Dynamics of [n.3]paracyclophanes (n = 2 – 4) as studied by NMR. Obtaining separate Arrhenius parameters for two dynamic processes in [4.3]paracyclophane. (27th May 2013)

• Record Type: Journal Article
• Authors: Szymański, Sławomir; Dodziuk, Helena; Pietrzak, Mariusz; Jaźwiński, Jarosław; Demissie, Taye Beyene; Hopf, Henning
• Is Part Of: Journal of physical organic chemistry. Volume 26:Number 7(2013:Jul.)
• Page Start: 596
• Page End: 600
• Publication Date: 2013-05-27
• Subjects: Chemistry, Physical organic -- Periodicals; 547.1
• DOI: 10.1002/poc.3137
• Languages: English
• ISSNs: 0894-3230

Abstract

<abstract abstract-type="main"> <title> <x xml:space="preserve">Abstract</x> </title> <p>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 <italic>E</italic><sub>a</sub> = 11.6 ± 0.1 kcal/mol and preexponential factor log (<italic>A</italic>/s<sup>−1</sup>) = 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 <named-content id="d136e493" content-type="reactionType" xlink:type="simple" xmlns:xlink="http://www.w3.org/1999/xlink">inversion</named-content> of the propano bridge with rate constant <italic>k</italic><sub>1</sub>, the enantiomers mutually rearrange with rate constant <italic>k</italic><sub>2</sub> due to <named-content id="d136e507" content-type="reactionType" xlink:type="simple" xmlns:xlink="http://www.w3.org/1999/xlink">inversion</named-content> of the butano bridge. The determination of Arrhenius parameters from dynamic <sup>1</sup>H spectra of the aromatic protons for these two conformational processes (<italic>E</italic><sub>a</sub> = 11.2 ± 0.5 kcal/mol and log (<italic>A</italic>/s<sup>−1</sup>) = 13.6 ± 0.5 for the former, and <italic>E</italic><sub>a</sub> = 9.7 ± 0.4 kcal/mol and log (<italic>A</italic>/s<sup>−1</sup>) = 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.</p> </abstract>