Daniel S. Leniart

An ESR and ENDOR study of spin relaxation of semiquinones in liquid solution

A quantitative test of the ENDOR theory for free radicals in solution has been developed experimentally using the semiquinone (SQ) radical anions of parabenzo‐ (PBSQ), duro‐ (DSQ), and 2,5‐dimethyl‐para‐benzo‐ (2,5‐DPMBSQ) dissolved in ethyl alcohol (EtOH) and dimethoxyethane (DME) solvents. It is shown that, in general, an ENDOR signal arising from a molecule containing four or less equivalent nuclei, such as PBSQ, can be analyzed rigorously whereas a molecule containing more than four equivalent nuclei, such as DSQ, which, in principle, could be analyzed rigorously, practically is best analyzed using approximate forms of the ENDOR theory. It is shown that 2,5‐DMPBSQ may be analyzed using a combination of both the rigorous and approximate forms of the ENDOR theory. The analysis involves calculation of the ENDOR relaxation parameters, T2n, Ωn, and Ωe,n, from the experimental ENDOR percent enhancement and linewidth studies. A comparison was made by performing the ESR linewidth and saturation studies to obt...

Theory of saturation and double resonance in ESR spectra. V. The average ENDOR and ELDOR lines

The general theory for the analysis of saturation and double resonance effects in the ESR spectra of dilute solutions of free radicals developed in the earlier papers in this series, has been solved approximately to obtain relatively simple formulas for the general case of free radicals with many nuclei. The solution is based on an expansion linear in b ≡ Wn/We (where Wn and We are, respectively, the lattice‐induced pure nuclear‐spin and electron‐spin flip relaxation rates), but it is asymptotically valid for Heisenberg and (polarized) chemical exchange processes. Also, coherence effects are neglected. The resulting expressions for ENDOR take the simple form of a single average (saturated) Lorentzian for each distinct ENDOR line obtained from a particular group of equivalent nuclei; while for ELDOR one similarly obtains an average ELDOR line shape. Expressions are given for the average saturation parameters that are needed in terms of the various relaxation mechanisms. The range of validity of these solut...

Liquid phase nitrogen endor in nitroxide radicals

Abstract Electron-nuclear double resonance (ENDOR) signals have been observed from nitrogen nuclei of several nitroxide radicals in the liquid phase. The rf enhancement factor, which increases the effective radio frequency field at the nucleus by a factor of 21 in the present case, makes these observations possible. Cross relaxation, arising from non-secular terms of the dipolar spectral density, is the dominant Δ m I = 1 relaxation mechanism. The data are sufficiently precise that dynamic second order shifts of the observed ENDOR transitions must be considered.

Applications of ENDOR spectroscopy to radicals of low symmetry: Radical anions of 2-phenylcycl[3.2.2]azines☆

Abstract The radical anions of 2-phenylcycl[3.2.2]azine and 6-methyl-2-phenyl-5-azacycl-[3.2.2]azine were prepared by reaction with Na and with Li. Proton hyperfine couplings have been determined using electron-nuclear double resonance (ENDOR) spectroscopy. Assignments of couplings have been made by a combination of four methods: radio frequency coherence effects, calculations of relative ENDOR intensities, computer simulation of the ESR spectra, and MO calculations of π-spin distribution. Successful use of the two first methods has experimentally corroborated the theory of ENDOR line-shapes in the presence of both saturating and non-saturating nuclear radio frequency fields.

ESR and ENDOR studies on the 2,2′-binaphthyl radical anion in solution☆

ESR and ENDOR studies of the 2,2′-binaphthyl radical anion in tetrahydrofuran in the temperature range −50 to −90°C showed the presence of only one paramagnetic species with no evidence of ion pairing or of hindered rotation such as occurs in the phenyl naphthalenes. Extended Huckel molecular orbital calculations of energy as a function of the dihedral angle between the two naphthyl parts suggest that the species has four stable conformations with angles of 45, 140, 220, and 315° and jumps rapidly among them at a rate in the range of 107 to 109 sec−1. The proton coupling constants calculated by the McLachlan method, together with the experimental values, provided an independent estimate of the stable angles which agreed closely with the values derived from the energies. As in other cases involving the nonplanar bonding of aromatic rings these calculations are only partially successful in predicting the coupling constants. Neither inversion or near degeneracy of the orbitals nor breakdown of σ−π separability seems able to account for the discrepancies. The g value of the radical was measured. It is slightly larger than the value predicted by Stones theory in contrast to a number of non-planar radicals which have smaller values.

Instrumentation and Experimental Methods in Double Resonance

The endor technique was introduced by Feher(1) for the investigation of single crystals in 1956 and continued through its embryonic stages to be used as a tool for the investigation of solid state phenomena. In 1963, the technique was extended to systems in the liquid phase by Cederquist(2) as applied to metal-ammonia solutions. In 1964, Maki chose the organic radical galvanoxyl as a candidate for endor in an organic solvent, n-heptane, and collaborating with Hyde at Varian Associates, a prototype instrument was constructed that produced the first endor spectrum of an organic free radical in solution.(3)

Electron spin resonance studies of reduced and unreduced lead silicate glasses

Abstract A measurement of the electron spin resonance spectrum of unreduced and reduced lead silicate glasses containing 50 mole per cent of PbO indicates that in addition to iron impurity signals occuring with g-values of 4.28 and 2.01, there is a signal at g = 1.915 which only occurs in the reduced samples. The intensity of the signal varies directly with the induced surface conductivity and, hence can be used as a tool to study the surface conductivity variation as a function of reducing schedules. At present the origin of this signal has not been definitely established.