title: Impact of electron-electron spin interaction on electron spin relaxation of nitroxide diradicals and tetraradical in glassy solvents between 10 and 300 k.
abstract: To determine the impact of electron-electron spin-spin interactions on electron spin relaxation rates, 1/T1 and 1/Tm were measured for nitroxide monoradical, diradical, and tetraradical derivatives of 1,3-alternate calix[4]arenes, for two pegylated high-spin nitroxide diradicals, and for an azine-linked nitroxide diradical. The synthesis and characterization by SQUID (superconducting quantum interference device) magnetometry of one of the high-spin diradicals, in which nitroxides are conformationally constrained to be coplanar with the m-phenylene unit, is reported. The interspin distances ranged from about 5-9 A, and the magnitude of the exchange interaction ranged from >150 to >0.1 K. 1/T1 and 1/Tm were measured by long-pulse saturation recovery, three-pulse inversion recovery, and two-pulse echo decay at X-band (9.5 GHz) and Q-band (35 GHz). For a diradical with interspin distance about 9 A, relaxation rates were only slightly faster than for a monoradical with analogous structure. For interspin distances of about 5-6 A, relaxation rates in glassy solvents up to 300 K increased in the order monoradical < diradical < tetraradical. Modulation of electron-electron interaction enhanced relaxation via the direct, Raman, and local mode processes. The largest differences in 1/T1 were observed below 10 K, where the direct process dominates. For the three diradicals with comparable magnitude of dipolar interaction, 1/Tm and 1/T1 were faster for the molecules with more flexible structures. Relaxation rates were faster in the less rigid low-polarity sucrose octaacetate glass than in the more rigid 4:1 toluene/chloroform or in hydrogen-bonded glycerol glasses, which highlights the impact of motion on relaxation.
authors.affiliation: Department of Chemistry and Biochemistry, 2101 East Wesley Avenue, University of Denver, Denver, CO 80208-2436, USA.
authors.forename: Hideo
authors.initials: H
authors.lastname: Sato
authors.forename: Velavan
authors.initials: V
authors.lastname: Kathirvelu
authors.forename: Gaëlle
authors.initials: G
authors.lastname: Spagnol
authors.forename: Suchada
authors.initials: S
authors.lastname: Rajca
authors.forename: Andrzej
authors.initials: A
authors.lastname: Rajca
authors.forename: Sandra S
authors.initials: SS
authors.lastname: Eaton
authors.forename: Gareth R
authors.initials: GR
authors.lastname: Eaton
mesh_terms: D004583:Electrons; D005609:Free Radicals; D005898:Glass / Q000737:chemistry*; D015394:Molecular Structure; D009589:Nitrogen Oxides / Q000737:chemistry*; D012997:Solvents / Q000737:chemistry*; D013057:Spectrum Analysis; D013696:Temperature
chemical_list: D005609:Free Radicals; D009589:Nitrogen Oxides; D012997:Solvents