Multiple Substituent Shift Additivity

Abstract

The relatively simple table of shift effects provided in Chapter 1 can be used to make approximate predictions of chemical shifts which in turn are useful in making first inspection assignments between carbons and absorptions. It is of course implicit in such a treatment that the magnitude of the shift-of-shift effect for a substituent be relatively independent of the presence of other groups. Thus, the shift of a given carbon might be considered to be the sum of the independent effects of all of the neighboring groups. This does indeed hold true in those situations where substituents do not directly interact with each other and where the conformation of the system is relatively constant. In theory it should be possible to arrive at predicted spectra for all possible isomers of an unknown compound and comparison of these with the observed values would narrow the possible candidates to one or at most two choices. However, it should be clear from the discussion in Chapter 2 that the magnitude of the shift effect of a substituent varies with the carbon framework to which it is attached, and therefore, no simple collection of generalized substituent effects will provide accurate predictions in all situations. Nonetheless, with appropriate spectral data for model compounds available, the total shift effect of several, noninteracting substituents can be predicted to be the sum of the Δδ effects for each individual substituent.