EMPIRICAL FORCE-FIELD AND AB-INITIO CALCULATIONS ON ALLYL CATIONS

Allyl cation geometries optimized using an extended version of MMP2, n ewly parameterized for localized and delocalized classical cations, co mpare favorably with those obtained at the MP2(full)/6-31G level. Hen ce, the force field should provide good starting structures for ab ini tio calculations. The pi-electron densities obtained by these two very different methods are quite similar. The relative energies of various isomers at MP4/6-31G//MP2(full)/6-31G* are reproduced well by the fo rce-field calculations. The heats of formation calculated by MMP2, as well as those predicted from the ab initio-data, agree with experiment ally determined values. The force-field method provides interpretive c apabilities. Energy differences between isomers can be separated into electronic and steric contributions, reasonable estimates of resonance energies are given, and nonbonded resonance energies in delocalized c ations can be evaluated. The stabilizing 1-3 pi-interactions in allyl cations are quite significant, but are reduced by alkyl, groups hyperc onjugatively and sterically. (C) 1997 by John Wiley & Sons, Inc.