*2.3. Charge-Transfer Energies*

The complexes **1**:acid are stabilized by charge-transfer interactions. The nature of charge transfer and the associated charge-transfer energies are reported in Table 2. Given the nature of the lone-pair orbitals illustrated in Figure 4, it is not surprising that charge transfer from lp2 is the dominant charge-transfer interaction, with energies ranging from 158 kJ·mol−<sup>1</sup> in **1**:LiF(OH) to 189 kJ·mol−<sup>1</sup> **1**:BeF2(OH). The charge-transfer energies involving lp1 are much less, with values between 15 and 31 kJ·mol−1. The total charge-transfer energies vary from 100 to 213 kJ·mol−1. It is interesting to note that the strongest complexes occur in **1**:acid(CO) (Table 1 and Figure 2), while the strongest intramolecular hydrogen bond can be seen in **1**:acid(OH). Figure 5 illustrates a linear dependence of these energies on the O–O distance, with a correlation coefficient of 0.965.

**Table 2.** Intramolecular charge-transfer stabilization energies (kJ·mol−1) for complexes of C3H4O2 with acids.


**Figure 5.** Charge-transfer energies versus the O–O distance.
