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 d 3 C are pointed out. The variations of the excitation energy AE, 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 613C 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 P C and the electronic charge, electronegativity, HammettTaft 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.WITH the rapid development of Fourier transform spectroscopy, the literature concerning carbon resonance is growing considerably. Two excellent works have been published recently on this to pi^.^^^*'*^ A carbon spectrum is usually (but not necessarily) obtained in the proton noise decoupled mode, which results in a nuclear Overhauser enhancement of the carbon resonance but also in a loss of spectral information.From this point of view, knowing the mechanisms which govern the screening of a carbon atom in a molecule is of prime importance and we intend to discuss here the present state of our understanding of the electronic dependence of the carbon chemical shift. The first part will be devoted to a consideration of the theoretical determination of 613C and the relationship between the electronic density and the screening constant. We shall then look at the various ways in which the carbon chemical shift may be correlated with the empirical parameters expressing the electronic influence of the substituents. It is possible by both approaches to predict the screening of a carbon nucleus on the basis of structural features and/or physicochemical parameters.This review is concerned only with the quantitative approaches of 8I3C and therefore the numerous works devoted to the assignment and qualitative discussions of carbon shifts are not taken into account.In order to compare literature results, all the data have been converted to the following convention: a positive * Author to whom correspondence should be addressed.
I-THEORETICAL EXPRESSION OF THE CARBON SCREENING CONSTANT d 3 CMost of the endeavours to calculate the chemical shift of 13C on theoretical grounds are based on the Ramsey e q u a t i~n '~' -~~~ which was developed from second order perturbation theory. This equation is the difference between two terms of comparable magnitude and its practical use is somewhat difficult since the results are very dependent upon t...
