Dynamics of [n.3]paracyclophanes (n = 2 – 4) as studied by NMR. Obtaining separate Arrhenius parameters for two dynamic processes in [4.3]paracyclophane

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.