**5. Conclusions**

We presented an analysis of aromaticity and antiaromaticity modulated hydrogen bonds using quantum chemical topology tools, namely the QTAIM, the IQA energy partition as well as the electronic delocalisation indicators FLU and MCI. For this purpose, we considered rings containing either the H-bond acceptor (ACR) or the H-bond donor (DCR). Our results show how the formation of the investigated H-bonds can trigger subtle electronic rearrangements with a quite significant impact in the stability and properties of the involved interacting systems. We described large changes in QTAIM charges and electron delocalisation indices along with their accompanying classical and exchange-correlation components of the IQA interaction energies related with the formation of these HB clusters. We also found fundamental differences within the ACR and DCR systems, for example, the weakening and strengthening of double bonds within the cyclic structures of ACR and DCR, a condition which leads to the destabilisation and stabilisation of the rings in these systems. Additionally, we related the enhancement and impairment of the examined Hbonds with respect to non-aromatic (i.e., non-cyclic) structures with changes in the aromatic and antiaromatic character of the system. We observe that reductions in aromaticity can be interpreted as increases in antiaromaticity and vice versa. Therefore, our results indicate that aromaticity and antiaromaticity can be considered on a common scale using QCT tools. Our results also point that the deviation from planarity of specific AMHB clusters could be related with a trend of the system to ameliorate a reduction in aromaticity. Overall, we expect the results of our investigation to provide novel useful insights about the intricate interplay among H-bond and *π* systems.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/molecules27186039/s1, Table S1. Aromaticity metrics, Table S2. Electronic energies, Tables S3 and S4. Binding energies, Tables S5 and S6. IQA and QCT descriptors, Table S7. IQA group energies, Table S8. IQA ring energies, Table S9. Atomic charges, Tables S10–S22. Optimised geometries, Table S23. Electronic energies for the isomers of the AZH (DCR) dimer, Table S24. Atomic charges for the isomers of the AZH (DCR) dimer, Tables S25–S28. IQA and QCT descriptors for the isomers of the AZH (DCR) dimer, Figures S1 and S2. Change in the IQA interaction energies upon dimerisation, Figure S3. DFT dimerisation energy as a function of the electron density at the BCP of the intermolecular HB contacts.

**Author Contributions:** Conceptualization and methodology, J.M.G.-V., M.G. and T.R.-R.; software, Á.M.P.; validation, formal analysis; investigation, resources, data curation, D.B.-E., M.G. and J.M.G.-V.; writing and visualization, M.G., J.M.G.-V. and T.R.-R.; supervision, project administration, J.M.G.-V., T.R.-R. and Á.M.P.; funding acquisition, T.R.-R. and Á.M.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** We gratefully acknowledge financial support the Spanish MICINN (grant PGC2018-095953- B-I00). M. Gallegos specially acknowledges the Spanish MICIU/MIU for the FPU19/02903 grant. We are also grateful to DGTIC/UNAM (project LANCAD-UNAM-DGTIC-250) for computer time.

**Data Availability Statement:** Structures of the studied systems are reported in the Electronic Supplementary Information.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Sample Availability:** Samples of compounds are not available from the authors.
