Interpretation of conformational effects on 2-endo-norborneol by natural chemical shielding analysis.
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.
Seidl
Peter R
PR
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
Carneiro
J Walkimar de M
JW
Tostes
J Glauco R
JG
Koch
Andreas
A
Kleinpeter
Erich
E