Conformational analysis. Part 14. A lanthanide-induced shift n.m.r. analysis of indan-1-one and norcamphor

Abstract

A model for the analysis of the lanthanide-induced shift (LIS) of molecules with asymmetric binding sites is presented and used to analyse the LIS of indan-1-one and norcamphor. The spatial environment of the carbonyl groups has been modelled using non-symmetry-related binding sites which can be varied independently to achieve the required agreement between the observed and calculated pseudo-contact shifts. The model proved very successful for indan-1-one with an essentially three-site complexation geometry reflecting the environment around the carbonyl group. The bond localisation in indan-1-one was investigated using this model and other theoretical and experimental methods. MNDO gives the opposite bond fixation to that predicted from both ab initio and n.m.r. coupling constants. The LIS excludes an idealised structure but gives good agreement for both calculated structures. A comparable LIS analysis of norcamphor illustrated again the sensitivity of the method to the correct substrate geometry. Of the three geometries obtained (MNDO, crystal structure of an analogue, and ab initio) the 3-21G ab initio structure gave significantly better agreement for both LIRAS3 and HARDER(four-site) than any other analysis. Both LIRAS3 and HARDER solutions are acceptable in this case, probably due to the asymmetric environment about the carbonyl group in norcamphor which is not completely reflected in either the HARDER or LIRAS3 models.