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Dynamics of [n.3]paracyclophanes (n = 2 – 4) as studied by NMR. Obtaining separate Arrhenius parameters for two dynamic processes in [4.3]paracyclophane
Author(s) -
Szymański Sławomir,
Dodziuk Helena,
Pietrzak Mariusz,
Jaźwiński Jarosław,
Demissie Taye Beyene,
Hopf Henning
Publication year - 2013
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.3137
Subject(s) - chemistry , arrhenius equation , enantiomer , activation energy , reaction rate constant , ring flip , computational chemistry , stereochemistry , proton nmr , kinetics , thermodynamics , crystallography , organic chemistry , physics , ring (chemistry) , quantum mechanics
Extending our earlier findings for [3.3]paracyclophane, NMR line shape studies of the conformational dynamics in [3.2] and [4.3]paracyclophanes are reported, of which the former is conformationally homogeneous and the latter occurs in two enantiomeric forms. For [3.2]paracyclophane, the Arrhenius activation energy E a = 11.6 ± 0.1 kcal/mol and preexponential factor log ( A /s −1 ) = 12.92 ± 0.07 were found. In [4.3]paracyclophane, the conformational dynamics are quite complicated because, apart from interconversions of each enantiomer into itself proceeding via inversion of the propano bridge with rate constant k 1 , the enantiomers mutually rearrange with rate constant k 2 due to inversion of the butano bridge. The determination of Arrhenius parameters from dynamic 1 H spectra of the aromatic protons for these two conformational processes ( E a = 11.2 ± 0.5 kcal/mol and log ( A /s −1 ) = 13.6 ± 0.5 for the former, and E a = 9.7 ± 0.4 kcal/mol and log ( A /s −1 ) = 13.2 ± 0.4 for the latter) is the highlight of this work. In the investigated temperature range, in [4.3]paracyclophane, the occurrence of other conformational processes beyond those mentioned above can be excluded, because they would produce different line shape patterns than those actually observed. Copyright © 2013 John Wiley & Sons, Ltd.