The Influence of Substituents Nature on the Chemical Shifts Values of Methyl Protons in the 1H NMR Spectra of 1,1,2,2-Tetrasubstituted Arylpropanes

Abstract

In our previous researches (see ECSOC-16, 2012) we found the interesting dependencies of the protons chemical shift values in unsubstituted, 1-monomethyl- and 1,1-dimethylsubstituted arylpropanes Ar-C1R1R2-C2H22–C3H33 NMR 1H spectra upon the number of methyl groups R. In continuation of this topic we investigate the similar dependencies in 1,1,2,2-tetrasubstituted arylpropanes. In this paper we describe results for forty families of 1,1,2,2-tetrasubstituted arylpropanes of general formula Ar-C1R1Y-C2R2Z–C3H33 with various types of R1, R2, Y, Z substituents. We suppose that under recording spectra conditions the intramolecular interaction between unbound by chemical bonds aryl fragment (Ar) and studied methyl group (C3H33) take place. This interaction leads to change of studied methyl group (C3H33) chemical shift value, which we investigate. For this purpose we have analyzed the literature 1H NMR spectral data of C3H33 proton chemical shifts in these compounds. We calculate the differences between the C3H33 chemical shifts of studied 1,1,2,2-tetrasubstituted arylpropane and two different etalon (reference) compounds. Previously, we found that the optimal "desired" standard etalon compound and additional etalon compound are respectively tetrasubstituted pentane of general formula H5C2– C1YR1 – C2ZR2 – C3H3 and propanol of general formula HO – C1YR1 – C2ZR2 – C3H3. In the absence in used literature sources the spectral data for the "desired" etalon compounds (standard and additional), we used the appropriate parameter of similar in structure substances, named as "forced" etalon compounds (standard and additional). The chemical shifts (base spectral parameters) of the studied methyl group of all 40 families of compounds Ar-C1R1Y-C2R2Z–C3H33 are in the range from 0.4 ppm to 1.7 ppm. Using differential parameters allows us to compare such compounds families, in which studied chemical shifts are equal to, for example, 0.5 ppm and 1.5 ppm. We calculate and discuss the mean base and differential spectral parameter values for all 40 families of compounds Ar-C1R1Y-C2R2Z–C3H33. There were founded some interesting regularities of influence of substituents (particularly those containing heteroatoms) R1, R2, Y, Z location and structure on studied methyl group (C3H33) chemical shift value.