Structural studies of analgesics and their interactions. I. The crystal and molecular structure of antipyrene

Abstract

Antipyrine, C₁₁H₁₂N₂O, one of the best known pyrazole derivatives used as pain-relieving medicines, crystallizes in the monoclinic space group C2/c, with eight molecules in a unit cell of dimensions a = 16.919, b = 7.425, c = 17.796 Å and β = 117.03°. Three-dimentional intensoty data from the crystal were collected on a 4-circle diffractometer. The structure was solved by the symbolic addition method and the atomic parameters, including those of the hydrogen atoms, were refined to an R value of 0.600 for 1775 reflexions. The molecular geometry of antipyrine in this structure differs substantially from that observed in some metal-antipyrine complexes. First, the differences in the dimensions of the pyrazolone ring suggest that antipurine is considerably less polar in the free state than in the complexes. Secondly, the two nitrgen atoms in the pyrazolone ring are more pyramidal in free antipyrine than in its complexes. Consequently, the conformation of the molecule is signifucantly different in two cases. The molecular structure of antipyrine is also compared with that of 3-methyl-3-pyrazoline-5-one. Antipyrine is one of the few molecules without internal symmetry which crystallize in the space group C2/c. The molecular packing in the structure can be most adequately described as consisting of layers of molecules parellel to the (204̅) plane. The molecular coordination number in this arrangement is 11.