An EPR and MS Investigation of Hexa-Substituted Pyrrolidine-l-oxyl Aminoxyl Stable Radicals. Unexpectedly Large y-Hydrogen Splittings

Abstract

The y-hydrogen hyperfine splitting constant (γ-H hfsc) for stable aminoxyl (nitroxide) spin labels such as 2,2,5,5-tetramethylpyrrolidine-1-oxyl and their derivatives is usually very small (<1.0 G) and not distinguished with EPR spectrometry. Surprisingly, large γ-H hfsc’s (≥2 .0 G) have been detected with EPR for the first time from ten 2-alkyl-2-phenyl-3,3,5,5- tetramethylpyrrolidine-1-oxyl stable aminoxyl radicals. It is discovered that γ-H hfsc’s are very sensitive to the size and the substitution pattern of 2-alkyl groups. When the 2-alkyl group is CH3 or CD3, γ-H hfsc’s are not resolved in the EPR spectra. But if the 2-alkyl group is C2H5, one γ-H hfsc is very large, equal to 4.72 G in C6H6. If the substituent is longer than C2H5, such as n-C3H7, n-C4H9, n-C5H11, n-C6H13 and CH2=CHCH2 - substituents, the γ-H hfsc is slightly smaller, equal to 4.59 G. For secondary substituents such as sec-C4H9 and cyclo-C6H11 , the γ-H hfsc decreases to 2.00 G. Intermediate γ-H hfsc’s correspond to C6H5CH2 (3.18 G) or a tertiary alkyl group such as t-C4H9 (3.47 G). Variation of γ-H hfsc’s is based on the change of the pyrrolidine ring conformation which is a result of the 2-alkyl group influence. The structures of these aminoxyl radicals are characterized also with mass spectrometry. Possible MS fragmentation mechanisms are discussed.