Substituent Effects on 13C Chemical Shifts

Duddeck, H., Substituent Effects on 13C Chemical Shifts in Aliphatic Molecular Systems. Dependence on Constitution and Stereochemistry, 16, 219. 

Table 4.6. Methyl Substituent Effects on 13C Chemical Shifts of Cyclopentane and Methylcyclopentane Resonances [210]...

In contrast, 13C nmr spectra of ring-substituted /J-bromocumyl cations [2] can be unambiguously interpreted in terms of open /1-bromocarbocations, since the ring substituent effects on the chemical shifts are similar to those on the corresponding non-brominated cations [1], even for the electron-attracting p-trifluoromethyl group (Olah et al, 1972). 

In general, <5 effects on 13C chemical shifts across four bonds are negligible. However, there is one exception, namely, if the substituent and the d-carbon are in the jyw-diaxial position. In that case, pronounced deshielding effects can be observed (numbers are 5 SCS determined for the carbons marked 25 66-68 ... 

Saturated compounds. The position of absorptions of methyl, methylene, meth-ine and quaternary carbon atoms in the alkanes is shown in Fig. 3.49. Within each group the exact position of absorption is determined by the number and nature of substituents on the p and y carbons. Replacement of a proton by CH3 results in a downfield shift of c. 8 p.p.m. at C-l, and c. 10 p.p.m. at C-2, and an upfield shift at C-3 of c. 2 p.p.m. Polar substituents result in a downfield shift in the position of absorption Table A3.12 in Appendix 3 shows the effect on 13C chemical shifts of replacing a methyl group by various polar substituents. 

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. 

C chemical shifts depend, in part, on the amount of electron density around the 13C nucleus. Since benzene ring substituents perturb the electron density at selected carbons around the ring, one might expect these substituents to exert a noticeable effect on the chemical shifts of these nuclei. 

Carbon-13 chemical shifts of the a- and (8-carbon atoms of various unsubstituted and 3-substituted thietane oxides and dioxides have been recorded and correlated by the equations S = ay + bx and Sf = ax + by where a and b are parameters characteristic of the sulfoxide or sulfone (y) and the substituent (x)216. The values of the substituent parameters were found to parallel those which determine the effect on the 13C chemical shifts when hydrogen is replaced by a substituent224. 

Singh et al [54] used a 13C NMR spectrometric method and reported the chemical shifts of primaquine and chloroquine. The signals are assigned on the basis of substituent effects on benzene shifts, intensities, multiplicities in single-frequency off-resonance decoupled and the comparison with structurally related compounds. 

A series of 3-aryl-5,8-dihydro-6,6,8-trimethyl-5,8-ethano-6H-pyrano [4, 3-e][l,2,4]triazine-4-oxides 193 was prepared (79JHC1389) by boiling a mixture of 192 and trimethyl orf/io-arylates. The effect of substituents on the chemical shifts of their 13C NMR spectra was studied. 

The effect on 15N and 13C chemical shifts in anilines75-77 and nitrobenzenes74 of a reduced resonance interaction caused by ortho substituents which sterically twist the amino and nitro groups from coplanarity with the aryl ring has been extensively investigated and is fairly well documented. 

The correlation analysis of spectroscopic properties in terms of o/ and a -type parameters has been very important. Substituent effects on 19F NMR shielding in fluorobenzenes have been studied in great detail by Taft and colleagues90,96 97. For <5 linear regression on a/ is on the whole satisfactory, but a term in a with a small coefficient is sometimes introduced. The correlation analysis of <5°, however, requires terms in both o/ and os-type parameters, with rrR being widely applicable. Many new values of these parameters have been assigned from fluorine chemical shifts. In recent years there has also been extensive use of correlation analysis of 13C NMR data98,99. 

There is considerable need for exploration of interaction effects If SCSs are to be used for signal assignments or structure determinations, it is essential to know about alterations of SCSs by interactions with other substituent(s) to avoid misinterpretations. Additionally, interaction effects provide valuable information about the o-electron distribution and its dependence on structure, since it is well known that 13C chemical shifts are highly sensitive to changes in the geometry and/or electronic state of the molecule. This research area is not easily accessible experimentally by other spectroscopic methods, at least for larger molecules, which are also beyond the reach of most theoretical calculations. 

In the previous sections the main focus of the discussion of substituent effects was their dependence on structure (especially stereochemistry) and the underlying transmission mechanisms. Admittedly, it would still be veiy difficult to predict 13C chemical shifts in more complicated and highly substituted aliphatic compounds with sufficient precision even if our knowledge about the physical back-... 

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