Nuclear singlet relaxation by chemical exchange

The population imbalance between nuclear singlet states and triplet of strongly coupled spin-1/2 pairs, also known as order, is well protected against several common relaxation mechanisms. We study the 13C pairs in aqueous solutions 1,2-13C2 squarate over a range pH values. order accessed by introducing 18O nuclei to break chemical equivalence. dianion equilibrium with hydrogen-squarate (SqH?) squaric acid (SqH2) characterized dissociation constants pK1 = 1.5 pK2 3.4. Surprisingly, we observe striking increase decay time TS when solution exceeds ?10, which far above acid–base points. derive general rate expressions for chemical-exchange-induced provide qualitative explanation behavior dianion. identify kinetic contribution constant, explicitly depends on constants. Qualitative agreement achieved theory experimental data. This shows that infrequent events may have strong effect order.