Structures and vibrational spectra of indolecarboxylic acids. Part II. 5-Methoxyindole-2-carboxylic acid

Abstract

The crystal and molecular structure of 5-methoxyindole-2-carboxylic acid (5-MeOICA) are determined by single crystal X-ray diffraction analysis, infrared spectra and density functional (B3LYP) calculations. The title compound crystallizes in the monoclinic system, space group C2/ c, with a=13.079(3), b=7.696(2), c=35.185(7) A ̊ , β=91.1(1)°, V=3541.0(1) A ̊ 3 and z=16. There are two independent molecules of 5-MeOICA in the asymmetric unit cell. The molecular ribbons, constituted by two independent molecular chains, are held together by intermolecular O–H⋯O and N–H⋯O hydrogen bond interactions. The carboxylic O atom is the acceptor of two hydrogen bonds. In molecular ribbon, the relative orientation of the A and B carboxylic groups is nearly perpendicular, which leads to formation of the zig-zag pattern of H-bonds. The molecular layers, separated by about 3.344 Å, are arranged in stacks. Interestingly, the layers in the adjacent stacks are oriented in a herringbone-like pattern. The B3LYP-calculated bond lengths and angles are in good agreement with experimental data. Examination of the infrared spectrum of 5-MeOICA supports the conclusions from the X-ray diffraction studies. The characteristic bands at 3336, 1695 and 1206 cm −1 have been assigned to the stretching vibrations of the N–H, CO and C–O groups, respectively.