Nuclear singlet relaxation by chemical exchange.

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