For the transferred state in Fig. , the three-way interaction energy E(Li-N 2 -PD) shows a strong negative correlation with the Kamlet-Taft α parameter (-0.72). As α is a measure of the hydrogen bond acidity, this implies increasing bond acidity of the PD results in more negative interaction energies and a stabler protonated system. Furthermore, the Dimroth and Reichardt E T , another measure of Lewis acidity, also shows a strong negative correlation with the interaction energy (-0.83) whereas the pKa shows a moderately positive correlation (0.55). We do not consider the high correlations from the Gutmann acceptor and donor numbers here because they are made with fewer than 5 points, and thus cannot be considered statistically significant.