Aromaticity and Chemical Bonding

Aromaticity is a core concept in chemistry that has extended from organic to inorganic species, opening the door to a series of new aromatic molecules that could be used in various applications, including the preparation of photovoltaic cells, nonlinear optical materials and electroluminiscene displays [1-8]. In order to identify species with salient aromatic properties one should design appropriate characterization tools. Our group has worked on the development of electron-delocalization aromaticity indices [9-17], the study of the role of the delocalization error in aromaticity [5-6] and some recent applications [1-5]. Our group has contributed with five aromaticity measures (FLU [16], ING [14], INB [14], AV1245 [10] and AVmin [9]) and the first aromaticity goodness tests [12-13]. ING and INB are the most reliable indicators of aromaticity found thus far, whereas AV1245 and AVmin are especially designed to study the aromaticity of large rings, such as nanorings and porphyrins. Lately, we have also turned our focus to the analysis of Hilbert-space atomic partitions that can be employed to compute multicenter indices such as Iring and MCI in large macrocycle rings, where real space partitions produce either too large numerical errors (Iring and MCI) or carry a gargantuan computational cost (MCI) [17]. We are currently exploring the limitations and capabilities of Robust Hilbert space partitions.

Six-porphyrin nanoring
Beyond aromatic systems, our interest extends to molecular species with atypical electron distribution that challenges traditional concepts of chemical bonding and are known as molecular electrides. These compounds are characterized by the presence of localized electrons occupying spatial regions not associated with any atomic nucleus, effectively functioning as anions. This unconventional electronic arrangement endows electrides with remarkable properties, including strong reducing power, superconductivity, and magnetic susceptibility, and high nonlinear optical (NLO) responses. As a result, electrides are highly attractive for a wide range of applications, such as catalysis, electron emission devices, reversible hydrogen storage, advanced NLO materials, and next-generation energy conversion technologies. Our group has devoted significant effort to the characterization and analysis of electrides [18–20], as well as to the design of new electride structures [21].

Six-porphyrin nanoring
Six-porphyrin nanoring

Recent publications:
1. Toldo J., Staab J., Matito E., Foroutan-Nejad C., Ottosson H.; Deciphering the molecular origin of the 19.3 eV electronic excitation energy of H3+. Chem. Sci. (accepted)
2. Orozco-Ic M., Soriano-Agueda L., Sundholm D., Matito E., Merino G.; Core-Electron Contributions to the Magnetic Response of Molecules with Heavy Elements and their Significance in Aromaticity Assessments. Chem. Sci. 15, 12906 (2024)
3. Escayola S., Labella J., Szczepanik D.W., Poater A., Torres T., Solà M., Matito E.; From (Sub)Porphyrins to (Sub)Phthalocyanines: Aromaticity Signatures in the UV-Vis Absorption Spectra. Inorg. Chem. 63, 18251 (2024)
4. Wang Z.-C., Tkachenko N. V., Qiao L., Matito E., Muñoz-Castro A., Boldyrev A. I., Sun Z.-M.; All-Metal σ-Antiaromaticity in Dimeric Cluster Anion {[CuGe9Mes]2}4−. Chem. Commun. 56, 6583 (2020).
5. Casademont-Reig I., Ramos-Cordoba E., Torrent-Sucarrat M., Matito E.; How Do the Hückel and the Baird Rules Fade Away in Annulenes? Molecules 25, 711 (2020).
6. Casademont-Reig I., Woller T., Contreras-García J., Alonso M., Torrent-Sucarrat M., Matito E.; New Electron Delocalization Tools to Describe the Aromaticity in Porphyrinoids. Phys. Chem. Chem. Phys. 20, 2787 (2018).
7. Popov I. A., Pan F.-X., You X. R., Li L.-J., Matito E., Sun Z.-M., Liu C., Zhai H.-J., Boldyrev A. I.; Peculiar All-Metal σ-Aromaticity of [Au2Sb16]4− Anion in Solid State. Angew. Chem. Int. Ed. 55, 15344 (2016).
8. Min X., Popov I. A., Pan F.-X., Li L.-J., Matito E., Sun Z.-M., Wang L.-S., Boldyrev A. I.; All-Metal Antiaromaticity in Sb4-Type Lanthanocene Anions. Angew. Chem. Int. Ed. 55, 5531 (2016)

Design of aromaticity indices:
9. García-Fernández C., Sierda E., Abadía M., Bugenhagen B., Prosenc M. H., Wiesendanger R., Bazarnik M., Ortega J. E., Brede J., Matito E., Arnau A.; Exploring the Relation between Intramolecular Conjugation and Band Dispersion in One-Dimensional Polymers. J. Phys. Chem. C 121, 27118 (2017)
10. Matito E.; Electronic Aromaticity Index for Large Rings. Phys. Chem. Chem. Phys. 18, 11839 (2016)
11. Feixas F., Matito E., Poater J., Solà M.; Quantifying Aromaticity with Electron Delocalization Measures. Chem. Soc. Rev. 44, 6434–6451 (2015)
12. Feixas F., Jiménez-Halla J. O. C., Matito E., Poater J., Solà M.; A Test to Evaluate the Performance of Aromaticity Descriptors in All-Metal and Semi-Metal Clusters. An Appraisal of Electronic and Magnetic Indicators of Aromaticity. J. Chem. Theory Comput. 6, 1118 (2010)
13. Feixas F., Matito E., Poater J., Solà M.; On the Performance of Some Aromaticity Indices: A Critical Assessment Using a Test Set. J. Comput. Chem. 29, 1543–1554 (2008)
14. Cioslowski J., Matito E., Solà M.; Properties of Aromaticity Indices Based on the One-Electron Density Matrix. J. Phys. Chem. A 111, 6521–6525 (2007)
15. Matito E., Solà M., Salvador P., Duran M.; Electron Sharing Indexes at the Correlated Level. Application to Aromaticity Measures. Faraday Discuss. 135, 325–345 (2007)
16. Matito E., Duran M., Solà M.; The Aromatic Fluctuation Index (FLU): A New Aromaticity Index Based on Electron Delocalization. J. Chem. Phys. 122, 014109 (2005); J. Chem. Phys. 125, 059901 (2006)
17. Grèbol-Tomàs J., Matito E., Salvador P.; Can Aromaticity be Evaluated Using Atomic Partitions based on the Hilbert-space? Chem. Eur. J. 30, e202401282 (2024)

Electrides:
18. El Bakouri O., Postils V., Garcia-Borràs M., Duran M., Luis J.M., Calvello S., Soncini A., Matito E., Feixas F., Solà M.; Chem. Eur. J. 24, 9853 (2018)
19. Sitkiewicz S.P., Ramos-Cordoba E., Luis J.M., Matito E.; J. Phys. Chem. A 125, 4819-4835 (2021)
20. Postils V., Garcia-Borràs M., Solà M., Luis J.M., Matito E.; Chem. Commun. 51, 4865 (2015)
21. Xu J., Yao S., Postils V., Matito E., Lorent C. Driess M.; Chem. Sci. 16, 10826-10832 (2025)