Visualization and quantification of the anisotropic effect of C=C double bonds on 1H NMR spectra of highly congested hydrocarbons-indirect estimates of steric strain.

The anisotropic effect of the olefinic C=C double bond has been calculated by employing the NICS ( nucleus independent chemical shift) concept and visualized as an anisotropic cone by a through space NMR shielding grid. Sign and size of this spatial effect on (1)H chemical shifts of protons in norbornene, exo- and endo-2-methylnorbornenes, and in three highly congested tetracyclic norbornene analogs have been compared with the experimental (1)H NMR spectra as far as published. (1)H NMR spectra have also been calculated at the HF/6-31G* level of theory to get a full, comparable set of proton chemical shifts. Differences between delta( (1)H)/ppm and the calculated anisotropic effect of the CC double bond are discussed in terms of the steric compression that occurs in the compounds studied.