Crystal and molecular structure and infrared spectra of tetraethylammonium hydrogen phthalate and tetrabutylammonium hydrogen phthalate

The structures of two acid tetraalkylammonium phthalates were determined with X-rays. Tetraethylammonium hydrogen phthalate (C 16 H 25 NO 4 ) ( 1 ) has space group P 2 1 /c with a= 10.456(8) A, b =9.984 (6) A c =15.630 (5) A,β /1=97.13 (3)°, Z =4. Tetrabutylammonium hydrogen phthalate (C 24 H 41 N0 4 ) ( 2 ) has spacegroup P 2 1 / n with a =15.418 (7) A, b =9.142 (2) A, c =17.217 (6) A, /gb=98.75 (3)°, Z=4. Both compounds exhibit extremely short intramolecular hydrogen bonds between the neighbouring carboxylic groups with O⋯0 distances of 2.374 (4) A and 2.385 (3) A respectively. This particular bonding shows characteristic features in the IR spectra, which differ distinctly from the IR spectrum of tetramethylammonium hydrogen phthalate, the structure of which was previously found to form intermolecular hydrogen bonds. From spectroscopic analogy, it is concluded that tetrapropylammonium hydrogen phthalate forms intramolecular hydrogen bonds. The study of the homologous series of tetraalkylammonium hydrogen phthalates with the change of intermolecular (methyl compound) to intramolecular hydrogen bonding (ethyl, propyl and butyl compounds) allows conclusions to be drawn on the reasons for the establishment of the different types of conformations. The decreasing Coulomb force with increasing cation radius (going from the methyl to the butyl compound) reflects a decreasing tendency to rotate the carboxylic groups out of the coplanar configuration.