*3.5. Computational Studies*

The theoretical study is firstly focused on analyzing electronic nature of the 9ETADE ring and, more importantly, how changes upon protonation and formation of the corresponding salts with hydrogenoxalate and oxalate. Secondly, we have evaluated the H-bonding networks that are formed in the solid state of compounds **1** and **3**, as described above in Figures 3a and 4a.

Figure 6 shows the MEP surfaces of 9ETADE and its salts. In neutral 9ETADE, the MEP minimum is located at N1 (see Scheme 1a for numbering of atoms) in line with the protonation site observed in compounds **1** and **3**. The MEP values at N3 and N7 are similar (–25.1 and –24.7 kcal/mol, respectively). The MEP maximum is located at the exocyclic NH2 group (+26.9 kcal/mol). The MEP is also large and positive (+20.0 kcal/mol) at the aromatic CH group of the five membered ring. The MEP surface of the 9-ethyladeninium– hydrogenoxalate salt extracted from the solid state of compound **1** is represented in Figure 6b. The protonation increases the MEP values at the NH2 (+59.8 kcal/mol) and CH (+48.9 kcal/mol) groups with respect to the neutral 9ETADE, as expected. It is interesting to note that the MEP at the OH group is large and positive, despite the negative charge of hydrogenoxalate moiety. The MEP minimum is located at the O-atom of hydrogenoxalate (–81.6 kcal/mol). For the 9-ethyladeninium oxalate salt (Figure 6c), the MEP minimum is located at the oxalate anion, as expected (–77.2 kcal/mol) and the maximum at the exocyclic NH2 group (+36.7 kcal/mol). It is worth mentioning that in this salt, the MEP value at the

CH group of the five membered ring is very large, similar to that at the NH2 group, thus revealing an enhanced ability to establish H-bonds, as further discussed below.

**Figure 6.** MEP surfaces (isovalue 0.01 a.u.) of 9ETADE (**a**), 9-ethyldeninimum hydrogenoxalate salt (**b**) and 9-ethyldeninimum oxalate salt (**c**) at the PBE0-D3/def2-TZVP level of theory. The energies at selected points of the surfaces are given in kcal/mol.

In the solid state of compound **1**, the protonated 9ETADE rings form centrosymmetric *R*2 2(10) H-bonded synthons, as highlighted in Figure 7, which are surrounded by four hydrogenoxalate monoanions (see Section 3.2 for distances and other geometric features). The distribution of bond and ring critical points (CPs, red and yellow spheres, respectively) and bond paths is represented in Figure 7 for the whole assembly, where the superimposed noncovalent interaction plot (NCIPlot) index isosurfaces are also represented. The NCIplot index method is conveniently used to characterize noncovalent interactions in real space. It also reveals the attractive or repulsive nature of the interactions by using a color scale. The QTAIM distribution shows that the two symmetrically equivalent N6–H7···N7 bonds that generate the *R*<sup>2</sup> 2(10) synthon are characterized by bond CPs and bond paths connecting the H-atoms to the N-atoms. Moreover, they are also characterized by small and blue NCIplot isosurfaces, revealing a moderately strong interaction. The formation of this assembly agrees well with the MEP analysis commented above that evidences the strong H-bond donor ability of the NH2 group in the salts. The dissociation energy of each individual H-bond has been computed using the potential energy density values at the bond CPs (Vr) and the equation proposed by Espinosa et al. (*E*dis = –0.5 × Vr) [39]. The *E*dis values are indicated in Figure 7 next to the bond CPs in red, showing that the binding energy of the *R*<sup>2</sup> 2(10) synthon is 7.8 kcal/mol, in line with other studies related to the energetics of Hoogsteen base pair [41,42]. Regarding the H-bonds of the *R*<sup>2</sup> 2(10) dimer with the counterions, the combined QTAIM/NCIplot analysis shows a quite intricate H-bonding network where each hydrogenoxalate anion establishes three H-bonds with the base pair, each one characterized by the corresponding bond CP and bond path. The strongest one corresponds to the charge assisted H-bond (N1+–H···O, 7.3 kcal/mol) and the rest of H···<sup>O</sup> contacts range from 1.4 to 2.6 kcal/mol. In line with the MEP surface analysis, the CH group also participates in the H-bonding network with a moderately strong interaction energy (2.4 kcal/mol), comparable to other NH···O contacts. The total dissociation energy is

42.4 kcal/mol, thus evidencing the importance of the H-bonding network on the solid-state architecture of compound **1**.

**Figure 7.** QTAIM distribution of bond and ring critical points (CPs, red and yellow spheres, respectively) corresponding to the H-bonds in the *R*<sup>2</sup> 2(10) synthons and its interaction with four surrounding [C2O4H]– anions. The dissociation energies of the H-bonds are indicated next to the bond CPs. Superimposed NCIplot isosurfaces [*s* = 0.5, cut-off = 0.04 a.u., color scale –0.04 a.u. (blue) ≤ (signλ2)ρ ≤ 0.04 a.u. (red)] are also represented.

In the literature, the ability of adenine derivatives and cocrystals to form *R*<sup>2</sup> 2(10) homodimers or a variety of heterodimers via the Hoogsteen face has been described and energetically evaluated using the QTAIM analysis [41–43]. For instance, the reported energies associated with NH···N H-bonds in *R*<sup>2</sup> 2(10) ethyl-adenine homodimers in its co-crystals with malonic and fumaric acid are 3.6 and 4.0 kcal/mol [18], respectively. These values are comparable to those obtained for compound **1**. Similar interaction energies have been also reported for Hoogsteen (adenine)··· Watson–Crick (N7-pyrimidyladenine) heterodimers (3.7 and 4.2 kcal/mol) [44].

Figure 8 shows a similar QTAIM/NCIplot analysis performed for compound **3**. In this case, the *R*<sup>2</sup> 2(10) synthon via the Hoogsteen face is not formed. Each oxalate anion is surrounded by four 9-ethyladeninium cations establishing a network of H-bonding, characterized by the corresponding bond CPs, bond paths and blue NCIplot isosurfaces. The individual dissociated energies shown in Figure 8 reveal that two strong H-bonds are formed. One corresponds to the charge assisted HB (N1+–H···O, 7.1 kcal/mol) and the other one corresponds to the N6–H6···O H-bond (6.1 kcal/mol) formed through the Watson–Crick face. The N6–H6···O H-bond formed through the Hoogsteen face is weaker (4.5 kcal/mol). These values are comparable to previously reported energies for adenine co-crystals with several carboxylic acids [18]. The QTAIM analysis also reveals the existence of a much weaker H-bond established between the aromatic CH bond of the six membered ring and N7 (2.0 kcal/mol), in line with the MEP surface plot depicted in Figure 6c. The total dissociation energy of this H-bonding network is very large (40.0 kcal/mol), specially taking into consideration that involves only one oxalate dianion, thus providing a great stability to compound **3**.

**Figure 8.** QTAIM distribution of bond and ring critical points (CPs, red and yellow spheres, respectively) corresponding to the pentameric assembly of compound **3**. The dissociation energies of the H-bonds are indicated next to the bond CPs. Superimposed NCIplot isosurfaces [*s* = 0.5, cut-off = 0.04 a.u., color scale −0.04 a.u. (blue) ≤ (signλ2)ρ ≤ 0.04 a.u. (red)] are also represented.
