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Mobilité conformationnelle et migration des liaisons π dans le [24]annulène
Author(s) -
Oth Jean F. M.,
de Julien de Zélicourt Yves
Publication year - 1999
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/(sici)1522-2675(19990310)82:3<435::aid-hlca435>3.0.co;2-h
Subject(s) - conformational isomerism , chemistry , annulene , crystallography , enthalpy , spectral line , stereochemistry , molecule , thermodynamics , physics , organic chemistry , astronomy
Conformational Mobility and Migration of the π Bonds of the [24]annulene. The configuration and the conformation of [24]annulene have been determined after a detailed analysis of its 1 H‐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 C 3 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]‐7 3 annulene is negative and of the order of −9 to −10 kcal⋅mol −1 .