title: 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.
authors.affiliation: Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland. dodziuk@ichf.edu.pl
authors.forename: Helena
authors.initials: H
authors.lastname: Dodziuk
authors.forename: Sławomir
authors.initials: S
authors.lastname: Szymański
authors.forename: Jarosław
authors.initials: J
authors.lastname: Jaźwiński
authors.forename: Maciej
authors.initials: M
authors.lastname: Ostrowski
authors.forename: Taye Beyene
authors.initials: TB
authors.lastname: Demissie
authors.forename: Kenneth
authors.initials: K
authors.lastname: Ruud
authors.forename: Piotr
authors.initials: P
authors.lastname: Kuś
authors.forename: Henning
authors.initials: H
authors.lastname: Hopf
authors.forename: Shaw-Tao
authors.initials: ST
authors.lastname: Lin
mesh_terms: D009682:Magnetic Resonance Spectroscopy / Q000592:standards; D015394:Molecular Structure; D011083:Polycyclic Compounds / Q000737:chemistry*; D011789:Quantum Theory; D012015:Reference Standards; D013237:Stereoisomerism
chemical_list: D011083:Polycyclic Compounds; C515344:(2.2)paracyclophane