Empirical force field andab initio calculations on allyl cations

✍ Scribed by Reindl, Bernd; Clark, Timothy; Schleyer, Paul v. R.

• Publisher: John Wiley and Sons
• Year: 1997
• Tongue: English
• Weight: 784 KB
• Volume: 18
• Category: Article
• ISSN: 0192-8651
• DOI: 10.1002/(sici)1096-987x(199703)18:4<533::aid-jcc7>3.0.co;2-u

✦ Synopsis

Allyl cation geometries optimized using an extended version of MMP2, newly parameterized for localized and delocalized classical cations, compare favorably Ž . with those obtained at the MP2 full r6-31G* level. Hence, the force field should provide good starting structures for ab initio calculations. The -electron densities obtained by these two very different methods are quite similar. The relative Ž . energies of various isomers at MP4r6-31G*rrMP2 full r6-31G* are reproduced well by the force-field calculations. The heats of formation calculated by MMP2, as well as those predicted from the ab initio data, agree with experimentally determined values. The force-field method provides interpretive capabilities. 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 cations can be evaluated. The stabilizing 1-3 -interactions in allyl cations are quite significant, but are reduced by alkyl groups hyperconjugatively and sterically.