Separation of anisotropic and steric substituent effects-nuclear chemical shielding analysis of H-4 and C-4 in phenanthrene and 11-ethynylphenanthrene.

The anisotropic effect of a proximally introduced ethynyl group on the chemical shifts of H-4 and C-4 of the phenanthrene skeleton was calculated using GIAO-HF/NICS methodology. The anisotropic effect, long considered to be the source of the considerable downfield shift of H-4 in 11-ethynylphenanthrene in comparison to the chemical shift value of the corresponding proton in phenanthrene, was determined to be only negligible in magnitude on the basis of these calculations. Partitioning of the natural chemical shieldings of H-4 and C-4 by the NCS-NBO method into various contributions from the C-C and C-H bonds present in each molecule revealed that steric compression was able to account for the large downfield shifts of both H-4 and C-4 in 11-ethynylphenanthrene relative to phenanthrene. Thus, the substituent effect is almost totally permeated by this latter interaction and not by the aforementioned process, which was previously presumed to be the sole underlying cause.