2.5.2. 1hJ(H–O)

The values of the second coupling constant 1hJ(H–O) across the hydrogen bond are also reported in Table 3. Its value of 7.9 Hz in **1** changes minimally upon complex formation, ranging from 7.7 to 8.1 Hz when complexation occurs at the O–H group. It decreases to between 5.8 and 7.4 Hz when the acid interacts at the C=O group. Much larger changes are observed in the transition structures as the H–O distance across the hydrogen bond contracts, and 1hJ(H–O) has a value of −19.7 Hz in **1**. This coupling constant varies from −10.3 Hz in the complex with BeF2 as the acid to −15.5 Hz with LiF as the acid. The dependence of 1hJ(H–O) on the H–O distance is illustrated in Figure 7, which has a secondorder trendline with a correlation coefficient of 0.997. The vertical 1hJ(H–O) axis was reversed to account for the negative magnetogyric ratio of 17O. This plot also illustrates the changing nature of the hydrogen bond, from a traditional hydrogen bond in the equilibrium complexes with bond formation at the O–H or C=O groups to a proton-shared hydrogen bond in the transition structures.

**Figure 7.** 1hJ(O–H) versus the O–H distance for complexes **1**:Acid.
