An EPR/ENDOR study of aminoxyls (nitroxides) capable of intramolecular bonding: hydroxyalkyl radical spin adducts of nitrones

1988 ◽  
Vol 66 (8) ◽  
pp. 1901-1911 ◽  
Author(s):  
D. Larry Haire ◽  
Yashige Kotake ◽  
Edward G. Janzen
Keyword(s):  
Hydroxyl Radicals ◽  
Spectroscopic Method ◽  
Paramagnetic Resonance ◽  
Endor Spectroscopy ◽  
Hydroxyalkyl Radical ◽  
Close Proximity ◽  
Carbon Centered Radical ◽  
Intramolecular Bonding ◽  
Hyperfine Splittings ◽  
Electron Paramagnetic

A comparison of the effect of solvent and 13C labeling (of the radical addend) on the EPR (electron paramagnetic resonance) spectra of hydroxyalkyl vs. alkyl radical adducts of α-phenyl-N-tert-butyl nitrone (PBN) and 5,5-dimethyl-1-pyrroline-1-oxide (DMPO) has been investigated. The solution ENDOR spectra in toluene and in ethanol are the first examples studied of aminoxyls with hydroxyl substituents in close proximity to the free radical centre. Diastereomeric mixtures of hydroxyalkyl radical adducts are clearly resolved by ENDOR spectroscopy. Conformations of these radicals have been assigned based on the 1H and 13C hyperfine splittings (HFS's) and by differential optimum ENDOR temperatures. Definitive assignments of carbon-centered radical adducts of DMPO and PBN are shown to be feasible by monitoring the β-13C HFS's of the radical addend. Long-range γ-H HFS's from the added radical group can be observed when the deuterated spin trap PBN-d14 is used. The production of 13C labeled hydroxyalkyl adducts of nitrones (e.g., DMPO–13CH2OH, from the reaction of hydroxyl radicals with added 13CH3OH) is shown to be useful as an improved EPR spectroscopic method for the verification of the presence of hydroxyl radicals.

PREFORMED HOLE-PAIRS IN CUPRATE SUPERCONDUCTORS

2009 ◽  
Vol 23 (01) ◽  
pp. 53-76 ◽  
Author(s):  
R. J. SINGH
Keyword(s):  
Magnetic Field ◽  
Cuprate Superconductors ◽  
Charge Carriers ◽  
Nuclear Spins ◽  
Paramagnetic Resonance ◽  
Special Cases ◽  
Hyperfine Splittings ◽  
Single Formula ◽  
Electron Paramagnetic ◽  
Cu Ions

This paper consists of two parts: (1) mechanism of formation of hole-pairs and quantum stripes in cuprate superconductors, and (2) resistivity versus temperature variation in YBa 2 Cu 3 O 7-δ superconductor with different doping levels in the range 400–Tc. (1) On deoxygenation, CuO 2 plane of high-temperature cuprate superconductors are broken into small magnetically isolated fragments, predominantly Cu -tetramer (CuO) 4. Its electron paramagnetic resonance spectra show fine and hyperfine splittings. It was concluded that these splittings are due to FM coupling of the electronic spins of the four holes on the four Cu -ions in (CuO) 4 as well as the FM coupling of the nuclear spins of the four Cu -ions. A magnetic field is generated perpendicular to CuO 2 plane due to the spin magnetic moment of four holes and their orbiting around the (CuO) 4 frame. In the nondeoxygenated superconductors, this magnetic field leads to the formation of quantum stripes of free charge carriers and in special cases to the formation of preformed hole-pairs. This is the first model of a hole-pair, though its existence has been conjectured from various experimental results. (2) The electrical conduction in superconductors along c-axis seems to arise from the movement of free holes and along a-axis from the parallel combination of resistivities arising from the movement of free holes and paired holes, but no single formula connecting ρ and T is applicable to explain resistivities either along c- or a-axis in the entire temperature range. Nature of charge carriers seems to change continuously as the temperature is lowered. In the range 200–Tc, both along the c- and a-axes, the experimental resistivities depart significantly from those calculated by the formula applicable in the range 400–200 K. Plausible explanation has been suggested for the departure of experimental resistivities from the calculated ones in the low temperature region.

An Electrolytically Generated, Localized Hole Center in Quartz

1989 ◽  
Vol 44 (4) ◽  
pp. 278-282
Author(s):  
A. B. Vassilikou-Dova ◽  
K. Eftaxias ◽  
G. Lehmann
Keyword(s):  
Activation Energy ◽  
Electron Paramagnetic Resonance ◽  
Temperature Dependence ◽  
Free Electron ◽  
Room Temperature ◽  
Hole Center ◽  
Paramagnetic Resonance ◽  
Tentative Model ◽  
Hyperfine Splittings ◽  
Electron Paramagnetic

Abstract In natural smoky quartz and neutron-irradiated, initially colorless quartz a new hole center was formed electrolytically at temperatures near 1100 K in addition to the well-known smoky quartz center. Unlike the latter its electron paramagnetic resonance spectra can already be measured at room temperature due to firm localization of the hole on one oxygen. It is characterized by fairly small hyperfine splittings due to Al impurity and significant deviations of all three principal g factors from that of a free electron. A tentative model for the structure of this center is proposed.An activation energy of 215 kJ/mol was determined for this electrolytic coloration from the temperature dependence of the electrolysis currents.

Electron paramagnetic resonance spectroscopic study of nitroxide radicals formed from oximes

1969 ◽  
Vol 47 (12) ◽  
pp. 2227-2236 ◽  
Author(s):  
P. Smith ◽  
W. M. Fox
Keyword(s):  
Hydroxyl Radicals ◽  
Continuous Flow ◽  
Flow System ◽  
Functional Structure ◽  
Methyl Radicals ◽  
Paramagnetic Resonance ◽  
A Value ◽  
Butanedione Monoxime ◽  
Structure Formula ◽  
Electron Paramagnetic

The addition of hydroxyl radicals to each of the oximes, acetaldoxime, acetoxime, and 2,3-butanedione monoxime, has been studied by mixing titanous chloride, hydrogen peroxide, and the oxime in an aqueous continuous-flow system. In each case a β-hydroxy nitroxide (functional structure [Formula: see text] is formed. In confirmation of these results, amino and hydroxymethyl radicals were added to the oximes using similar procedures to give in each case the radicals of functional structure [Formula: see text] and [Formula: see text], respectively. Attempts to add methyl radicals, prepared by the reaction between titanous chloride and t-butyl hydroperoxide, to these oximes gave no observable products save the methyl radicals. However, use of titanous chloride and peracetic acid gave results consistent with the addition of hydroxyl radicals. The addition of hydroxyl radicals to the oxime-O-methyl ethers of acetaldehyde and acetone gave radicals of functional structure [Formula: see text][Formula: see text] which have not previously been identified unambiguously. On the assumption that |QN| = |QHNH| = 23 G in all these radicals, ρNπ has been calculated and used to obtain a value for [Formula: see text] of ca. 11 G which is within the range of other estimates in the literature.

The first 1H and 14N ENDOR spectra of an oxygen-centered radical adduct of DMPO-type nitrones

1988 ◽  
Vol 66 (9) ◽  
pp. 2395-2402 ◽  
Author(s):  
D. Larry Haire ◽  
Uwe M. Oehler ◽  
H. Duane Goldman ◽  
Robert L. Dudley ◽  
Edward G. Janzen
Keyword(s):  
Electron Paramagnetic Resonance ◽  
Long Range ◽  
Double Resonance ◽  
Electron Nuclear Double Resonance ◽  
Paramagnetic Resonance ◽  
Spectral Signatures ◽  
Radical Adduct ◽  
Sequence Patterns ◽  
Hyperfine Splittings ◽  
Electron Paramagnetic

We report the first 1H and 14N electron nuclear double resonance (ENDOR) spectra of oxygen-centered radical adducts (i.e., derived from the addition of tert-butoxyl radicals) of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as well as the related cyclic nitrones: 4-phenyl-5,5-dimethyl-1-pyrroline-N-oxide (4-Ph-DMPO) and 3,3,5,5-tetramethyl-1-pyrroline-N-oxide (M4PO). Features of the electron paramagnetic resonance (EPR) and ENDOR spectra of these cyclic (pyrrolidine) aminoxyl (nitroxide) species along with some deuterated analogues in liquid toluene solutions are highlighted. For instance, we found several long-range γ-H hyperfine splittings (HFS's) detectable by ENDOR that provide unique spectral signatures for these radical adducts. Conformational assignments of the pyrrolidine aminoxyl structures based upon the EPR and ENDOR findings are also discussed. Signs of the various hyperfine splittings (i.e., aN, [Formula: see text]) investigated by cross-relaxation intensity sequence patterns (CRISP) in the ENDOR spectra are also presented.

Sign in / Sign up

Export Citation Format

Share Document