title: Multifrequency EPR study of the mobility of nitroxides in solid-state calixarene nanocapsules.
abstract: Multifrequency continuous wave (cw) and echo-detected (ED) electron paramagnetic resonance (EPR) was employed to study the mobility of nitroxides confined in nanocapsules. The complexes p-hexanoyl calix[4]arene with 4-methoxy-2,2,6,6-tetramethylpiperidine-N-oxyl (MT) and N-(2-methylpropyl)-N-(1-diethylphosphono-2,2-dimethylpropyl)-aminoxyl (DEPN) were studied by X-, W-band and 360 GHz cw EPR at temperatures between 90 and 370 K. Thereby we were able to extract the canonical values of the hyperfine and g-tensors of the encapsulated radicals as well as information on restricted orientational dynamics of the caged spin probes. Comparing cw and ED-EPR data obtained on MT@C6OH we found that between 90 and 200 K the caged nitroxide undergoes isotropic small-angle fluctuations (librations), whereas at higher temperatures restricted rotations of the radical with correlation times of 0.75 x 10(-10) s and 1.2 x 10(-10) s dominate at 325 and 300 K, respectively. The activation energy of the rotational motion of encapsulated MT radicals was evaluated as E(a) = 1.0 kcal mol(-1), which is comparable to the magnitude of van der Waals interactions. Compared to MT, the reorientational motion of DEPN was found to be slower and more isotropic.
authors.affiliation: International Tomography Center SB RAS, Novosibirsk, Russia. elena@tomo.nsc.ru
authors.forename: Elena G
authors.initials: EG
authors.lastname: Bagryanskaya
authors.forename: Dmitriy N
authors.initials: DN
authors.lastname: Polovyanenko
authors.forename: Matvey V
authors.initials: MV
authors.lastname: Fedin
authors.forename: Leonid
authors.initials: L
authors.lastname: Kulik
authors.forename: Alexander
authors.initials: A
authors.lastname: Schnegg
authors.forename: Anton
authors.initials: A
authors.lastname: Savitsky
authors.forename: Klaus
authors.initials: K
authors.lastname: Möbius
authors.forename: Anthony W
authors.initials: AW
authors.lastname: Coleman
authors.forename: Gennady S
authors.initials: GS
authors.lastname: Ananchenko
authors.forename: John A
authors.initials: JA
authors.lastname: Ripmeester
mesh_terms: D047250:Calixarenes / Q000737:chemistry*; D003497:Cyclic N-Oxides / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy; D053769:Nanocapsules / Q000737:chemistry*; D009589:Nitrogen Oxides / Q000737:chemistry*; D013113:Spin Labels; D013696:Temperature
chemical_list: D003497:Cyclic N-Oxides; D053769:Nanocapsules; D009589:Nitrogen Oxides; D013113:Spin Labels; D047250:Calixarenes