Dephasing of electron spin echoes for nitroxyl radicals in glassy solvents by non-methyl and methyl protons

Abstract

Dephasing of two-pulse electron spin echoes for 0.1–0.4 mM solutions of nitroxyl radicals in glassy solvents at 11 K and 40 K is dominated by nuclear spins in the solvent. Below about 70 K the rate of dephasing is the same for 4-oxo-2,2,6,6-tetramethylpiperin-1-yl (tempone) and for Fremy's salt (peroxylaminedisulphonate). In solvents that do not contain methyl groups the rate constant for echo dephasing increases linearly with increasing proton concentration. The shape of the echo decay curve could be simulated with a model based on dephasing due to proton flip-flops with a rate constant of 30 ± 5 Hz M⁻¹. At the same total proton concentration the electron spin echo decay rate is faster in solvents that contain methyl groups than in solvents without methyl groups. Several observations indicate that the effect of the methyl groups in the solvent on the echo dephasing is due to quantum mechanical tunnelling: (a) the echo dephasing is approximately independent of temperature between 11 K and 40 K, (b) the apparent spin flip-flop rate, W _m, for the methyl protons is independent of the concentration of methyl protons, (c) the estimated values of W _m for the methyl protons increase in the order hindered aliphatics < less hindered aliphatics < aromatic methyls, which is the order of decreasing barriers to classical rotation and therefore increasing tunnelling frequency, and (d) dephasing due to fluorines in CF₃ groups is comparable with that for non-methyl protons. These results indicate that the nitroxyl spin echo dephasing can be used to probe the proton environment of the radical.