Effect of Lanthanide and Cobalt Ions on Electron Spin Relaxation of Tempone in Glassy Water:Glycerol at 20 to 200 K

Abstract

The enhancement of X-band spin–lattice and spin–spin relaxation rates for the nitroxide tempone (2,2,6,6-tetramethyl-4-oxo-piperidin-1-oxyl) in 1:1 water:glycerol by Dy³⁺, Er³⁺, Tm³⁺ or Co²⁺ was examined between 20 and 200 K. Nitroxide relaxation rates were measured by two-pulse spin echo and three-pulse inversion recovery. The impact of the rapidly relaxing metal aquo ions on 1/T₁ of the nitroxide increases in the order Co²⁺ ~ Er³⁺ < Dy³⁺ < Tm³⁺. The maximum spin–lattice relaxation enhancement occurs at about 35 K for Dy³⁺, 40 K for Er³⁺, and 60 K for Co²⁺. When the metal ion is bound to the chelator diethylenetriamine pentaacetic acid (DTPA) the maximum enhancements for Dy(DTPA)²⁻ and Er(DTPA)²⁻ shift to about 80 K. The maximum enhancement is proposed to occur when 1/T₁ for the metal ion is approximately equal to the resonance frequency for the nitroxide. Interaction with the paramagnetic metal ion causes a much larger fractional change in 1/T₁ than for 1/T₂. Below about 20 K the enhancement of nitroxide 1/T₂ increases, which is attributed to relaxation of the metal ions at rates comparable to the electron–electron dipolar coupling, expressed in frequency units.