| 1. |
- Escayola, Silvia, et al.
(författare)
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Guidelines for Tuning the Excited State Huckel-Baird Hybrid Aromatic Character of Pro-Aromatic Quinoidal Compounds**
- 2021
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Ingår i: Angewandte Chemie International Edition. - : John Wiley & Sons. - 1433-7851 .- 1521-3773. ; 60:18, s. 10255-10265
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Tidskriftsartikel (refereegranskat)abstract
- Pro-aromatic molecules have higher-energy diradicaloid states that are significantly influenced by resonance structures in which conjugated rings take on Huckel-aromatic character. Recently, it has been argued that there are also pro-aromatic molecules that adopt central units with 4n pi-electron Baird-aromatic character in the T-1 state, although detailed analysis suggests that these compounds are better labelled as T-1 Huckel-Baird hybrid molecules where Huckel-aromaticity dominates. Herein, we consider a series of symmetrically substituted conjugated rings with potential Baird aromaticity in the lowest excited triplet and singlet states. Our computational results allow us to establish general guidelines for the rational design of molecules with excited state Huckel/Baird aromaticity in pro-aromatic quinoidal compounds. We found two main strategies to promote high Baird aromatic character: 1) anionic and small conjugated rings with electron donating groups as substituents and small exocyclic groups with electron withdrawing substituents, or 2) electron deficient conjugated rings with exocyclic electron-donor substitution.
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| 2. |
- Escayola, Silvia, et al.
(författare)
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In the quest of Huckel-Huckel and Huckel-Baird double aromatic tropylium (tri)cation and anion derivatives
- 2023
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Ingår i: Journal of Physical Organic Chemistry. - : John Wiley & Sons. - 0894-3230 .- 1099-1395. ; 36:1
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Tidskriftsartikel (refereegranskat)abstract
- Besides the most common form of aromaticity involving a pi-ring, hexaiodobenzene and hexakis(phenylselenyl)benzene dications also present sigma-aromaticity in the outer ring formed by the main group substituents. These two compounds are considered sigma- and pi-double aromatic, and their characterization is of special interest to the fields of organic and structural chemistry. In this work, we decided to explore the double aromaticity in substituted tropylium cations for three reasons: (i) the seven neutral halogen substituents of the tropylium cations will, without oxidation, lead to 14 sigma-electrons (a 4n + 2 Huckel number); (ii) tropylium cations are highly stable and can be easily generated experimentally; and (iii) whereas in substituted benzenes the distances between substituents in the optimized structures or X-ray crystals are too large to allow strong sigma-aromaticity, these distances are expected to be shorter in substituted tropylium cations. Yet, instead of the expected sigma-aromaticity, we found that the most stable geometries are highly puckered, meaning that delocalization in both pi- and sigma-systems is lost. Our results, which include also the tropylium anion and trication in the singlet and triplet state, show that there is a need to open a lone pair hole by oxidation to generate sigma-aromaticity. Among the systems studied, only triplet C7Br7+3 with an internal Huckel aromatic tropylium ring and an external incipient Baird aromatic Br-7 ring shows double pi- and sigma-aromaticity. This result, however, is functional-dependent and reveals that (C7Br73+)-C-3 is at the borderline for onset of double aromaticity.
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| 3. |
- Poater, Jordi, et al.
(författare)
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Single-Not Double-3D-Aromaticity in an Oxidized Closo Icosahedral Dodecaiodo-Dodecaborate Cluster
- 2023
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 145:41, s. 22527-22538
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Tidskriftsartikel (refereegranskat)abstract
- 3D-aromatic molecules with (distorted) tetrahedral, octahedral, or spherical structures are much less common than typical 2D-aromatic species or even 2D-aromatic-in-3D systems. Closo boranes, [BnHn](2-) (5 = n = 14) and carboranes are examples of compounds that are singly 3D-aromatic, and we now explore if there are species that are doubly 3D-aromatic. The most widely known example of a species with double 2D-aromaticity is the hexaiodobenzene dication, [C6I6](2+). This species shows p-aromaticity in the benzene ring and s-aromaticity in the outer ring formed by the iodine substituents. Inspired by the hexaiodobenzene dication example, in this work, we explore the potential for double 3D-aromaticity in [B12I12](0/2+). Our results based on magnetic and electronic descriptors of aromaticity together with B-11{H-1} NMR experimental spectra of boron-iodinated o-carboranes suggest that these two oxidized forms of a closo icosahedral dodecaiodo-dodecaborate cluster, [B12I12] and [B12I12](2+), behave as doubly 3D-aromatic compounds. However, an evaluation of the energetic contribution of the potential double 3D-aromaticity through homodesmotic reactions shows that delocalization in the I-12 shell, in contrast to the 10s-electron I-6(2+) ring in the hexaiodobenzene dication, does not contribute to any stabilization of the system. Therefore, the [B12I12](0/2+) species cannot be considered as doubly 3D-aromatic.
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