Natural abundance 15N and 13C spectroscopy. Aminobenzoic acids, substituted anilines, and related compounds

Abstract

Combined use of carbon-13 and nitrogen-15 NMR spectral data can give unique insight into the molecular dynamics of nitrogen-containing organic compounds and ions. 13C and 15N spin-lattice relaxation data and chemical shifts are reported for aniline and also for o-, m-, and p-aminobenzoic acids in solution as the cations, anions, and neutral compounds. 15N and 13C dipolar T 1 values obtained as a function of temperature for aniline and anilinium ion allowed estimation of the activation energy for overall molecular reorientation ( 13C T 1's) and for the composite internal-overall motion of the amine functions ( 15IN T 1's). The calculated values using both nuclei were the same: 3 kcal mole −1 for aniline and 4.8 kcal mole −1 for anilinium ion. '3C spin-lattice relaxation times T 1's for the neutral and ionized aminobenzoic acids probe inter- and intramolecular association through hydrogen bonding and ion pairing. 13C and 15N relaxation and shielding data give evidence for possible intramolecular association between the amine and carboxyl functions in ortho-aminobenzoic acid and its mono-ions. 15N T 1 and nuclear Overhauser enhancement (NOE) data indicate that aniline or substituted aniline NH 2 or NH 3 + groups undergo rapid but not free internal rotation (and/or inversion in the case of NH 2 groups), at rates at least comparable with the rate of overall reorientation for these systems ( τ c ≈ 10 −10 sec). o- and m-Aminobenzoic acids are effectively diprotonated in CF 3SO 3H solution. 15N T 1's) and NOE's for these two species indicate that the NH 3 + groups spin essentially freely in this highly acidic, nonnucleophilic medium.