title: (1) H NMR Spectra. Part 28: Proton chemical shifts and couplings in three-membered rings. A ring current model for cyclopropane and a novel dihedral angle dependence for (3) J(HH) couplings involving the epoxy proton.
abstract: The (1) H chemical shifts of selected three-membered ring compounds in CDCl(3) solvent were obtained. This allowed the determination of the substituent chemical shifts of the substituents in the three-membered rings and the long-range effect of these rings on the distant protons. The substituent chemical shifts of common substituents in the cyclopropane ring differ considerably from the same substituents in acyclic fragments and in cyclohexane and were modelled in terms of a three-bond (γ)-effect. For long-range protons (more than three bonds removed), the substituent effects of the cyclopropane ring were analysed in terms of the cyclopropane magnetic anisotropy and steric effect. The cyclopropane magnetic anisotropy (ring current) shift was modelled by (a) a single equivalent dipole perpendicular to and at the centre of the cyclopropane ring and (b) by three identical equivalent dipoles perpendicular to the ring placed at each carbon atom. Model (b) gave a more accurate description of the (1) H chemical shifts and was the selected model. After parameterization, the overall root mean square error for the dataset of 289 entries was 0.068 ppm. The anisotropic effects are significant for the cyclopropane protons (ca 1 ppm) but decrease rapidly with distance. The heterocyclic rings of oxirane, thiirane and aziridine do not possess a ring current. (3) J(HH) couplings of the epoxy ring proton with side-chain protons were obtained and shown to be dependent on both the H-C-C-H and H-C-C-O orientations. Both density functional theory calculations and a simple Karplus-type equation gave general agreement with the observed couplings (root mean square error 0.5 Hz over a 10-Hz range).
authors.affiliation: Department of Chemistry, University of Liverpool, Crown St, Liverpool, L69 7ZD, UK. rabraham@liv.ac.uk
authors.forename: Raymond J
authors.initials: RJ
authors.lastname: Abraham
authors.forename: Paul
authors.initials: P
authors.lastname: Leonard
authors.forename: Cláudio F
authors.initials: CF
authors.lastname: Tormena
mesh_terms: D016880:Anisotropy; D003521:Cyclopropanes / Q000737:chemistry*; D004852:Epoxy Compounds / Q000737:chemistry*; D060526:Magnetic Fields; D009682:Magnetic Resonance Spectroscopy; D008956:Models, Chemical; D011789:Quantum Theory; D012997:Solvents
chemical_list: D003521:Cyclopropanes; D004852:Epoxy Compounds; D012997:Solvents; C030797:cyclopropane