The spin—orbit/Fermi contact effect on ¹³C substituent chemical shifts in 1-halo-bicyclo[1.1.1]pentanes

In the title compounds, the relativistic spin-orbit corrections to ¹³C magnetic shielding constants are calculated as a quadratic response to the spin-orbit/Fermi contact (S-O/FC) perturbation. To carry out such calculations, the Gaussian 98 package of programs was properly modified. The FC operator was introduced using the finite perturbation (FPT) approach, while the paramagnetic shielding operator was taken into account within the coupled perturbed Kohn-Sham approach, using a gauge single origin. In all cases the single origin was taken at the halogen nucleus. XH and CH₃X (X = F, Cl, Br, I) compounds were taken as test cases to compare results obtained with this modified program to other values taken from the literature. Results obtained with different functionals are also compared. In 1-halo-bicyclo[1.1.1]pentanes, the calculated S-O/FC contributions to ¹³C magnetic shielding constants correspond to shielding effects for C_α and C_β, and to deshielding effects for C_γ. Calculated S-O/FC contributions to the Cα magnetic shielding constants are larger than those calculated in CH₃X. Such a result is in line with the high s character at the C atom of the C-X bond for the title compounds.