18446-45-2Relevant academic research and scientific papers
Conformational mobility and migration of the π bonds of the [24]annulene
Oth, Jean F. M.,De Julien De Zelicourt, Yves
, p. 435 - 483 (2007/10/03)
The configuration and the conformation of [24]annulene have been determined after a detailed analysis of its 1H-NMR spectrum recorded at - 95°. At this temperature, molecular dynamics is practically frozen, and the spectrum can be correctly simulated considering eight magnetic sites with the relevant couplings. The [24]annulene exhibits alternation of the double and the single bonds with the CTTTCTTTCTTT sequence (C = cis, T= trans) expressing the connectivity of the double bonds. The signal of the 9 protons pointing inside the ring is 7.72 ppm at lower field than the signal of the 15 outer protons; this indicates a marked paramagnetic ring current. Molecular dynamics is revealed by the dependence of the spectrum upon the temperature; the simulation of the line shape of these spectra indicates that the [24]annulene in Solution exists as an equilibrium of two conformers A and B (B/A ≤ 0.05), both having the same configuration. Each of these conformers undergoes two isodynamic processes: a migration of the π bonds on the adjacent single bonds (bond shift) described by V and a conformational mobility described by K. The two conformers interconvert extremely rapidely. Conformer A complies with C(3h) symmetry, conformer B with C3 symmetry. The enthalpy, entropy, and free energy of activation for the processes described by V and K in the major conformer A have been determined: these processes are slower than those observed in [16]annulene. From their values, we could deduce that the resonance energy in the [24]-73annulene is negative and of the order of - 9 to - 10 kcal · mol-1.
