The ¹⁴N and ²H nuclear quadrupole resonance spectra of anthranilic acid

The ¹⁴N quadrupole resonance spectra have been obtained at 77 K for all three polymorphic forms of anthranilic acid (o-aminobenzoic acid). In addition, ²H spectra were obtained for forms I and II in which the exchangeable hydrogens were replaced by deuterium. It was not possible to obtain a sample of deuterated form III. In form I, nonequivalent nitrogens with e²Qq_zz/h values of 1602 and 2999 kHz were observed. These correspond to the nitrogens of the zwitterionic and neutral molecules, respectively. Forms II and III give rise to only one type of nitrogen each, with e²Qq_zz/h values of 3607 and 3705 kHz, respectively. The NQR observations are entirely consistent with previous X-ray studies of the polymorphs. The ²H NQR spectra are complex due to the presence of nonequivalent deuteriums which are dipolar-coupled. In form I the ND₃⁺ group gives rise to e²Qq_zz/h values of 130.2, 143.4 and 155.3 kHz, respectively. For the ND₂ groups of this polymorph, the values are 203.2 and 212.4 kHz, respectively. Form II yields deuterium quadrupole coupling constant values for the -ND₂ group of 227.1 and 229.8 kHz, respectively. The O-D groups in forms I and II give rise to quadrupole coupling constants of 184.0 and 172.2 kHz, respectively. The O-D groups in forms I and II give rise to quadrupole coupling constants of 184.0 and 172.2 kHz, respectively. In the deuterated solids satellite lines are seen on the ¹⁴N transitions, as a result of simultaneous spin flips of the ¹⁴N and directly bound ²H. These lines are of help in assigning the complex spectra in the region of the pure ²H transitions, where dipolar coupling of closely located, slightly nonequivalent dueteriums gives rise to a multiplet structure. Several examples of double transitions, involving simultaneous spin flips of deuteriums, are also reported. The deuterium quadrupole transitions can be assigned to deuteriums bound to specific atoms, and to specific hydrogen bond locations within the structures of forms I and II.