title: Rotational barriers of biphenyls having heavy heteroatoms as ortho-substituents: experimental and theoretical determination of steric effects.
abstract: The free energies of activation for the aryl-aryl rotation of 17 biphenyl derivatives, bearing a heavy heteroatom (S, Se, Te, P, Si, Sn) as ortho substituent, have been measured by variable temperature NMR. These numbers, so called B values, represent a meaningful measure of the steric hindrance exerted by the selected substituents. DFT computations match quite satisfactorily the experimental barriers and the ground state geometries as well (determined, in two cases, by X-ray diffraction). The present values extend the available list of B values and thus provide an enlarged basis for the compilation of the space requirements of standard substituents, based solely on experimental determinations.
authors.affiliation: Department of Organic Chemistry A. Mangini, University of Bologna, Viale Risorgimento 4, I-40136 Bologna, Italy.
authors.forename: Lodovico
authors.initials: L
authors.lastname: Lunazzi
authors.forename: Michele
authors.initials: M
authors.lastname: Mancinelli
authors.forename: Andrea
authors.initials: A
authors.lastname: Mazzanti
authors.forename: Susan
authors.initials: S
authors.lastname: Lepri
authors.forename: Renzo
authors.initials: R
authors.lastname: Ruzziconi
authors.forename: Manfred
authors.initials: M
authors.lastname: Schlosser