The influence of the methyl group position on the chemical shifts values of all protons in mono-methylalkane molecules in the 1H NMR spectra.

Abstract

In our previous work (see ECSOC-16, 2012) we found the interesting dependencies of the alkyl protons chemical shift values in 1-monosubstituted linear alkanes NMR 1H spectra upon the nature of substituent group, including the alkyl groups as substituents. In continuation of this topic we started more detailed study of the methyl group as a substituent introduced into the molecule of a linear alkane, "moving" it from the beginning to the middle of the alkane molecule. In this paper we describe the effect of the methyl group position in monomethylsubstituted alkane molecule on chemical shift values ​​of all the nuclei of hydrogen 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 proton chemical shifts in the 1H 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 "long-chain" methyldodecanes, for which we can found the "credible" spectral literature data. We compared the protons chemical shift values ​​ of the same type protons, for example, of triprotonic signal of terminal methyl groups in the unsubstituted and studied monomethyl 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 0.02 ppm. These values ​​of the differences we have identified as "significant" (ie, to be discussed) and randomly selected them for the reason that it is the value of 0.02 ppm we estimate a possible error of experimental determination of protons chemical shift values ​​in the studied molecules. When researching all four families of N-monomethyl alkanes we reveal that a "significant" difference in protons chemical shift values ​​are found only in the so-called "tetracarbon fragments": -СN-1HN-12-CNHN(CH3)-СN+1HN+12-. Each of such fragment comprises a carbon atom N with attached to it "methyl substituent", the previous (N-1) and the following (N +1) carbon atoms. We calculate the mean values ​​for each protons type in these "tetracarbon fragments" in each of the four families of the monosubstituted alkanes. It is shown that the mean chemical shift values ​​for each protons type in these "tetracarbon fragments" slightly differ for families of 3-, 4- and 5-methylalkanes (where N = 3, 4 or 5), in which the "tetracarbon fragment" under consideration is situated in the midchain position, but differ significantly in the case of the family of 2-methylalkanes (where N = 2) in which it is situated at the beginning of the carbon chain.