Theoretical and empirical calculations of the carbon chemical shift in terms of the electronic distribution in molecules

Abstract

Abstract The principles of the theoretical calculations of the carbon atom screening constant are reviewed. The Independent Electron, SCFMO and Valence Bond formulae are briefly discussed, and the relative importance of the different terms involved in an approximate decomposition of σ 13 C are pointed out. The variations of the excitation energy Δ E , the mean distance between the nucleus and 2p electrons, and the electronic charges and bond orders with the structure are presented. The nature of the steric effect is also explained. In a second part the various empirical correlations which make an evaluation of δ 13 C from structural and physical properties possible are shown. The actual substituent parameters of the main classes of compounds are listed and worked examples given to make the use of the tables easier. The correlations between δ 13 C and the electronic charge, electronegativity, Hammett‐Taft constants, electric field effects, geometrical parameters, spectroscopic data (electronic transitions, screening of other nuclei) and pH are also discussed with a view to appraising the carbon screening.