Substitution of any organic compound by a certain substituent causes changes of the chemical shifts of carbon atoms up to 3 bonds away. The semiempirical methods, still commonly used for prediction of chemical shifts for various kinds of compounds, are based on the assumption that the substituent induced change in the 13C NMR is an additive property. The chemical shifts of carbon atoms in a compound containing given substituent are calculated by addition of certain constant values to the chemical shifts of carbon atoms in the parent unsubstituted compound. These parameters are characteristic for the substituent and its position with respect to the carbon atom in question. Such aproach implies that for each type of compound and, moreover, for each site of substitution a separate set of parameters has to be used. For given substituent the values of these parameters, even for analogous substitutions site (e.g. for the substituted carbon atom) are considerably different.

Any substituent, however, has a definite electron donating or electron withdrawing properties. Thus it seemed reasonable to assume that chemical shifts of carbon atoms in all compounds of certain kind, where polar effects of a substituent are transmitted by analogous sequence of bonds, could be calculated with just one common set of parameters, but these parameters will be multiplied by the factor "a_s" specific for the kind of the compound and the position of substituent with respect to the substituent R, as shown by the equation:

δ C(k) = δ^oC(k) + a_sA(R)

Above hypothesis was recenly confirmed on the example of six-membered heteroaromatics [1] alicyclic hydrocarbons [2], as well as condensed aromatics, or five-membered heterocycles and alkenes [3].

In this work we have investigated the factors influencig the term a_s for a substituted carbon atom. It was found that its value depends mainly on hybrydization, electron density on this atom and the distortion of the bond angle. In the case of the sp² hybridization the a_s value increases with the increase of the electron density characterized by the 13C NMR chemical shift before substitution, whereas for the sp³ hybridization, opposite, it decreases.

For both hybrydization cases all correlations between the chemical shifts of carbon atoms and their changes involved by substitution are of at least good quality (r > 0.95). For bulky substituents, and in the case of occurence of the gamma-effect another relations are observed, as it was previously found [1, 2].

[1] J. Oszczapowicz, Int. J. Mol. Sci. 2005, 11-17.

[2] J. Oszczapowicz, I. Oszczapowicz, J. Phys. Org. Chem. 2005, 818-824.

[3] J. Oszczapowicz, I. Oszczapowicz, in preparation.

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