title: Interpretation of conformational effects on 2-endo-norborneol by natural chemical shielding analysis.
abstract: This paper represents an extension of our work on the (1)H and (13)C NMR chemical shifts of norbornane and 2-endo-norborneol. NCS-NBO analysis was employed to probe contributions of bond orbitals and orbitals of lone pairs to nuclear shielding in conformers of the alcohol generated by rotation of the C-O bond. Variations in (1)H and (13)C chemical shifts with the dihedral angle are discussed in terms of Lewis and non-Lewis partitioning and their respective importance is evaluated. In addition to hyperconjugation of the lone pair in a p orbital of oxygen that was previously reported, a sizable participation of the lone pair which is in an sp orbital is also observed and their combined effect dominates the carbon chemical shifts of the C(1)-C(2)-OH and C(3)-C(2)-OH fragments. Both lone pairs on oxygen also contribute to localized, though-space effects on nuclei in the vicinity, these effects answering for the largest deviations in hydrogen chemical shifts on rotation around the C-O bond. On the other hand, for conformers in which nonbonded repulsions lead to distortions in the molecular framework, variations in chemical shifts may be attributed to angular effects.
authors.affiliation: Universidade Federal do Rio de Janeiro, Escola de Química, Centro de Tecnologia, Bloco E, CEP 21949-900, Rio de Janeiro, RJ, Brazil. pseidl@eq.ufrj.br
authors.forename: Peter R
authors.initials: PR
authors.lastname: Seidl
authors.forename: J Walkimar de M
authors.initials: JW
authors.lastname: Carneiro
authors.forename: J Glauco R
authors.initials: JG
authors.lastname: Tostes
authors.forename: Andreas
authors.initials: A
authors.lastname: Koch
authors.forename: Erich
authors.initials: E
authors.lastname: Kleinpeter