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Effects of temperature on the ESR and ENDOR spectra of the DMPO–OBut spin adduct; the problems of identifying the trapped radical

The ESR and ENDOR spectra and hyperfine coupling constants (hfi) of the tertiary butoxyl spin adduct of 5,5‐dimethyl‐1‐pyrroline‐N‐oxide (DMPO) change markedly, but reversibly, with increase in temperature from 140 to 280 K, and the spectra can be cycled repeatedly between these temperature limits. The variation with temperature of the hfi of nitrogen, β and γ protons arises through a change in equilibrium concentrations of the planar and puckered conformers of the adduct pyrroline ring which are interconverting rapidly on the ESR time‐scale (107 MHz). A simple mathematical model has been derived which successfully simulates the experimental temperature variation of the coupling constants, gives their values in the two conformers and estimates of 4.3 ± 0.5 kJ mol−1 for the energy difference between the conformers in three solvents. Accurate hfi measurements by ENDOR spectroscopy across a narrow temperature range (140–160 K) corroborated ESR experiments and afforded the assignment of all magnetically inequivalent protons that have previously eluded observation. By using the di‐tert‐butyl tetroxide equilibrium, tROOOORt ⇌ 2tROO· as a clean source of tert‐peroxyl radicals, it was shown that these radicals are not trapped singly by DMPO to give a peroxyl spin adduct but give an ESR‐silent complex which decomposes at higher temperatures to the DMPO–OBut spin adduct. These studies highlight the problems of identifying the trapped radical from ESR spectra of spin adducts especially when the spectra are observed at only one temperature. Copyright © 2000 John Wiley & Sons, Ltd.