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The Influence of the Second Methyl Group Position on the Chemical Shifts Values of all Protons in Dimethylalkane Molecules in the 1H NMR Spectra
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1  Ukrainian Academy of Printing

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 study the subsequences of the second methyl group introduction into the different types of  monomethyl substituted  linear alkanes molecules, "moving" it from the beginning to the middle of alkyl chain. In this paper we describe the effect of the mutual arrangement of methyl group position in dimethylsubstituted alkane molecule on chemical shift values ​​of all the nuclei of hydrogen atoms (including the newly introduced methyl group). The position of the each methyl group in the alkyl chain denoted by the symbol «N1 and N2», indicating the number from the beginning of the chain of carbon atoms to which two "methyl substituents" are attached. For this purpose we have studied and analyzed the literature values ​​of proton chemical shifts in the 1H NMR spectra of different "dimethylalkane families": (where N1 and N2 = 2, 3, 4 or 5). Each family consists of molecules, ranging from "short-chain" dimethylpropanes to the most "long-chain" dimethyloctanes, 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 linear alkane (or monosubstituted methylalkane) and studied dimethylalkanes. 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 families of N1,N2-dimethyl alkanes we reveal that a "significant" difference in protons chemical shift values ​​are found only in two nonoverlapping so-called "tetracarbon fragments": -СN-1HN-12-CNHN(CH3)-СN+1HN+12- in the cases when N2 ≥ N1 +3. Each of such fragment comprises a carbon atom N (N1 or N2) with attached to it "methyl substituent", the previous (N-1) and the following (N +1) carbon atoms. When both of these fragments overlap, depending on the relative position of the two methyl groups in the molecule must be considered "heptacarbon" [-СN-1НN-12-СNНN(СN'НN'3)-СN+1НN+12-СN+2НN+2(СN+2'НN+2'3)-СN+3НN+32-] (N2 = N1 +2), "hexacarbon" [-СN-1НN-12-СNНN(СN'НN'3)-СN+1НN+1(СN+1'НN+1'3)-СN+2НN+22-] (N2 = N1 +1) or "pentacarbon" [СN-1НN-12-СN(СН3)(СН3)–С N+1Н N+12-] (N1 = N2),fragments. We calculate and tabulate the mean values ​​for each protons type in these "polycarbon fragments" in each of the different families of the disubstituted alkanes. It is shown that the mean chemical shift values ​​for each protons type in these "polycarbon fragments" are very simslar for simslar families of dimethylalkanes. For instance, the corresponded mean chemical shift values are similar to each other in two families of 3,3- and 4,4-dimethylalkanes, in which the "pentacarbon fragment" under consideration is situated in the midchain position.