2.5.1. 2hJ(O–O)

Spin–spin coupling constants 2hJ(O–O) across the O–H ... O hydrogen bond are reported in Table 3. This coupling constant has a value of 12 Hz in the isolated monomer **1** and then decreases to about 10 Hz in the complexes when the acid interacts with **1** at the C=O bond. When interaction occurs at the O–H bond, 2hJ(O–O) increases to between 13 and 15 Hz. The value of 2hJ(O–O) obviously depends on the O–O distance, so it is not surprising that 2hJ(O–O) increases to about 24 Hz in the transition structures that have the shortest O–O distances. These relationships are seen most easily in Figure 6, which is a plot of 2hJ(O–O) versus the O–O distance. The second-order trendline has a correlation coefficient of 0.992.

**Figure 6.** 2hJ(O–O) versus the O–O distance for complexes **1**:acid.
