SOLVENT EFFECT ON RELATIVE N-ACIDITY AND O-ACIDITY - INVERSION OF THEDEPROTONATION SITE OF 2-[(2,4,6-TRINITROPHENYL)AMINO]BENZOIC AND 4-[(2,4,6-TRINITROPHENYL)AMINO]BENZOIC ACIDS

The deprotonation equilibrium of the title compounds (1 and 2, respect ively) was experimentally investigated in DMSO-water(DMSO = dimethyl s ulfoxide) mixtures by means of N-15 NMR relaxation, and theoretically by semiempirical (AM1) and ab initio (HF/3-21G) calculations, as isola ted species and in dimethyl sulfoxide or water using continuum solvati on models (AM1-SM2 and SCRF). Calculations predict the ionization to t ake place at the nitrogen atom for both acids in the gas phase. AM1-SM 2 results again indicate the nitroanion to be more stable in water, wh ereas SCRF calculations predict the oxyanion to be slightly favoured o ver the nitroanion in both solvents. NMR results indicate that the pre ferred ionization site of 2 is the amine nitrogen in Me,SO and the car boxylic oxygen in Me,SO-water 60/40, whereas deprotonation of 1 leads to a nitroanion with an intramolecular N-... H-O hydrogen bond.