The interpretation of carbon nuclear magnetic resonance shifts. 3.. The variation of the local diamagnetic shielding term in 13C N.m.r., as determined by calculation and by X‐ray photoelectron spectroscopy

Abstract

AbstractExperimental values of ‘chemical shifts’The term ‘chemical shift’ is used in the general sense, as by physicists and physical chemists (cf. chemical shifts in Mössbauer spectroscopy, or as here, X‐ray photoelectron spectroscopy). The chemical shift δ, as measured in n.m.r. spectroscopy, is then the algebraic sum of the ‘chemical shift’ in the local diamagnetic shielding term, ΔσAp in the paramagnetic term, ΔσAp and, the contribution from the rest of the molecule, in the local term approximation. in the local diamagnetic term σAd for carbon in a wide range of groupings have been obtained from X‐ray photoelectron spectroscopic measurements of chemical shifts in the C‐Is binding energy. Eb. This conversion is possible because σAd and Eb both depend directly on the electrostatic potential within the valence shell; thus Eb increases, and σAd calculated by an extended Flygare method (in which CNDO‐calculated atomic charges are used to interpolate Hartree‐Fock values of σd for the free atom A and ± ions) show that the extended Flygare method values are reliable, differential relaxation effects in the photo‐emission process being small. Agreement is also good with published ab initio values of σAd. The range of variation in σAd for carbon is 7–8 ppm from C2H2 to CF4, or 10 ppm if one includes negative ions such as CN−. Some problems of the local term approximation are discussed, and comparison made with the atom‐plus‐ligand ‘local’ term approximation.