Details

Autor(en) / Beteiligte

• Sato, Hideo
• Kathirvelu, Velavan
• Spagnol, Gaëlle
• Rajca, Suchada
• Rajca, Andrzej
• Eaton, Sandra S
• Eaton, Gareth R

Titel

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

Ist Teil von

• The journal of physical chemistry. B, 2008-03, Vol.112 (10), p.2818-2828

Ort / Verlag

United States: American Chemical Society

Erscheinungsjahr

2008

Quelle

MEDLINE

Beschreibungen/Notizen

• To determine the impact of electron−electron spin−spin interactions on electron spin relaxation rates, 1/T 1 and 1/T m 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 Å, and the magnitude of the exchange interaction ranged from >150 to >0.1 K. 1/T 1 and 1/T m 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 Å, relaxation rates were only slightly faster than for a monoradical with analogous structure. For interspin distances of about 5−6 Å, 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/T 1 were observed below 10 K, where the direct process dominates. For the three diradicals with comparable magnitude of dipolar interaction, 1/T m and 1/T 1 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.