NMR solvent effect study of pyrroline‐N‐oxide spin traps: Microenvironments in sodium dodecyl sulfate micelles

Abstract

Complete 13C NMR assignments for ten five-membered ring nitrone (pyrroline-N-oxide) spin traps in CHCl, are presented. A good correlation between the nitronyl "C NMR chemical shift (6) and the substituent constant (a) for the substituted phenyl DMPOs was found. The "C NMR chemical shift of the nitronyl carbon of these 5 , 5dimethyl-2-phenylpyrroline-N-oxides (or phenyl-DMPOs) exhibited a large solvent effect (14 ppm from cyclohexane to water) which could be correlated with the solvent polarity parameter EJ30). Using a paramagnetic relaxation agent, the weighted time-averaged fast exchange of the nitrone between the interior of a sodium dodecyl sulfate micelle and the bulk aqueous phase could be determined. It was found that the 2-phenylpyrroline-N-oxides reside in microenvironments with polarities close to that of ethanol, methanol and between methanol and water. The fraction of nitrone completely inside the micelle at any given time was found to be around 77 0.05%. This is the first time that the paramagnetic relaxation enhancement method has been used with "C NMR spin-lattice relaxation time measurements.

Deuterium isotope effects on 13C NMR chemical shifts of some α‐(2‐hydroxyaryl)‐N‐phenylnitrones (Schiff base N‐oxides)

“…N-(Salicylidene)aniline N-oxide (1), N- (3,5-dichlorosalicylidene) aniline N-oxide (2) and N- (3-methoxysalicylidene)aniline N-oxide (3) have been obtained previously. 4 N- aniline N-oxide (4) was synthesized by the reaction of 2-hydroxynaphthaldehyde with phenylhydroxylamine in ethanol 19 and crystallized from ethanol (m.p.…”

“…The chemical shifts of the C-˛atom in˛-aryl-N-phenylnitrones are very sensitive to the intramolecular hydrogen bond and intermolecular interactions. 1,2,4 The signal assigned to C-˛in N-oxides of Schiff bases is shifted to lower frequencies by about 20 ppm relative to that in the spectra of the parent bases (Table 3), but at the same time is it less shielded than in˛-aryl-N-phenylnitrones without the intramolecular hydrogen bond.…”

“…In this and some other Mannich base N-oxides in solution, a single-minimum intramolecular hydrogen bond is present. 6,7 In order to verify the existence of a proton transfer equilibrium in the Schiff base N-oxides N-(salicylidene)aniline N-oxide (1), N- (3,5-dichlorosalicylidene)aniline N-oxide (2), N- (3-methoxysalicylidene)aniline N-oxide (3) and N-(2-hydroxynaphthylidene)aniline N-oxide (4) in solution, we have reinvestigated their 1 H and 13 C NMR spectra (Table 2) and undertaken an investigation of the deuterium isotope effect on the 13 C chemical shifts. Measurement of the deuterium isotope effect is known to be a very good tool in studying intramolecular hydrogen bond and proton transfer equilibria.…”