The Influence of the Methyl Group Position on the Chemical Shifts Values of all Carbon Atoms Nuclei in 13C NMR Spectra of Monomethylalkane Molecules

In our previous work (see ECSOC-17, 2013) we found some interesting dependencies of the alkyl protons chemical shift values in 1-monosubstituted linear alkanes 1H NMR spectra upon the position of methyl group (as substituent) in molecule. In continuation of this topic we studied 13C NMR spectra of the same types of monomethyl alkanes.In this communication we describe the effect of the methyl group position in monomethylsubstituted alkane molecule on chemical shift values of all the nuclei of carbon atoms in the molecule of this methylalkane (including the newly introduced methyl group). The position of the methyl group in the alkyl chain denoted by the symbol «N», indicating the carbon atom to which is attached "methyl substituent" number from the beginning of the chain. For this purpose we have studied and analyzed the literature values of carbon chemical shifts in 13C NMR spectra of four "methylalkane families": 2-, 3-, 4- and 5-monomethylalkanes (where N = 2, 3, 4 or 5). Each family consists of molecules, ranging from "short-chain" methylbutanes to the most "longchain" methyldodecanes, for which we can found the "credible" spectral literature data.We compared the carbons chemical shift values of the same type carbon nuclei, for example, of signal of terminal methyl groups in the unsubstituted and studied monomethylsubstituted alkanes. We fixed the differences between the compared chemical shifts only for the cases when the difference is equal or exceeds a value equal to 0.05 ppm. These values of the differences we have identified as "significant" (ie, to be discussed).When researching all four families of N-monomethyl alkanes we reveal that a "significant" difference in carbon chemical shift values are found only in the so-called "decacarbonic fragments": -СN-4HN-42-СN-3HN-32-СN-2HN-22-СN-1HN-12-CNHN(CN'HN'3)-СN+1HN+12-СN+2HN+22-СN+3HN+32-СN+4HN+42-. This fragment comprises a carbon atom N with attached to it "methyl substituent" (N'), the previous (N-1), (N-2) (N-3), (N-4) and the following (N +1), (N +2), (N +3), (N +4) carbon atoms. For other types of carbon atoms nuclei (-СN-5HN-52, -СN+5HN+52, etc.) there is not found the "significant" differences between the values of the chemical shifts in the longchain unsubstituted linear alkanes and the same longchain monomethylalkanes.We calculate and compare the mean values for each carbon types chemical shift in this "decacarbonic fragment" in longchain compounds of abovementioned four families of the monosubstituted alkanes: 2-, 3-, 4- and 5-methylalkanes (where N = 2, 3, 4 or 5). 13C NMR spectral regularities are also found and discussed for short- and midchain monomethylalkanes.