, for the NH donor groups in hydrogen bonds, nitrogen atoms in barbital are more negative than those in urea and imidazole, and vice versa for the hydrogen atoms.
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