A new method for empirical force field calculations on localized and delocalized carbocations

Abstract

A new empirical force field method for localized and delocalized carbocations is described. Additional geometry parameters for carbocations were added to Allinger's MMP2 molecular mechanics program, which can treat delocalized π‐systems. The effect of hyperconjugation in carbocations is introduced via a quantum chemical term into force field calculations for the first time. The calculated heats of formation are in excellent agreement with a wide range of experimental data; the largest deviations are about 3.5 kcal/mol. The calculated structures agree very well with those computed at correlated ab initio levels (MP2(full)/6‐31G*). The relative energies and geometries of different conformations of representative carbocations also were in good agreement with MP4/6‐31G*//MP2(full)/6‐31G* results. © 1996 by John Wiley & Sons, Inc.

Modern Molecular Mechanics and ab Initio Calculations on Benzylic and Cyclic Delocalized Cations

“…The Parent Benzyl Cation. MMP2 and MP2(full)/6-31G* optimizations gave the expected C 2 v symmetry for the benzyl cation 1 , as computed earlier. ,49b,− Although the thermochemistry of C 7 H 7 + ions is still not well established, the MMP2 heat of formation of 1 (215.4 kcal/mol) excellently corresponds with experimental data (215 kcal/mol 21b ) and a recently reported ab initio estimate (217.0 kcal/mol 54 ). Table shows C−C bond lengths and π-electron densities obtained by the MMP2 and MP2 methods, which agree remarkably well.…”

“…This paper employs a newly parametrized empirical force field, which has been shown to compute carbocation geometries and π-electron densities well, to examine the benzyl, tropylium, and homotropylium cations. Allinger's MMP2 program, which combines a classical molecular mechanics treatment with an SCF procedure for π-systems of unsaturated molecules, was used as the basis for the parametrization.…”

“…Allinger's MMP2 program, which combines a classical molecular mechanics treatment with an SCF procedure for π-systems of unsaturated molecules, was used as the basis for the parametrization. Hyperconjugation was introduced via a quantum chemical term . This new method allows a force field treatment of delocalized carbocations, such as allyl, benzyl, and 1,3-pentadien-5-yl as well as the calculation of resonance energies, − which are quite large in such species. , In contrast, early force field parametrizations were not devised to reproduce carbocation geometries and hyperconjugation effects, but rather to compute strain energies …”

Chapter 2. Theoretical organic chemistry

“…Empirical force fields for delocalized carbocations have been improved by Reindl et al by introduction of additional terms into Allingers MMP2 program, and a quantum chemical term is implemented into force field calculations for the first time. 64 The calculated heats of formation are in excellent agreement with a wide range of experimental data, with the largest deviation of 3.5 kcal mol-'. Calculated structures and conformations are found to be in good accord with those obtained from ab initio MP2(fu11)/6-3 lG* calculations.…”