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Theoretical studies of [ n ]paracyclophanes and their valence isomers. I. Geometries, strain energies, and enthalpies of the inter‐conversions of [ n ]paracyclophanes and their Dewar benzee isomers
Author(s) -
Bockisch F.,
Rayez J.C.,
Liotard D.,
Duguay B.
Publication year - 1992
Publication title -
journal of computational chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.907
H-Index - 188
eISSN - 1096-987X
pISSN - 0192-8651
DOI - 10.1002/jcc.540130903
Subject(s) - mindo , mndo , chemistry , isomerization , standard enthalpy of formation , valence (chemistry) , enthalpy , benzene , computational chemistry , ring strain , thermochemistry , thermodynamics , organic chemistry , molecule , physics , catalysis
Four semiempirical methods (AM1, MNDO, PM3, and MINDO/3) are used to calculate the deformation angles of [ n ]paracyclophanes and their Dewar benzene isomers for n = 3… 10. The results obtained by all these methods are in good agreement with data from X‐ray studies. We have determined the strain energies that, in both series of compounds, are due to two components: (1) the strain energy of deformation of the cycle (aromatic or Dewar Benzene skeletons) and (2) the strain energy of the oligomethylene chain. In [6]paracyclophane, the strain energy [ SE ring (MNDO) ≈ 32.9 kcal/mol] almost compensates the resonance energy ( E resonance ≈ 36 kcal/mol) so that its chemical properties are closer to alkenes than to benzenic compounds. To better reproduce the enthalpy of the valence isomerization [ n ]Dewar bezene → [ n ]paracyclophane, which is poorly calculated with these methods, a correction is proposed and the reaction enthalpy of [6]paracyclophane is estimated to be about Δ H r ≈ 15 ± 15 kcal/mol. It is found that MNDO and MINDO/3 need the smallest corrections, but MNDO leads to better geometries than MINDO/3. In conclusion, MNDO seems to be the best technique for further studies of these compounds. © 1992 by John Wiley & Sons, Inc.