The theory given here may also be of value for other phenomena involving a combination of chemical and spin dynamics, such as Signal Amplification by Reversible Exchange (SABRE) 101-103 , Chemically Induced Dynamic Nuclear Polarization (CIDNP) 104,105 , and spin-isomer conversion of dissolved H 2 gas 106 . One additional observation should be mentioned: The theory given above relies on the existence of a Larmor frequency difference between the 13 C nuclei in the protonated squarate species. This effect is field-dependent and should disappear in a sufficiently small magnetic field. However, experimental results do not fully verify this prediction. There appears to be a small pH-dependent contribution to R S which does not disappear at very low magnetic field. At the time of writing, it is not known whether this observation is due to a relatively trivial effect, such as relaxation by paramagnetic impurities, or whether it indicates a further relaxation mechanism of unknown origin.