| Summary: | A new monoclinic form of 4-nitrophenylacetic acid, C8H7NO4, (I), differs from the known orthorhombic form both in its molecular conformation and in its intermolecular contacts. The conformation is different as the plane of the carboxylic acid group in (I) is more nearly perpendicular to the plane of the aromatic ring [dihedral angle = 86.9 (3)°] than in the previous form (74.5°). Both polymorphs display hydrogen-bonded R22(8) carboxylic acid dimeric pairs, but in (I), neighbouring dimers interact through nitro–nitro N...O dipole–dipole contacts rather than the nitro–carbonyl contacts found in the orthorhombic form.
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