An Assessment of Alternative Low Level Calculation Methods for the Initial Selection of Conformers of Diastereomeric Esters

Critical assessment of performance of alternative molecular modeling methods depending on a specific object and goal of the investigation is a question of continuous interest. This prompted us to demonstrate the origin of the guidelines we have used for a rational choice and use of a proper low level calculation method (LLM) for an initial geometry optimization of generated conformers, with the aim of selecting a set for further optimization. What was performed herein was a comparison of LLMs: MM3, MM+, UFF, Dreiding, AM1, PM3, and PM6 on the optimization of conformers’ geometry of α-methoxyphenylacetic acid (MPA) 2-butyl esters as a set of typical diastereomeric esters of a chiral derivatizing agent. This set of esters calculated represents only compounds of this certain type in the current work. The LLM conformer energies were correlated with benchmark energies found by using higher level reference method B3LYP/6-311++G** on the geometries gained previously by optimization with LLMs. In an alternative treatment, the energy range to be covered and corresponding number of LLM optimized conformers obligatory for submitting to further optimization using a high level optimization cascade were considered on the basis of determination of the cut-off conformer (COFC).