Relative Energies and Structural Features of Small Amines and Their Ammonium Analogs: Results From 6-31G* Optimizations and an Mm2 Ammonium Force Field

An MM2 force field that reproduces the structural fea tures and relative energies of quaternary aliphatic amines as derived from ab initio calculations at the 6-31G* level has been completed. The purpose of the study is to provide Medicinal Chemists with a method of evaluating the structures and conformational ener gies of ammonium species typically found in pharma ceuticals. Full Cartesian coordinate geometry optimiza tions at the 6-31G* level are followed by single-point Møller-Plesset Perturbation calculations through the MP3 level, which were performed on CRAY X-MP/4, CRAY Y-MP/464, and VAX 8800 machines using Gaus sian 80(UCSF). Methyl-, ethyl-, propyl-, and ethylmethyl- amines, N-Methylpiperidine, 2-Methylpiperidine, and corresponding ammonium systems are used to evalu ate changes in structure, barrier heights, and relative energies resulting from quaternization.

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