o-Fluorobenzoic acid crystallizes in the monoclinic system, space group P21/c, with four molecules in a unit cell of dimensions a=6.71, b=3"81, c=25.05 A;/~=99"9 °.The detailed crystal structure was determined by normal and generalized electron-density projections along the a and b axes and three-dimensional parameter refinement. The calculations are based on 356 observed structure factors. The molecules occur as centrosymmetrical dimers with normal hydrogen bonds between adjacent carboxyl groups. The molecular structure of o-fluorobenzoic acid is overcrowded. The strain in a planar model for the molecule is relieved by a 21 ° twisting of the carboxyl group out of the plane of the aromatic ring, and by valency-angle distortions. The presence of hydrogen atoms including that in the hydrogen bond has been demonstrated. There is no intermolecular hydrogen bridge between the fluorine atom and the carboxyl group, but a strong van der Waals interaction can be assumed between the fluorine and the adjacent hydrogen atom.
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