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The way benzannelation affects the degree of aromatic character in mobius annulenes is described. Computations on other mobius systems show that benzannelation clearly reduces the degree of bond-length equalization.

A report is presented with respect to the study conducted, which includes optically active sulfuranes rapid isomerization, and the lever mechanism is explained, and found that both the Berry as well as the lever mechanisms is necessary to achieve complete ligand exchange. An ideal monkey saddle can be approached to a high degree of approximation by an efective monkey saddle point adjusted by small variations in the chemistry.

Discusses effective monkey saddle points and Berry and lever mechanisms in the topomerization of SF[sub 4] and related tetracoordinated AX[sub 4] species. Sulfuranes; Interhalogens; Trends in the lever and Berry activation barriers; Effective monkey saddle points in AX[sub 4] species.

The first experimental example of a [4n]annulene derivative with one MObius twist, 1, was synthesized recently [Ajami, D.; Oeckler, O.; Simon, A.; Herges, FR. Nature 2003, 426, 819] and was purported to possess aromatic character. However, critical analysis of the published crystallographic data indicates that the MObius [16]annulene core of 1 shows large bond alternation (Δr up to 0.157 Å). Delocalization in this core is inhibited by large dihedral angles, which hinders effective π overlap. This conclusion is supported by computational results (B3LYP/6-31 1 +G**) on 1 and several less benzannelated derivatives, based on geometric (Δr, Δrm, Julg A, HOMA) and magnetic (NICS, magnetic susceptibility exaltation) criteria of aromaticity. That benzannelation results in bond localization in the [16]annulene core is shown by additional computations on benzannelated derivatives of other Möbius aromatic species. Additionally, the aromatic stabilization energy (ASE) of 1 has been reinvestigated using two different procedures. Evaluation of uncorrected ISE11 values of just the polyene bridge portion of 1 and its Hückel counterpart suggests that stabilization of 1 relative to its Hückel isomer is confined to the polyene bridge and is not due to a delocalized iv circuit. Furthermore, application of s-cis/s-trans corrections lowers the ISE11 value of 1 from 4.0 kcal/mol to 0.6 kcal/mol, suggesting that 1 is nonaromatic. [ABSTRACT FROM AUTHOR]

In 1966, Zimmerman proposed a type of Möbius aromaticity that involves through-space electron delocalization; it has since been widely applied to explain reactivity in pericyclic reactions, but is considered to be limited to transition-state structures. Although the easily accessible hexahelicene radical anion has been known for more than half a century, it was overlooked that it exhibits a ground-state minimum and robust Zimmerman-Möbius aromaticity in its central noose-like opening, becoming, hence, the oldest existing Möbius aromatic system and with smallest Möbius cycle known. Despite its overall aromatic stabilization energy of 13.6 kcal mol -1 (at B3LYP/6-311+G**), the radical also features a strong, globally induced paramagnetic ring current along its outer edge. Exclusive global paramagnetic currents can also be found in other fully delocalized radical anions of 4N+2 π-electron aromatic polycyclic benzenoid hydrocarbons (PAH), thus questioning the established magnetic criterion of antiaromaticity. As an example of a PAH with nontrivial topology, we studied a novel Möbius[16]cyclacene that has a non-orientable surface manifold and a stable closed-shell singlet ground state at several density functional theory levels. Its metallic monoanion radical (0.0095 eV band gap at HSE06/6-31G* level) is also wave-function stable and displays an unusual 4π-periodic, magnetically induced ring current (reminiscent of the transformation behaviour of spinors under spatial rotation), thus indicating the existence of a new, Hückel-rule-evading type of aromaticity., (© 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

In sigma complexes, intermediates in nucleophilic and electrophilic aromatic substitution and other reactions, delocalization in the aromatic ring is formally disrupted. Unexpectedly, computational evidence is presented that favorable processes contain aromatic sigma complexes. Tetracoordinated carbon therein surprisingly employs orbitals that are more similar to sp 2 than to sp 3 hybrids in sigma bonds with adjacent ring atoms. Both leaving groups and nucleo- or electrophiles may donate electrons to the π-system depending on the availability of p-type orbitals to fulfill Hückel (4N+2) or Möbius (4N) rules of aromaticity in analogy to conjugated transition-metal metallacycles., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Magnetic aromaticity and antiaromaticity of closed shell metalloaromatics with 4d transition metals (Nb, Tc, Rh) is strictly correlated with the orbital topology (Möbius or Hückel) of their π-HOMO, investigated computationally with DFT methods. A surprisingly simple rule emerged: the metallacycle is aromatic (antiaromatic) when the number of π MOs is even and the π-HOMO is of Möbius (Hückel) topology-and vice versa when the number of π MOs is odd., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Inspired by recent reports of low-valent iron-complex-catalyzed formal [2+2] cycloaddition of olefins, we demonstrate computationally that with such low-valent iron complexes and with "strong" ligands, the olefin metathesis is also preferred over the undesired cyclopropanation side-reaction, competition already studied by Hoffmann and co-workers almost 40 years ago (J. Am. Chem. Soc. 1981, 103, 5582). The [2+2] cycloaddition step in metathesis propagation, which gives a Chauvin-type metallacyclobutane intermediate, is proposed to proceed either via a planar four-electron Craig-Möbius aromatic [π2 s +π2 s ] transition-state structure with a low barrier of 4.7 kcal mol -1 or, alternatively, via a twisted Zimmerman-Möbius aromatic [π2 s +π2 a ] transition state with a 5.5 kcal mol -1 activation-energy barrier, with respect to an "encounter" π-complex minimum obtained from an Fe II alkylidene and the entering olefin, while the corresponding triplet pathways are all disfavored., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Herein, the Zimmerman Möbius/Hückel concept is extended to pericyclic reactions involving transition metals. While sigmatropic hydrogen shifts in parent hydrocarbons are either uniquely antarafacial or suprafacial, we have shown by theoretical orbital topology considerations and quantum chemical computations at DFT level that both modes of stereoselectivity must become allowed in the same system as a consequence of Craig-Möbius-type orbital arrays, in which a transition metal d orbital induces a phase dislocation in metallacycles. This may have fundamental implications for the understanding of reactivity and bonding in organometallic chemistry., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)