title: Frequency dependence of electron spin relaxation times in aqueous solution for a nitronyl nitroxide radical and perdeuterated-tempone between 250 MHz and 34 GHz.
abstract: Electron spin relaxation times of perdeuterated tempone (PDT) 1 and of a nitronyl nitroxide (2-(4-carboxy-phenyl)-4,4,5,5-tetramethylimidazoline-3-oxide-1-oxyl) 2 in aqueous solution at room temperature were measured by 2-pulse electron spin echo (T(2)) or 3-pulse inversion recovery (T(1)) in the frequency range of 250 MHz to 34 GHz. At 9 GHz values of T(1) measured by long-pulse saturation recovery were in good agreement with values determined by inversion recovery. Below 9 GHz for 1 and below 1.5 GHz for 2,T(1)~T(2), as expected in the fast tumbling regime. At higher frequencies T(2) was shorter than T(1) due to incomplete motional averaging of g and A anisotropy. The frequency dependence of 1/T(1) is modeled as the sum of spin rotation, modulation of g and A-anisotropy, and a thermally-activated process that has maximum contribution at about 1.5 GHz. The spin lattice relaxation times for the nitronyl nitroxide were longer than for PDT by a factor of about 2 at 34 GHz, decreasing to about a factor of 1.5 at 250 MHz. The rotational correlation times, τ(R) are calculated to be 9 ps for 1 and about 25 ps for 2. The longer spin lattice relaxation times for 2 than for 1 at 9 and 34 GHz are due predominantly to smaller contributions from spin rotation that arise from slower tumbling. The smaller nitrogen hyperfine couplings for the nitronyl 2 than for 1 decrease the contribution to relaxation due to modulation of A anisotropy. However, at lower frequencies the slower tumbling of 2 results in a larger value of ωτ(R) (ω is the resonance frequency) and larger values of the spectral density function, which enhances the contribution from modulation of anisotropic interactions for 2 to a greater extent than for 1.
authors.affiliation: Department of Chemistry and Biochemistry, University of Denver, Denver, CO 80208, USA.
authors.forename: Joshua R
authors.initials: JR
authors.lastname: Biller
authors.forename: Virginia M
authors.initials: VM
authors.lastname: Meyer
authors.forename: Hanan
authors.initials: H
authors.lastname: Elajaili
authors.forename: Gerald M
authors.initials: GM
authors.lastname: Rosen
authors.forename: Sandra S
authors.initials: SS
authors.lastname: Eaton
authors.forename: Gareth R
authors.initials: GR
authors.lastname: Eaton
mesh_terms: D016880:Anisotropy; D003903:Deuterium; D004578:Electron Spin Resonance Spectroscopy; D007202:Indicators and Reagents; D009589:Nitrogen Oxides / Q000737:chemistry*; D010100:Oxygen / Q000737:chemistry; D012996:Solutions; D013113:Spin Labels; D014216:Triacetoneamine-N-Oxyl / Q000737:chemistry*; D014867:Water
chemical_list: D007202:Indicators and Reagents; D009589:Nitrogen Oxides; D012996:Solutions; D013113:Spin Labels; D014867:Water; D014216:Triacetoneamine-N-Oxyl; D003903:Deuterium; C039900:nitroxyl; D010100:Oxygen