Solvation of neutral nitroxides

Nitroxides have become of very great importance to biologists, but nevertheless, have not been extensively studied by solvation chemists. They are widely used as spin labels , first introduced by McConnell, because of their stability, resistance to dimerisation or disproportionation, and large hyperfine anisotropy [17]. This last property controls the widths of the lines, especially the M, = — 1 feature, the width being a function of tumbling rate. Hence correlation times can be estimated over a very wide time span (ca. 10 -10- s). 

In marked contrast with RjNO molecules, the dianion, (OjS)2NO , has an isotropic hyperfine splitting that is remarkably insensitive to changes in solvation [18]. Also, the electronic spectrum is insensitive, in contrast with those for neutral nitroxides. This difference seems to be largely due to very strong hydrogen bonding to the two -SO J units in protic solvents, which dominates the overall solvation. 

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