Intramolecular non-bonded interactions and β- and γ-hydroxy substituent 13C NMR chemical shifts

Abstract

A systematic study of β- and γ-substituent 13C chemical shifts (SCS) in terms of intramolecular non-bonded interactions has been carried out. It is demonstrated that for a variety of molecules substituted with an OH substituent (cf. some conformationally non-rigid multifunctional alcohols ( I–IX in Table 1), 1-hydroxy ( 1a) and 2-hydroxy ( 1b) substituted adamantanes, exo 2-hydroxy ( 2a), endo 2-hydroxy ( 2b), 1-hydroxy ( 2c), and 7-hydroxy ( 2d) substituted norbornanes, and the hydroxy derivative of caged compound ( 3)), β- and γ-SCS for primary, secondary and tertiary carbon atoms can be compared to the total local van der Waals and electrostatic non-bonded interaction energies in the same correlation. Furthermore, it is shown that the relationship can be extended to molecules bearing two such substituents, e.g. in the dihydroxy derivatives of certain caged compounds ( 4a) to ( 4c) and ( 5). The local electrostatic potentials are mostly responsible for the variations of β- and γ-SCS in the series I–IX, and ( 1)–( 5) substituted molecules. On the other hand, β- and γ-SCS in the strained ( 6) and ( 7) molecules are mainly influenced by changes in the local van der Waals interaction energies. This approach presents a more consistent treatment of the influences of geometric distortions and electronic interactions on chemical shifts of sp 3 hybridized carbon atoms.