2.5.3. 1J(O–H)

The third coupling constant associated with the O–H ... O hydrogen bond is 1J(O–H) for the proton donor O–H group. Its value in **1** of −78 Hz minimally changes in complexes in which the acid interacts with the C=O group, varying between −79 and −82 Hz. However, interaction with the O–H group leads to a significant increase in the absolute value of 1J(O–H) to between −81 and −90 Hz. As the O–H distance increases dramatically in the transition structures, 1J(O–H) decreases significantly in absolute value to −20 Hz in **1** and from −30 to −47 Hz in the complexes with the acids. Once again, the variation in this coupling constant can be readily seen in the plot of 1J(O–H) versus the O–H distance, shown in Figure 8. The correlation coefficient of the second-order trendline is 0.973.

**Figure 8.** 1J(O–H) versus the O–H distance for complexes **1**:acid.
