Modelling of the alkyl environment effects on the 13C chemical shift

Abstract

AbstractUsing a topological parameter, λR, to characterize the effects of a variable mono or polyalkyl environment, R, on widely varied types of 13C sites gives rise to a homogeneous description of their chemical shifts over the entire range of δ13C variation (220 ppm/TMS). The proposed model, δ13C=ωcλR+ηc expresses the chemical shift in terms of this parameter together with the sensitivity, ωc, of the perturbed 13C site, bonded to a constant environment in a given family, and with ηc as the reference shift. In the case of polyalkyl environments, R=∑ Ri (n ⩽ 4); since the use of λR presupposes that groups Ri coexist in R without interacting, it is expressed as the sum of the intrinsic effects of each of these groups. Thus, this model demonstrates the general character of most of the common alkyl environment effects and the specific behaviour of the various types of 13C atoms. The homogeneity of this description relies on an assessment of the Ri contribution by the DARC/PELCO topological analysis of alkyl sites in a series of reference alkynes. The δ13C values calculated for 138 compounds, belonging to the alkane, alkene, allene, alcohol, amine, epoxide, ester and ketone families, are highly satisfactory (r̄=0.979, s̄=0.98).