Effects of proton exchange on the lifetimes of long-lived states in aliphatic chains

Long lived spin states (LLS) were created in aliphatic chains containing several CH2 groups, with distinct couplings with the 1 H of a neighbouring methylene. We present a systematic study of the influence of the acid-base proton exchange on the relaxation time of delocalized LLSs. We observed the dependence of the LLS relaxation times upon titration in a large range of pH values for a series of compounds containing two or three methylene groups. The LLS relaxation times exhibit drastic changes around pKa values. These changes do not seem to obey a definite trend and vary from one compound to another, and empirically correlate with the chemical shift changes upon titration. Tentative interpretations are proposed.