An X-ray study of the p-n-alkoxybenzoic acids. Part II. The crystal structure of anisic acid

Abstract

A three-dimensional X-ray study has been made of the crystal structure of anisic acid (p-methoxybenzoic acid). Crystals are monoclinic, a= 16·98 ± 0·03, b= 10·95 ± 0·02, c= 3·98 ± 0·01 Å, β= 98° 40′± 10′, space-group P2₁/a, Z= 4. The analysis was based on 687 reflections measured, with Mo-K radiation and balanced filters, by scintillation counting. Atomic co-ordinates obtained in a previous two-dimensional analysis were used as a starting point and the structure refined by Fourier, difference Fourier, and least-squares methods to a final R of 0·082.

Seven of the eight hydrogen atoms were located and anisotropic thermal parameters were used for the heavy atoms. The molecule is distorted from a planar configuration by intermolecular stresses. Thus, although the carbon atoms of the phenyl ring and the attached oxygen atom are strictly coplanar the atoms of the carboxyl group and the methyl carbon atom are displaced from this plane by between 0·05 and 0·16 Å. In addition, intramolecular stresses between the methyl-hydrogen atoms and the hydrogen atom adjacent in the ring make the C–C–O angles at the point of attachment of the methoxy-group unequal, 114 and 126°. Within the ring the C–C bond distances lie in the range 1·379–1·396 Å, and the extra-ring C–C bond is 1·502 Å. The e.s.d.s. in these distances are 0·009 Å. Other distances are: C–OH, 1·290 Å; CO, 1·233 Å; C–O, 1·360 Å; and O–CH₃, 1·443 Å, with e.s.d.s of 0·008 Å. Angles in the ring lie between 118·9 and 121·2°. Around the carboxyl group, O–C–O = 123·4°, C–CO = 119·1°, and C–C–OH = 117·5°. At the methoxyl oxygen atom, C–O–C = 116·8°. In the crystal the molecules form hydrogen-bonded dimers, O–H ⋯ O = 2·643 Å, and these dimers are intermeshed closely, in the region of the O–H ⋯ O linkages with the methyl groups of neighbouring molecules. It is suggested that this feature prevents the two lowest members of the series from forming liquid crystals but that a similar interleaving of molecules is needed to cause the higher homologues to show such behaviour.