13C-Hyperfine Splitting in Semidiones

Abstract

A number of ketyls and semidiones containing 13C have been examined by ESR spectroscopy. Grossly different values of aCOC in the ketyls (∼ 50 G) and the semidiones (∼ 1 G) allow for an easy differentiation of the two species. This is important because strained ring ketyls can undergo a decomposition reaction leading to the semidione with one additional carbonyl group added to the ketyl. Values of aCOC, aαC, and aβC for keyls and semidiones are reporduced by the Karplus–Fraenkel equation if the term QCC′C is allowed to increase by 15% when C′ is altered from methyl to t-butyl. No evidence for the nonplanarity of di-t-butyl ketyl could be deduced. Some unusual long-range 13C hfsc (aγC) have been detected in certain bicyclic semidiones possessing a nearly coplanar, zigzag arrangement of three carbon–carbon bonds with a pz orbital.

Couplage hyperfin et relaxation du carbone 13 dans les complexes alkylanilines-Ni II

“…La variation angulaire du couplage hyperfin des carbones fl a 6t6 6tudi6e par RPE [18][19][20][21][22] et RMN [23,24] sur des radicaux libres. La validit6 de la loi en cos z 0 pr6sum6e par diff~rents auteurs [18][19][20][21][22][23][24] est remise en question dans un travail r6cent sur les complexes alkylanilines-ac6tylac6tonate de nickel [25]. Nous nous sommes propos6s de r6examiner ce point ~t l'aide d'une m6thode simple ne faisant intervenir que les d6placements de contact relatifs (ou les constantes de couplage hyperfin) du carbone a et des protons et carbones fl des groupernents alkyles de ces m~mes complexes, sans introduire explicitement la densit6 de spin sur le carbone adjacent du cycle ph6nyle.…”

Long-range proton hyperfine coupling

“…The experiment values (absolute) are given in brackets. For 119 the proton coupling constants are in satisfactory agreement, although the 13C splittings compare less favorably with the results of Russell et al 270 Part of the discrepancy for the 13C splittings may result from the neglect of inner-shell polarization in the INDO scheme. The calculated results shown are the best of several possible sets given by Underwood and Vogel for different geometries.…”

“…Such contributions could not be neglected in a discussion of other nuclei, for example, 13C hyperfine splittings.269 There is a scarcity of experimental work for such hyperfine splittings for situations of interest in long-range coupling. 270 Specific comments about the quantitative contributions of the various mechanisms depend on the computational approach employed. The results for semiempirical and ab initio studies are described below.…”

Long-range proton hyperfine coupling in bicyclic radicals

“…Contributions to the hyperfine coupling might also be expected to arise from polarizations of inner core electrons. While this type of effect will be important in 1~C hyperfine interaction [15] we will be primarily interested in the proton hyperfine coupling of bicyclic systems and such core polarizations need not be considered. The procedure to be followed is a non-empirical ASMO-CI approach, in which case the configurations corresponding to electron polarization or electron transfer can be readily formulated.…”