Structure and NMR Spectra of Some [2.2]Paracyclophanes. The Dilemma of [2.2]Paracyclophane Symmetry

Abstract

Density functional theory (DFT) quantum chemical calculations of the structure and NMR parameters for highly strained hydrocarbon [2.2]paracyclophane 1 and its three derivatives are presented. The calculated NMR parameters are compared with the experimental ones. By least-squares fitting of the (1)H spectra, almost all J(HH) coupling constants could be obtained with high accuracy. Theoretical vicinal J(HH) couplings in the aliphatic bridges, calculated using different basis sets (6-311G(d,p), and Huz-IV) reproduce the experimental values with essentially the same root-mean-square (rms) error of about 1.3 Hz, regardless of the basis set used. These discrepancies could be in part due to a considerable impact of rovibrational effects on the observed J(HH) couplings, since the latter show a measurable dependence on temperature. Because of the lasting literature controversies concerning the symmetry of parent compound 1, D(2h) versus D(2), a critical analysis of the relevant literature data is carried out. The symmetry issue is prone to confusion because, according to some literature claims, the two hypothetical enantiomeric D(2) structures of 1 could be separated by a very low energy barrier that would explain the occurrence of rovibrational effects on the observed vicinal J(HH) couplings. However, the D(2h) symmetry of 1 with a flat energy minimum could also account for these effects.

Structure and NMR spectra of cyclophanes with unsaturated bridges (cyclophenes)

“…experimental value is 0°. As discussed in our previous articles, a few theoretical works favor a considerable twist of the C sp2 C sp3 C sp3 C sp2 bridges.…”

“…NMR and electronic spectra are especially sensitive to such effects. We have previously reported NMR spectra and other properties of several cyclophanes with saturated bridges . In this article, we extend these investigations to a study of the structure and NMR spectra of the molecules 2–4 and 6–12 with one or more unsaturated bridges.…”

“…As mentioned on the section on Methods, in addition to the results of the calculations at the B3LYP/6‐311++G(2d,2p), ωB97X‐D/6‐311++G(2d,2p), and ωB97X‐D/cc‐pVQZ levels for molecules 1–4 , the B3LYP/6‐311G(d,p) and B3LYP/Huz‐IV values obtained earlier for molecule 1 together with the experimental data are given in Fig. .…”

“…Numerous experimental and theoretical studies of the structure and physicochemical properties of [2.2]paracyclophane 1 have been published. As discussed in our previous article, the symmetry of this molecule is, after half a century of studies, still debated. It is either D 2 with a small barrier between the two conformers or D 2 h with large amplitude torsional vibrations around the C sp3 –C sp3 bonds.…”

“…Chemical shifts and a few coupling constants of the known cyclophanes were reported together with their synthesis. We have recently carried out NMR, electronic absorption and magnetic circular dichroism studies of molecules 1 and 5 and X ray study of 5 . Ionization potentials of molecules 1–3 , determined by photoelectron spectroscopy, were reviewed by Heilbronner and Yang .…”