Nitrogen and carbon NMR of some benzofuroxans

Abstract

15N and 14N NMR spectra show that there is no valence tautomerism involving the nitro group in nitro derivatives of benzofuroxan systems and that the N-oxide function has a fixed position in the furoxan ring. Carbon chemical shifts of molecules with one, two or three furoxan rings attached to a benzene ring reveal additivity of effects which makes possible a complete and unambiguous assignment of the shifts.

Tautomerism and 1H, 13C and 15N NMR spectral assignments of some nitro derivatives of malonic acid diamide

“…Two other signals detected in the 13 C NMR spectrum at υ 110.1 and 151.5 ppm are characteristic of the furoxan carbon atoms C-6 and C-5, respectively. 14 -17 The structure is confirmed by the two lines of equal integration detected in the 15 N NMR quantitative spectrum at υ 4.7 and 18.5 ppm, which are compatible with nitrogen N-2 and N-4 respectively.…”

“…All experimental chemical shifts extracted from the 1 H, 13 C and 15 N NMR spectra of molecules 1-5 are given in Table 1. Chemical shifts of the tautomers 2-4 calculated with the ACD program are also given for protons (except exchangeable protons, which are generally dependent on solvent effects) and all carbons.…”

“…This is consistent with the existence of an almost symmetrical structure with the acid proton equally spaced between the two oxygen atoms (as in the solid state) or alternatively with a rapid equilibrium between the two enolic forms of 3 which makes positions 2 and 4 chemically equivalent. 15 …”

“…Depending on the spectrometer, excitation pulses in the range 6.5-7 µs (90°flip angle) and a recycle time of 10 s were used for direct 13 C measurements. For direct 15 N measurements, short pulses of about 7 µs (30°fl ip angle) and a long recycle time (>30 s) were used. The following parameters were used for 13 …”

Localization of N‐oxide groups by means of nitrogen chemical shifts

“…sample tubes, using the procedure described in detail elsewhere. 8 The data are quoted as nitrogen shieldings, referred to external nitromethane,' rather than as chemical shifts in order to avoid sign confusion.…”