Equation 45 has a straightforward physical interpretation. The factor k p 10 -pH represents the rate constant for protonation of the Sq 2-anions. If the resulting SqH - species has a small frequency difference between the two C sites (∆Ω → 0), the singlet state remains a good energy eigenstate in the SqH -species and there is no significant evolution of the singlet order before the molecule loses its proton again. The kinetic contribution to R S becomes very small in this limit, so that R S is dominated by the term R 0 S , which is the population-weighted average of the singlet relaxation rate constants in the individual species. This is the approach used previously for the studies of ligand binding by singlet NMR .