The proton exchange of squarate in solution is a complex problem since three chemical species, Sq 2-, SqH - and SqH 2 are involved (see figure ). Furthermore, each of the protonated 1,2-13 C 2 species displays several isotopomers, depending on the position of the 13 C 2 pair with respect to the protonation sites. For the sake of simplicity, we consider only the high-pH regime, ignoring the neutral SqH 2 species, and neglecting the distinction between the multiple 1,2-13 C 2 isotopomers of SqH -. Protonation (or deuteration) of squarate is taken into account solely through its influence on the 13 C chemical shifts. We ignore any direct effects of the proton or deuteron nuclear spins, through heteronuclear dipoledipole couplings or scalar couplings, and also ignore the effect of protonation on other interactions such as the 13 C-13 C J-coupling. We also ignore the effect of O or 2 H on the chemical equilibria. Although this model is highly simplified, we show below that it reproduces the main features of the observed data. We consider the following chemical reaction