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

Using a topological parameter, λ_R, 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=ω_cλ_R+η_c expresses the chemical shift in terms of this parameter together with the sensitivity, ω_c, of the perturbed ¹³C site, bonded to a constant environment in a given family, and with η_c as the reference shift. In the case of polyalkyl environments, R=∑ R_i (n ⩽ 4); since the use of λ_R presupposes that groups R_i 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 R_i 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).