Dynamics of [n.3]paracyclophanes (n = 2 – 4) as studied by NMR. Obtaining separate Arrhenius parameters for two dynamic processes in [4.3]paracyclophane |
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Authors: | S?awomir Szymański Helena Dodziuk Mariusz Pietrzak Jaros?aw Ja?wiński Taye Beyene Demissie Henning Hopf |
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Institution: | 1. Institute of Organic Chemistry, Polish Academy of Sciences, , 01‐224 Warsaw, Poland;2. Institute of Physical Chemistry, Polish Academy of Sciences, , 01‐224 Warsaw, Poland;3. Institute of Organic Chemistry, Technical University Braunschweig, , 38106 Braunschweig, Germany |
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Abstract: | 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 Ea = 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 k1, the enantiomers mutually rearrange with rate constant k2 due to inversion of the butano bridge. The determination of Arrhenius parameters from dynamic 1H spectra of the aromatic protons for these two conformational processes (Ea = 11.2 ± 0.5 kcal/mol and log (A/s?1) = 13.6 ± 0.5 for the former, and Ea = 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. |
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Keywords: | paracyclophanes conformational processes NMR line shape analysis Arrhenius parameters |
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