The effect of branching in alkyl substituents on the value of chemical shifts of nuclei of C-1 – C-4 atoms of phenyl ring in NMR <sup>13</sup>C spectra of monosubstituted para-alkylbenzenes

Abstract

In our previous works (see ECSOC-15, 2008 and also 2 our accompanied communication) we found interesting dependencies of ortho- and meta-protons chemical shift (δoH) in monoalkylbenzenes and disubstituted para-alkylaromatic compounds NMR 1H spectra upon the place and degree of branching of alkyl chain. If this branching takes place at the α-carbon atom, the shift of basic spectral parameter – δiH (δoH or δmH) - is observed and entitled as "α-effect"; the branching at β-carbon atom leads to another type shift ("β-effect"). The positive value of α- or β-effect means that δiH value increases with the increasing of amount of methyl groups in α- or β-position; contrary, negative value of such effect mean decrease δiH value. The natural question emerges: are α- and β-effects for phenyl ring carbon atoms existed too? We obtain the answer while studying NMR 1H spectra of 26 especially selected rows of disubstituted para-alkylaromatic compounds Nn of the general formula p-Y-C6H4-(CH2)r-CR1R2R3 (where r=0 or r=1). We examined as substituents "Y" 26 the most widespread groups from most electronegative (NO2) to most electropositive (NMe2). The variable fragments (R1, R2, R3) given in general formula are only hydrogen atom or methyl group. Depending upon the place of alkyl chain branching all compounds we divide into two types: the compounds of A type (r=0 in general formula), where the branching takes place at α-carbon atom and compounds of B type branched at β-carbon atom (r=1). The differential spectral parameters (ΔδiC) were used instead of basic ones (δiC). The ΔδiC parameter means a difference between the value of studied carbon chemical shift δiC,N(n) of the examined compound Nn and the same value δiC,N(0) of the standard compound, where all variable substituents R1 = R2 = R3 = Н. The δiC,N(0) and ΔδiC parameter values of all A and B type compounds are demonstrated in 5 tables. Averaged values of differential parameters were calculated, tabulated in additional 4 tables and pictured on 4 diagrams. In our communication we have a broad discussion of values and signs calculated for all types averaged differential parameters. The regular "structure-property" dependencies (α- and β-effects) which were founded only for δ0H,N(n) parameters, are also detected for 4 types of δiC,N(n) parameters of studied disubstituted para-alkylaromatic compounds. Such α- and β-effects were observed for all examined differential spectral parameters ΔδiC,N(n) in all investigated rows of disubstituted para-alkylaromatic compounds. It should be stressed that for the investigated rows of compounds 1-26 there are no facts contradicting with the predicting values and signs of both α- and β-effects. The similar regular "structure-property" dependencies are also examined for 1H NMR spectra parameters of other aromatic compounds in our accompanied communication, which we also want to sent now to ECSOC-15.