The strong pH-dependence of T S suggests that the reversible protonation of squarate in aqueous solution plays a role in the singlet relaxation. In aqueous solution, the symmetrical squarate dianion is in equilibrium with hydrogen squarate and squaric acid (see figure ). The possibility arises that this dynamic chemical process could act as a relaxation mechanism for nuclear singlet order. However, at first sight, this appears unlikely. The pKa values for the two deprotonation stages of squaric acid are 1.5 and 3.4 respectively . The equilibrium fractions of the monoprotonated and diprotonated species at a pH of 7.0 are only ∼250 ppm and ∼0.8 ppb respectively. Such minority species could only have a significant effect if a mechanism exists that leads to an extreme sensitivity of nuclear singlet order to rare protonation events.