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Dependence of the γ carbon‐13 shielding effect on the dihedral angle

1981; Wiley; Volume: 17; Issue: 4 Linguagem: Inglês

10.1002/mrc.1270170409

0030-4921

Joseph B. Lambert, Adelia R. Vagenas,

Molecular Spectroscopy and Structure

Abstract The γ effects on 13 C shielding have been examined for a series of rigid molecules containing the substituents CH 3 , OH, NH 2 , F, Cl, Br or I. Plots of the γ effect vs the CC‐αC‐βX dihedral angle are well scattered if the plots do not distinguish modes of substitution. For the series in which the resonating carbon is CH 2 , however, fairly linear correlations are observed for XCH 3 , OH, NH 2 , Cl and Br. It is concluded that within a structurally defined and substitutionally homogeneous series, there is a reasonably linear stereochemical dependence. Although substitution at C‐α does not seriously alter the linear relationship, hybridization of the intermediate carbons may be important. The slopes of the plots for the γ effect vs dihedral angle are remarkably similar. Neither a strictly steric (van der Waals) nor a strictly polar source can be the cause of the linear relationship, since our series of substituents ranges widely with respect to both size and electronegativity. The only two factors that are constant within the series are the removal of the hydrogen atom (CCCH to CCCX) and the addition of the bonding and nonbonding orbitals on X that were not present on H.