Solvent effects on the 14N hyperfine coupling constants and spin exchange rates for di-t-butyl nitroxide and peroxylamidisulphonate ions [(O3S)2NO2–]

Abstract

Solvent effects on the ¹⁴N hyperfine coupling for solutions of di-t-butyl nitroxide (DTBN) and (O₃S)₂NO^2–(Fremy's salt) are compared. The former is very sensitive, especially towards protic solvents whilst the latter is remarkably insensitive. Similarly, the electronic spectrum for DTBN is very solvent dependent whereas that for Fremy's salt is insensitive to solvent. We have previously established that DTBN forms two hydrogen bonds between the N—O group and water, but only ca. one to alcohols. We conclude that the N—O group for Fremy's salt does not readily form hydrogen bonds, all the solvation being at the two —SO^–₃ groups. However, in aqueous solutions one weak H-bond may be formed.

Spin exchange rates have been measured for these nitroxides in a range of solvents. Detailed comparison shows that the results for DTBN in water are anomalously low by a factor of ca. 2, whilst those for Fremy's salt in water are normal. It is unlikely that this effect is due solely to the presence of the two N—O hydrogen-bonds, since the results for alcoholic solutions correlate well with those for aprotic solvents. The suggestion that hydrophobic interactions are responsible is examined, and strongly supported by studies on the effect of changing the temperature and the addition of t-butyl alcohol.