*2.5. Spin–Spin Coupling Constants*

The total spin–spin coupling constants 2hJ(O–O), 1hJ(H–O), and 1J(O–H) are given in Table 3, and the paramagnetic spin–orbit (PSO), diamagnetic spin–orbit (DSO), Fermi contact (FC), and spin–dipole (SD) components are reported in Table S2 of the Supporting Information. For the equilibrium ground states of the complexes, the PSO, FC, and SD components of 2hJ(O–O) are positive. Both the PSO and SD components of 2hJ(O–O) are non-negligible, with the PSO component having values comparable to those of the FC term when interaction with the acid occurs at the carbonyl group. Even though the FC terms for the transition structures have values that are greater and closer to total J, the FC terms are poor approximations to the total coupling constant 2hJ(O–O).

Coupling constants 1hJ(H–O) are also small and positive for the equilibrium structures and are dominated by the FC terms. The PSO terms are smaller and positive for the equilibrium structures, but these are partially canceled by the negative DSO and SD terms. The net result is that the FC terms differ from 1hJ(H–O) by about 1 Hz. The coupling constant 1J(O–H) is negative and has a significantly greater absolute value than 2hJ(O–O) and 1hJ(H–O) since it refers to a covalent O–H bond. Values of this coupling constant are dominated by the negative FC terms, while the PSO terms make smaller, non-negligible negative contributions to 1J(O–H) in the equilibrium structures but positive contributions in the transition structures. Thus, all terms should be included in determining 1J(O–H).


**Table 3.** Spin–spin coupling constants 2hJ(O–O), 1hJ(H–O), and 1J(O–H) (Hz) for the O–H **...** O hydrogen bond in complexes C3H4O2:Acid.
