Determination of the carbon-13 shielding tensors for methyl halides and haloforms by N.M.R. in mixtures of liquid crystals

Abstract

The carbon-13 isotropic chemical shifts and anisotropies of the shielding tensors of methyl halides (CH₃F, CD₃Br, CH₃I) and haloforms (CHF₃, CDCl₃, CDBr₃, CHI₃) have been determined from carbon N.M.R. spectra in mixtures of liquid crystals with opposite anisotropy of diamagnetic susceptibility. By using these results the components of the chemical shielding tensors were determined. The tensor element σ _zz (z axis parallel to the C ₃ symmetry axis) was found to be approximately constant (ca. 200 ppm) for methyl halides. The element σ _xx (= σ _yy ) takes values between 84 ppm (for CH₃F) and 226 ppm (for CH₃I). On the contrary, the change of σ _zz for haloforms was about three times larger than the change of σ _xx . The experimental shielding tensors of methyl halides have been compared with the theoretical ones calculated by the Coupled Hartree-Fock method. There is a good agreement between the experimental and theoretical values for σ _zz , but not for σ _xx . The difference of these values increases when the electronegativity of the substituent decreases, i.e. from F to I.