The crystal structure of a third polymorph (γ) of C.I. Pigment Red 1, 1-[(4-nitrophenyl)azo]-2-naphthol

Abstract

Abstract The crystal structure of a third polymorph (γ) of C.I. Pigment Red 1, 1-[(4-nitrophenyl)azo]-2-naphthol has been determined by X-ray diffraction techniques. It crystallises in the monoclinic system with cell parameters a = 7.715 ± 0.025 Å, b = 25.998 ± 0.081 Å, c = 6.964 ± 0.025 Å, β = 107.58 ± 0.17°. The space group is P21/c and there are four molecules per unit cell. The structure was solved using direct methods and least-squares refinement has been completed on three-dimensional data, 689 reflections (CuKα radiation). The hydrogen atoms have been found and included but not refined. The final residual is 8.4%. The molecule exists as the hydrazone tautomer and is approximately planar; the benzene and naphthalene groups are planar but are inclined at an angle of 10.8° with respect to each other. The molecules are linked by van der Waals forces. There would appear to be no relationship between this crystal structure and previously reported polymorphs.