Impact of Electron-Electron Spin Interaction on Electron Spin Relaxation of Nitroxide Diradicals and Tetraradical in Glassy Solvents Between 10 and 300 K

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• 作者：Hideo Sato ; Velavan Kathirvelu ; Ga&euml ; lle Spagnol ; Suchada Rajca ; Andrzej Rajca ; Sandra S. Eaton ; Gareth R. Eaton
• 刊名：Journal of Physical Chemistry B
• 出版年：2008
• 出版时间：March 13, 2008
• 年：2008
• 卷：112
• 期：10
• 页码：2818 - 2828
• 全文大小：294K
• 年卷期：v.112,no.10(March 13, 2008)
• ISSN：1520-5207

文摘

To determine the impact of electron-electron spin-spin interactions on electron spin relaxation rates, 1/T₁and 1/T_m were measured for nitroxide monoradical, diradical, and tetraradical derivatives of 1,3-alternatecalix[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) magnetometryof one of the high-spin diradicals, in which nitroxides are conformationally constrained to be coplanar withthe m-phenylene unit, is reported. The interspin distances ranged from about 5-9 Å, and the magnitude ofthe exchange interaction ranged from >150 to >0.1 K. 1/T₁ and 1/T_m were measured by long-pulse saturationrecovery, three-pulse inversion recovery, and two-pulse echo decay at X-band (9.5 GHz) and Q-band (35GHz). For a diradical with interspin distance about 9 Å, relaxation rates were only slightly faster than for amonoradical with analogous structure. For interspin distances of about 5-6 Å, relaxation rates in glassysolvents 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 differencesin 1/T₁ were observed below 10 K, where the direct process dominates. For the three diradicals with comparablemagnitude of dipolar interaction, 1/T_m 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 4:1toluene/chloroform or in hydrogen-bonded glycerol glasses, which highlights the impact of motion on relaxation.