The effect of branching in alkyl substituents on the value of chemical shifts of nuclei of H-2 and H-3 atoms of phenyl ring in NMR <sup>1</sup>H spectra of monosubstituted para-alkylaromatic compounds

Abstract

In our previous works (see ECSOC-15, 2008; and also our accompanied communication №1) we found some interesting dependencies of ortho-protons chemical shift (δoH) in monoalkylbenzenes 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: which other classes of aromatic compounds (in addition to monoalkylbenzenes) the founded regularities apply to? Are α- and β-effects for meta-protons existed too? We obtain the answer while studying NMR 1H spectra of 21 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" 21 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 (ΔδНо) were used instead of basic ones (δоН). The ΔδНо parameter means a difference between the value of studied proton chemical shift δiН,N(n) of the examined compound Nn and the same value δоН,N(0) of the standard compound, where all variable substituents R1 = R2 = R3 = Н. The δiН,N(0) and ΔδiН parameter values of all A and B type compounds are demonstrated in 3 tables. Averaged values of differential parameters were calculated, tabulated in 2 tables and pictured on 2 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 earlier were founded only for monoalkylbenzenes δ0H,N(n) parameters, are also detected for δ0H,N(n) and δmH,N(n) parameters of studied disubstituted para-alkylaromatic compounds. Such α- and β-effects were observed for all examined differential spectral parameters ΔδiH,N(n) in all investigated rows of disubstituted para-alkylaromatic compounds. It should be stressed that for the investigated rows of compounds 1-21 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 13C NMR spectra parameters of the same disubstituted para-alkylaromatic compounds in our next communication №3, which we also want to sent now to ECSOC-15.