Aromaticity and antiaromaticity: what role do ionic configurations play in delocalization and induction of magnetic properties?

Abstract

epoc ABSTRACT: C m H m (m ¼ 4, 6, 8) species are analyzed in D mh and D (1/2)mh geometries by means of valence bond (VB) calculations. The fundamental factors that distinguish aromatic and antiaromatic modes of electron delocalization are elucidated by analysis of the mixing between the covalent-state and ionic structures that distribute the electrons in all possible modes available in the cycle. The major difference found is that, by contrast to the aromatic species where all the ionic structures mix into the covalent state, in the antiaromatic species the set of diagonal-ionic structures is excluded from mixing with the covalent-state, owing to its fundamental symmetry features. This exclusion of covalent-ionic mixing is expressed at the most fundamental building blocks of the wavefunction; the spin-alternant state. The spin-alternant state is a resonance hybrid of the two spin-alternant determinants. This resonance hybrid will support a collective motion of the p -electrons around the perimeter of the ring only if ionic structures can mix to mediate the electronic flow. It is shown that in aromatic species all the ionic structures mix and sustain a continuous electronic flow around the ring perimeter. By contrast, owing to the exclusion of the diagonal-ionic structures in antiaromatic compounds, the electronic flow in antiaromatic species is interrupted. Symmetry and angular momentum analyses of the ground state in the presence of an external magnetic field show that the properties of the spin-alternant state can qualitatively describe the magnetic properties of the two classes. The continuous flow of -electrons mediated by the ionic structures of aromatic species is responsible for the enhanced diamagnetism of these species. By contrast, paramagnetic -ring current in antiaromatic species becomes possible only in the presence of the magnetic field that allows the mixing of the otherwise excluded ionic structures. Scheme 1. Vertical resonance energies (VREs) calculated by valence bond (Ref. 10) for benzene, CBD and COT in their uniform D mh geometries (with equal C-C bond lengths) 732 A. SHURKI ET AL.Figure 5. The reduced resonance between the spin-alternant determinants describes a collective circular motion of the -electrons. The propensity of this 'motion' depends on the overlap and matrix element of the determinants 738 A. SHURKI ET AL.

Real Space Delocalization, Resonance and Aromaticity

“…These results contradict an earlier comparison [23], which however only took concerted six-electron movements in account. The importance of ionic structures is in good agreement with the results by Shurki et al [24] for the same direction movement. In general, the role of the ionic spin structures is, however, not qualitatively different from the role of the ionic structure in H 2 , since the dominant opposite direction movement is much less affected by the restriction to covalent structures.…”

“…In order to explore delocalization, the maximum probability paths connecting these structures thus have to be investigated. This is somewhat analogous to work by Shurki et al [24] and seemingly related to the concept of diamagnetic ring currents [20,3]. However, like Maynau and Malrieu [23] we do not restrict our search to paths, where all electrons move in the same circular direction (Fig.…”

“…Already before Hückel's work, the diamagnetic properties of aromatic systems were attributed to electronic 'ring currents' [2,19,20,3]. Theoretical studies used this notion of ring currents to explain aromaticity also in absence of a magnetic field and discussed the role of the ionic structures [21,22,23,24]. Notably, Shurki et al stated that the "flow of the electrons around the C 4 H 4 ring is, therefore,…”

“…an infinitely large probabilistic barrier). This discussion of parity is related to the 'electron-switch symmetry index' by Shurki et al [24] If two rings (spin-up and spin-down) are combined (cf. the alternating spin structure of benzene), the intermediate value theorem apparently does not help anymore: rotating two rings of the same size at once always preserves the sign of Ψ independent of the parity.…”

Electronic Structures and Spin Topologies of γ-Picoliniumyl Radicals. A Study of the Homolysis ofN-Methyl-γ-picolinium and of Benzo-, Dibenzo-, and Naphthoannulated Analogs

“…D, E & F in Chart 1) is avoided as much as possible and this is consistent with retaining a high degree of aromaticity. 64 Roberts and Szwarc reported bond dissociation energies of 75.5, 76.5, and 77.5 kcal/mol, respectively, for α-, βand γ-picoline, respectively, 65 relative to the bond dissociation energy of toluene which was ΔH 298 = 77.5 kcal/mol in 1948. Using ΔH 298 = 89.6 kcal/mol for the bond dissociation energy of toluene, Kromkin et al 62 reported bond dissociation energies for α-, βand γ-picoline of 87.2, 90.4, and 86.5 kcal/mol, respectively, and of 86.0 kcal/mol for 4-methyl-quinoline (4MQ).…”