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

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 E a = 11.6 AE 0.1 kcal/mol and preexponential factor log (A/s À1 ) = 12.92 AE 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 AE 0.5 kcal/mol and log (A/s À1 ) = 13.6 AE 0.5 for the former, and E a = 9.7 AE 0.4 kcal/mol and log (A/s À1 ) = 13.2 AE 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.

Tuning Supramolecular Polymer Assembly through Stereoelectronic Interactions

“…Although dt­[3.3]­pCps are well-known to the chemical literature, a detailed study of their strain energies and conformational dynamics has not been undertaken in the same way as that for [2.2]­pCp and [3.3]­pCps . − Alternative studies have been extensively carried out with [3.3]­metacyclophanes carrying heteroatoms in the bridge, revealing changes in barriers associated with analogous conformational processes to [3.3]­pCps as a function of linker identity. − Figure summarizes the structural differences of hydrocarbon [3.3]­pCp and sulfur bridged dt­[3.3]­pCp- S obtained by X-ray crystallography. A longer C–S bond compared to C–C bond (1.82 Å vs 1.53 Å) leads to less bent aromatic rings (4.7° compared to 6.4°) in dt­[3.3]­pCp- S vs [3.3]­pCp .…”

Structure, NMR and Electronic Spectra of [m.n]Paracyclophanes with Varying Bridges Lengths (m, n = 2–4)

“…Temperature effects on the proton chemical shifts have also been studied experimentally, with the chemical shifts of the ethano-bridge protons of 2 and 3 4 decreasing when the temperature decreases from 303 to 223 K.…”

“…Earlier, in our group, using line-shape analysis combined with B3LYP-computed NMR chemical shifts as well as previous studies in solution, , the interconversion barrier between the cis and trans conformations of 4 was determined to be 12.29 ± 0.08 kcal/mol. We also recently reported an analysis of the dynamics of [2.3]­paracyclophane, 2 , and [3.4]­paracyclophane, 5 , using temperature-dependent NMR measurements, obtaining an Arrhenius activation energy of 11.6 ± 0.1 kcal/mol for 2 . In the same report, two separate sets of Arrhenius parameters (activation energies 11.2 ± 0.5 kcal/mol for the propano and 9.7 ± 0.2 kcal/mol for the butano-bridges) were obtained for 5 .…”

“…We also recently reported an analysis of the dynamics of [2.3]­paracyclophane, 2 , and [3.4]­paracyclophane, 5 , using temperature-dependent NMR measurements, obtaining an Arrhenius activation energy of 11.6 ± 0.1 kcal/mol for 2 . In the same report, two separate sets of Arrhenius parameters (activation energies 11.2 ± 0.5 kcal/mol for the propano and 9.7 ± 0.2 kcal/mol for the butano-bridges) were obtained for 5 . In the case of 6 , rotation of the rings, as well as conformational motions of the bridges, give four conformational isomers ( vide infra ) of which only the two lowest ones have significant population at room temperature.…”

“…Because of these interesting properties and promising prospective applications of cyclophanes, a detailed analysis of the spectroscopic properties of these molecules is of value. We have previously reported a combined computational and experimental studies of NMR parameters in [2.2]- ,, and [3.3]­paracyclophanes and dynamic NMR spectra of [ n .3]­paracyclophanes ( n = 2–4), as well as electronic and magnetic circular dichroism spectra of some cyclophanes (among them those of [2.2]­paracyclophane 1 ), exploring the influence of nonparallel arrangement of the aromatic rings on their properties. In this paper, we extend our investigations of the paracyclophanes to the study of the structure, NMR, absorption and emission properties of [ m .…”