Application of electron spin resonance spectroscopy to problems of structure and conformational analysis: polysemiquinones and macrocyclic semidiones
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Unpaired electron spin distribution in polysemiquinones of varying geometric disposition is discussed. Particular emphasis is placed on the dependence of spin distribution on the acceptor strength, temperature, and reduction state of the radical ions. ESR spectra of diradical species in which separation between the unpaired electrons does not exceed 10 (ANGSTROM), and which nevertheless exhibit sharp ESR transitions in non-viscous media are displayed. The absence of significant line broadening in these spectra has been attributed to a minor contribution of dipolar relaxation to the overall linewidth;Conformational motion in macrocyclic semidiones derived from acyloins of paracyclophanes has been investigated. ESR spectra for the E-semidione derived from 1,4-(5'-hydroxy-6-ketodecamethylene)-benzene reveal a rigid, asymmetric structure at temperatures as high as 370 K. Higher homologs such as those derived from 1,4-(8'-hydroxy-9'-ketohexadecamethylene)-benzene are considerably more flexible, with partial equilibration of the (alpha)-hydrogens taking place at subambient temperatures. In the E-semidione derived from 1,4-(12'-hydroxy-13'-ketotetracosane-methylene)-benzene, internal rotation (a process which involves rotation of the semidione moiety through the cavity of the carbocycle) is rapid on the ESR time scale at 350 K;Efforts to detect valence isomerization by ESR spectroscopy in a number of 7-substituted bicyclo 4.1.0 hept-3-ene-2,5-semidiones were unsuccessful, due possibly to rapid decay of the paramagnetic ions, once formed. The ESR spectrum of bullvalene semidione at 330 K reveals a non-fluxional ion, with an upper limit of 1 x 10('7) sec('-1) for the rate constant of the degenerate Cope rearrangement.