Substituent Effects on l3C Chemical Shifts

Substituent Effects on l3C Chemical Shifts in Aliphatic Molecular Systems. 

This approach has also been applied to saturated C(i) atoms. Due to the much lower polarizabilities of C-H and C-C single bonds, however, the effects are clearly smaller (82), though still detectable. Thus, some effects of substituents X on S-positioned carbon atoms in cyclohexanes and cholestanes were attributed to LEF effects of the C-X dipoles by Schneider and colleagues (76,77,85). The finding that only minor effects are to be expected in saturated molecules was confirmed by INDO-SCF calculations of electric-field effects on l3C chemical shifts of some model compounds performed by Seidman and Maciel (86). These authors conclude, further, that conformational studies on such systems are not promising (86). 

Effects of various substituents on 13C chemical shifts can be found in textbooks (2-5,11) or reviews concerned with applications of l3C NMR spectroscopy to specific classes of chemical compounds (16-24,142). Therefore, this section is limited to recent information about effects of less common substituents and to reference data for various carbo- and heterocyclic molecules. It is not the author s intention to present a comprehensive survey rather, a number of typical examples have been selected. The reader may use this section as an entry into the original papers and the references cited therein. 

Nuclear Magnetic Resonance, l3C Chemical Shifts in Aliphatic Molecular Systems, Substituent Effects on. Dependence on... 

The quaternary carbons of the tertiary cations 97 have chemical shifts of <5 l3C 69.2,61.1 and 58.8 for the Me, Ph and cyclopropyl substituted ions, respectively, which shows that progressively less charge is delocalized into the spirocyclopropyl ring going from methyl to phenyl to cyclopropyl. Consequently, the order of the stabilizing effects of the substituents on the cationic center is cyclopropyl > Ph > Me. 

Almost all experimental work on pyrans has employed H and l3C NMR spectra. However, detailed assignments of all the measured signals are still rare [85MRC793 92JCS(P2)1301]. Typical spectral data for representative 2H- and 4//-pyrans are given in Tables XXII to XXV. Carbon-13 chemical shifts were used to study the transmission of substituent effects when applied to the classical Hammett equation (90MR212). 

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