Mobilité conformationnelle et migration des liaisonsπ dans le [24]annulène

Fully Conjugated [4]Chrysaorene. Redox-Coupled Anion Binding in a Tetraradicaloid Macrocycle

“…Furthermore, the ring current in 3a is apparently much stronger than in [24]annulene, for which the inner protons were found to resonate at 12.08 and 12.66 ppm. 27 The paratropicity is more likely to originate from [36]annulenoid pathways such as 3′'', which (a) do not imply complete AWA decoupling and (b) agree with the observed partial localization of double bonds outside the path (Figure 1). Importantly, 3′′′ and similar paths can be constructed by employing the same set of canonical structures that is used in the closed-shell benzenoid formulation 3 (see Scheme S6).…”

“…However, the lengths of radial C–C bonds in both the experimental and calculated geometries of [4]­chrysaorene (typically 1.43 to 1.44 Å) indicate that the outer and inner circuits in 3 are not fully decoupled. Furthermore, the ring current in 3a is apparently much stronger than in [24]­annulene, for which the inner protons were found to resonate at 12.08 and 12.66 ppm . The paratropicity is more likely to originate from [36]­annulenoid pathways such as 3′’’ , which (a) do not imply complete AWA decoupling and (b) agree with the observed partial localization of double bonds outside the path (Figure ).…”

Annulenes, 1980–2000

“…[24]Annulene 16 has been the subject of a recent detailed report, 54 investigating conformational mobility and migration of π-bonds, using temperature dependent 1 H NMR spectroscopy. [24]Annulene 16 is a blue-black crystalline substance which forms a violet solution.…”

Experimental and Theoretical Evidence for Aromatic Stabilization Energy in Large Macrocycles

“…Conversely, other factors may destabilize the transition state, so that Δ G ‡ overestimates the ASE, as indicated by the observation that antiaromatic [16] and [20]­annulenes also have significant barriers for this type of exchange process, even when the ASE is expected to be negative. Values of Δ G ‡ for [ N ]­annulenes ( N = 12–24) show a clear alternation, with higher barriers for N = 4 n + 2 and lower barriers for N = 4 n (Figure b). ,− If we assume that this alternating pattern reflects the underlying aromatic and antiaromatic ASEs, which make it alternately harder and easier to twist the molecule out of conjugation, then an estimate of the ASE can be made according to eq , as represented by the lengths of the red arrows in Figure b. …”

“…The Eyring equation gives the energy barrier Δ G ‡ in terms of the gas constant R , the temperature T , the Boltzmann constant k B , the Planck constant h , and the rate of exchange k . (b) Barriers for this type of exchange process for [ N ]­annulenes ( N = 12–24) from 1 H NMR spectroscopy. ,− The blue line is drawn between the averages for neighboring points. The lengths of the red arrows is the ASE­( N ) from eq .…”