Molecular mechanics calculations of several lanthanide complexes: An application of the random incremental pulse search

A new method for the evaluation of metal complexes with molecular mechanics calculations is described. The method has been employed to determine the global minimum energy conformations of three seven‐coordinate lanthanide complexes. The method searches the potential energy surface of the metal complexes for the global minimum structure using the recently reported Random Incremental Pulse Search (RIPS). The molecular mechanics calculations were performed using the MM2 metal‐extended force field (MM2MX). This force field excludes 1‐3 bending terms about the metal center and, instead, explicitly includes 1‐3 nonbonded van der Waals interactions. This affords a model based mainly on steric interactions about the ligating atoms, and removes the necessity of specifying ligand‐metal‐ligand bond angles for seven‐coordinate and higher metal complexes in which no preferred bond angle can be proficiently determined. The calculated minimum‐energy structures were well known seven‐coordinate conformations in all cases, and a comparison with X‐ray crystallographic is presented.