An NMR study and ab initio molecular orbital calculation of substituted benzofuroxans and the salt of 4,6-dinitrobenzofuroxan

C-13, N-15 and O-17 NMR data are reported for a series of substituted benzo furoxans in aprotic and acidic solutions and for a potassium salt of a subs tituted benzofuroxan. Some of the title compounds can exhibit fast furoxan valence equilibrium at room temperature regardless of a solvent used, where as for the others no evidence of above-mentioned process exists. The NMR pa rameters most sensitive to salt formation are the chemical shifts of the C7 , N1, N3 and all of the oxygen nuclei. Hence these are reported as the most satisfactory chemical shifts to be used in distinguishing between the salt and non-ionic forms of the substituted benzofuroxans studied. Calculated e nergies at the self-consistent field (SCF) level of theory for both tautome ric forms (N1- and N3-oxide) of some compounds studied are used for predict ing the tautomeric equilibrium constants. Absolute O-17 shieldings are empl oyed in the reversal of the assignments of O-17 NMR signals existing in the literature. (C) 1999 Elsevier Science B.V. All rights reserved.