Halogen substituted adamantanones. Structural studies with lanthanide shift reagents

Abstract

Europium shift reagents (both FOD and DPM) have been used to study a series of halogen substituted adamantanones by 1H NMR spectroscopy. Reliable correlations of molecular structure with the lanthanide induced shifts can be obtained only if certain precautions are taken. The source of atomic coordinates used to predict induced shifts with the pseudocontact equation is critical; coordinates by X-ray crystallography can be unreliable for this purpose, while those from empirical force field calculations yield satisfactory results. Determination of the europium-oxygen bond length by optimization of the agreement factor is shown to be inadequate. Even with a restricted Eu-O bond length, calculation of the best lanthanide position is shown to be dependent on the step size by which the lanthanide is moved in the optimization procedure. All of the compounds studied were found to give complexes with approximately linear C-O-Eu arrays, and the distortions from linearity are consistent with steric effects.