For the system considered here, the factor ∆ SO is given by ∼ 0.64 in a magnetic field of 9.4 T, i.e. slightly less than the 2 3 limit. Under these conditions, every protonation event destroys on average about 64% of the nuclear singlet order of a Sq 2-anion, and the kinetic contribution to R S is equal to about 64% of the protonation rate constant of squarate ions in solution.