The data in figure therefore indicate a strong chemical-exchange mechanism for singlet relaxation mechanism that has a much stronger effect than the mere averaging of the rate constant T -1 S over the participating species. We propose a mechanism of this kind below. The protonation of a squarate oxygen atom causes a substantial change in the electronic structure which leads in turn to a large isotropic chemical shift difference between the two 13 C sites. In high magnetic field the isotropic shift difference of the protonated species, measured in frequency units, greatly exceeds the scalar spin-spin coupling between the two 13 C nuclei. Hence, in the protonated form, the singlet state of the 13 C 2 pair is not even an approximate eigenstate of the coherent spin Hamiltonian. This leads to a rapid loss of singlet order upon each protonation event. As discussed below, within a certain kinetic regime, the singlet relaxation rate constant T -1 S becomes directly proportional to the rate of protonation of the squarate dianion. The protonation rate is proportional to hydronium concentration and is therefore strongly pH-dependent.