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

Abstract: In our previous researches (see ECSOC-16, 2012 and ECSOC-17, 2013) we found the interesting dependencies of the protons chemical shift values in unsubstituted, mono- and dimethylsubstituted linear alkanes NMR 1H spectra upon the position of methyl group. In continuation of this topic we begin the investigation of the similar dependencies in trimethylsubstituted linear alkanes. In this paper we describe only four simplest families of trimethylalkanes. The position of the each methyl group in the alkyl chain of trimethylalkanes denoted by the symbol «N1, N2 and N3», indicating the number from the beginning of the chain of carbon atoms to which three "methyl substituents" are attached. For this purpose we have studied and analyzed the literature data of proton chemical shifts in the 1H NMR spectra of four "trimethylalkane families": 2,2,5- (N1 = 2; N2 = 2; N3 = 5); 2,2,4-; 2,2,3- and 2,3,4-trimetyllalkanes. We compared the protons chemical shift values (base spectral parameters) of the same type of protons. For example, for triprotonic signal of terminal methyl groups in the studied trimethylalkane on the one hand, with the studied earlier the same parameters of terminal methyl groups in a) unsubstituted linear alkane, b) monomethylalkane and c) dimethylalrane on the other hand. We fixed the differences between the compared chemical shifts (ie, differential spectral parameters) only for the cases when this difference is equal or exceeds a value equal to 0.02 ppm. These values of the differential spectral parameters we have identified as "significant" (ie, to be discussed). When researching all four families of N1,N2,N3-trimethylalkanes we reveal that only in the case of 2,2,5-trimethylalkane family the chemical shifts of all protons in molecule we can considered as the sum of two previously studied parameters of nonoverlapping pentacarbonic [С1Н12-С2(СН3)(СН3)–С 3Н 32-] and tetracarbonic [–С4H42-C5H5(C5'H5'3)-С6H62-] fragments. In the other two cases of 2,2,N3- trimethylalkanes, when both of these fragments overlap, depending on the relative position of the third methyl group in the molecule must be considered the "octacarbonic" fragment [С1Н13-С2(С2'Н2'3)(С2''Н2''3)–С3Н32-С4Н4(С4'Н4'3)–С5Н52-] as the whole moiety in 2,2,4-trimethylalkanes and "heptacarbonic" fragment [С1Н13-С2(С2'Н2'3)(С2''Н2''3)–С3Н3(С3'Н3'3)-С4Н42–] in 2,2,3-trimethylalkanes. Similarly in the case of last forth family of 2,3,4-trimethylalkanes we consider as the whole moiety another "octacarbonic" fragment [С1Н13-С2H(С2'Н2'3)–С3Н3(С3''Н3''3)-С4Н4(С4'Н4'3)–С5Н52-].We calculate and discuss the mean base and differential spectral parameter values for each protons type in these "polycarbonic fragments" in each of the studied trimethylalkane families.
Keywords: Chemical shift; base spectral parameters; “standard” and additional differential parameters; linear alkanes; trimethylalkanes; dimethylalkanes; integral spectral changes; polycarbonic fragments of alkane molecule