Since the isotropic shift difference between the 13 C sites (generated by the 18 O isotope shift) is very small, the singlet state is a good eigenstate of the coherent spin Hamiltonian. Hence a 18 O-enriched 1,2-13 C 2 squarate molecule in the nuclear singlet state remains in that state indefinitely -or more precisely, for times short compared to the relaxation time constants. But now suppose that the singlet Sq 2-molecule is protonated at time t = 0, giving a SqH -molecule. The change in electronic structure induces a chemical shift difference of ∆δ between the 13 C sites, corresponding to a precession frequency difference of ∆Ω = -γB∆δ in the magnetic field B. If the molecule remains in the protonated state for an interval τ , the spin state of the 13 C 2 pair state at time t = τ is given by