AN EPR AND MS INVESTIGATION OF HEXA-SUBSTITUTED PYRROLIDINE-1-OXYL AMINOXYL STABLE RADICALS - UNEXPECTEDLY LARGE GAMMA-HYDROGEN SPLITTINGS

The gamma-hydrogen hyperfine splitting constant (gamma-H hfsc) for sta ble aminoxyl (nitroxide) spin labels such as 2,2,5,5-tetramethylpyrrol idine-1-oxyl and their derivatives is usually very small (<1.0 G) and not distinguished with EPR spectrometry. Surprisingly, large gamma-H h fsc's (greater than or equal to 2.0 G) have been detected with EPR for the first time from ten 2-alkyl-2-phenyl-3,3,5,5en tetramethylpyrroli dine-1-oxyl stable aminoxyl radicals. It is discovered that gamma-H hf sc's are very sensitive to the size and the substitution pattern of 2- alkyl groups. When the 2-alkyl group is CH3 or CD3, gamma-H hfsc's are not resolved in the EPR spectra. But if the 2-alkyl group is C2H5, on e gamma-H hfsc is very large, equal to 4.72 G in C6H6. If the substitu ent is longer than C2H5, such as n-C3H7, n-C4H9, n-C5H11, n-C6H13 and CH2=CHCH2- substituents, the gamma-H hfsc is slightly smaller, equal t o 4.59 G. For secondary substituents such as sec-C4H9 and cyclo-C6H11, the gamma-H hfsc decreases to 2.00 G. Intermediate gamma-H hfsc's cor respond to C6H5CH2 (3.18 G) or a tertiary alkyl group such as t-C4H9, (3.47 G). Variation of gamma-H hfsc's is based on the change of the py rrolidine ring conformation which is a result of the 2-alkyl group inf luence. The structures of these aminoxyl radicals are characterized al so with mass spectrometry. Possible MS fragmentation mechanisms are di scussed.