¹H and ¹⁴N ENDOR spectra of tert-butylphenylnitroxide

Abstract

This paper is dedicated to Professor Charles A. McDowell or1 the occasion of his seventieth birthdayJUN YAMAUCHI and Y~s u o DEGUCHI. Can. J. Chem. 66, 1862 (1988 IH and I4N ENDOR spectra of tert-butylphenylnitroxide were observed in ethylbenzene solution. The experimental conditions for detecting optimum 'H and I4N ENDOR lines were quite different. The 'H hyperfine coupling constants and their signs are discussed from the viewpoints of molecular orbital calculations, general triple resonance, CRISP (cross relaxation intensity sequence pattern) method, and steric hindrance. In view of the large anisotropic hyperfine interaction between the nitrogen and the unpaired electron, detection of I4N ENDOR in nitroxide radicals is rather difficult, so that a TMI mode cavity was utilized in this experiment. The observed 14N hyperfine coupling constant 11.86 G at -80°C is one of the smallest values observed to date for nitroxide radicals.JUN YAMAUCHI et Y~s u o DEGUCHI. Can. J. Chem. 66, 1862Chem. 66, (1988.Operant en solution dans I'Cthylbenzene, on a determint les spectres ENDOR 'H et I4N du nitroxyde de tert-butyle et de phenyle. Les conditions experimentales permettant des dttections optimales des raies ENDOR du 'H et du I4N sont trks differentes. On discute des constantes et des signes du couplage hyperfin du I H a I'aide de calculs d'orbitales moltculaires, de la rtsonance gtntrale triple, de la mtthode CRISP (patron de la sequence des intensites de la relaxation croiske) et de l'emp&chement sterique. A cause de l'interaction anisotropique hyperfine importante qui existe entre I'azote et 1'Clectron qui n'est pas apparit, la detection des spectres ENDOR du I4N, dans les radicaux nitroxydes, est plutbt difficile; pour cette experience, on a donc utilise un mode de cavitt TMllo. La constante de couplage hyperfin observe pour le I4N, i i -80°C, est tgale a 11,86 G et elle est l'une des plus faibles valeurs observtes a date pour des radicaux nitroxydes.[Traduit par la revue]