**4. Results and Discussion**

### *4.1. General Energetic Changes Induced by HB Formation*

We consider first the differences in dimerisation energies of the different systems shown in Figure 2. Table 1 reports the values of ΔΔ*E*,

$$
\Delta\Delta E(\mathbf{Y}\_2) = \Delta E(\mathbf{Y}\_2) - \Delta E(\mathbf{R}\_2),
\tag{3}
$$

in which Δ*E*(Y2) is the energy change associated to the process

$$\mathbf{Y} + \mathbf{Y} \rightleftharpoons \mathbf{Y} \cdots \mathbf{Y}, \quad \Delta E(\mathbf{Y}\_2), \tag{4}$$

and R is the corresponding reference system used for ACR and DCR, namely the dimers of formamide (NCO) and formamidine (NCN) in the corresponding ACR and DCR form. A negative/positive value of ΔΔ*E*(X2) indicates a stronger/weaker interaction in X···X with respect to the reference complex R···R.

The straightforward comparison of the DFT and CC values for ΔΔ*E* reveals that both levels of theory are in good agreement concerning the sign and magnitude of ΔΔ*E*. These observations indicate that our DFT results offer a reliable picture of the energetics of the binding phenomena under study. As the footnote of Table 1 reports, all the values for Δ*E* are negative, pointing, as expected, to stabilising dimerisation contacts in all the investigated dimers. Furthermore, the easiness of the complexation seems to be driven, as

expected, by the hydrogen bond formation as reflected by the correlation of the binding energies with the *ρ* at the bond critical point of the HB contacts (see SI Figure S3). We also note that the N–C=N bonding pattern leads to lower binding energies, due to the larger acidity of H atoms bonded to oxygen. The AMHB [32] interpretation of the sign of ΔΔ*E* in Table 1 states that the ACR dimers AZH and AZA display either an increase in aromaticity or a decrease in antiaromaticity, respectively, as a consequence of the formation of the investigated H-bonds. Ditto for the DCR clusters 2HP and 2AP. On the other hand, the ACR complexes 2HP and AZA along with the DCR systems AZH and AZA exhibit the opposite behaviour. We consider now QCT analyses to further dissect these energetic trends.

**Table 1.** Values of ΔΔ*E*, as defined in Equation (3), computed in the DFT and CC approximations described in the main text. NCO and NCN denote formamide and formamidine, respectively, the reference systems shown in Figure 2c. All values are reported in kcal/mol.


The corresponding values for <sup>Δ</sup>*E*/kcal· mol−<sup>1</sup> for the references in Formula (4) are NCO (ACR): <sup>Δ</sup>*E*DFT = −35.45, <sup>Δ</sup>*E*CC = −30.10; NCN (ACR): <sup>Δ</sup>*E*DFT = −16.75, <sup>Δ</sup>*E*CC = −13.64; NCO (DCR): <sup>Δ</sup>*E*DFT = −15.93, <sup>Δ</sup>*E*CC = −13.59; NCN (DCR): <sup>Δ</sup>*E*DFT = −16.75, <sup>Δ</sup>*E*CC = −13.64.
