As shown in figure , the central peak of the C triplet is associated with 1,2-13 C 2 -squarate species with an O atom attached to one C atom and a O atom attached to the other. These species contain strongly coupled C pairs with a small chemical shift difference between the two C nuclei, and are suitable candidates for singlet order excitation techniques. Figure shows a singlet-filtered O-enriched 1,2-13 C 2 squarate spectrum at pH = 14.0. The spectrum was acquired at 16.5 T using the pulse sequence protocol in figure with the M2S parameters given in table I. The signals from the outer peaks of the triplet are suppressed in the singlet-filtered C spectrum since these signals are generated by isotopologues with no O atoms directly bonded to both C atoms, or with no O atoms directly bonded to both C atoms. Although these species include isotopologues with asymmetric distributions of O atoms on the other side of the molecule to the 13 C 2 pair, these remote heavy atom substitutions are too far away to allow appreciable excitation of C singlet order, for the chosen pulse sequence parameters.