Calculation of steric interactions between a lanthanide shift reagent and substituted pyridines

Abstract

Steric interactions between a lanthanide-shift reagent (LSR) and ligands have been examined in detail by calculating the intramolecular non-bonded energy of interaction in Eu(dpm)₃py₂, (where py = pyridine or one of its methyl-substituted derivatives) as py is translated and/or rotated with respect to the remainder of the Eu(dpm)₃py complex. The various calculations performed indicate that pyridine and 3,5-dimethylpyridine bind well to the LSR with a minimum amount of steric interference in the crystal conformation; however, for 2-methylpyridine and especially for 2,6-dimethylpyridine, steric interference of the methyl groups with the LSR are significant. Furthermore, the non-bonded energy of interaction becomes very high when the unsubstituted pyridine ring is moved away from its crystal conformation. This shows that only a small pocket is available for the pyridine molecule or its derivatives in the octahedral LSR complex. The lanthanide-shift reagent studied here is widely used in conformational analysis by n.m.r.; our results show that steric interactions between this LSR and substrate molecules can be severe and hence may be expected to cause conformational alterations of some types of flexible ligands.