CONFORMATIONAL-ANALYSIS .25. THE EVALUATION OF MOLECULAR GEOMETRIES BY THE LANTHANIDE-INDUCED SHIFT (LIS) TECHNIQUE

A refined lanthanide induced shift (LIS) technique, using Yb(fod)(3) t o obtain the paramagnetic induced shifts of all the spin 1/2 nuclei in the molecule, together with complexation shifts obtained by the use o f Lu(fod)(3) has been used to test the accuracy of molecular geometrie s obtained by various theoretical methods, including molecular mechani cs and ab initio calculations with the RHF/6-31G basis set. The techn ique has been applied to a series of aromatic aldehydes, ketones and e sters. The relative strengths and weaknesses of the calculated geometr ies are clearly illustrated by this technique. Although the ab initio geometries generally gave better agreement with the observed LIS than geometries generated by molecular modelling programs, as expected, thi s was not always the case, particularly for the 2,6-difluoro compounds . The LIS were also used to differentiate between two experimental geo metries for 2,6-dimethylacetophenone. Thus the method outlined here is general and can in principle be applied to any derived molecular geom etry.