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Peculiarities of NMR 13C Spectra of Alkyl Groups in Functionalized Linear Alkanes of the General Formula ch3(CH2)mY
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Abstract: Literary data of NMR 13C spectra of linear alkanes X-(CH2) n-Y (I), (X = H; Y - 38 different substituents, including H and CH3) were considered. The universal way to estimate the chemical shifts of the methylene groups (δCCH2 = δCi, i = 1 ÷ 36) in I was proposed. It is based on the concept that considers changes in the values δCi of each carbon atom in I (called as increments ΔδCi) as a result of conversion to I of a hypothetical alkane with an infinitely long chain -(CH2)k-(CH2)n-(CH2)l- (II) by replacing infinitely long fragments -(CH2)k- and -(CH2)l- of it with the substituents X and Y. The values δCi of each methylene group in II are assumed the same and equal to that of the medium methylene groups in long-chain alkanes (29.75 ppm). Equal to or greater than 0.05 ppm values of ΔδCi increments are observed in only r closest to the X and s closest to Y carbon atoms in the H-(CH2)r-(CH2)t-(CH2)s-Y (I). Parameter r = 5 is a constant, but the parameter s = 4 ÷ 9 is variable, depending on the nature of the substituent Y. Value δCt = 29.75 ppm is constant. All ΔδCi increments for each substituent Y are calculated using the formula: ΔδCi = δCi - 29.75 ppm, and tabulated. Relationship between the parameters r, s and t determines the length of alkyl chain. All compounds I conditionally divided into three groups: long-chain (n> r + s + t), medium-chain (n