Molecular mechanics andab initio calculations on cyclopentadienyl cations

Abstract

We report ab initio [MP2(full)/6-31G*] and force field (MMP2 extended to carbocations) results on the parent and alkylated singlet cyclopentadienyl cations. The ab initio results are complemented by force field calculations of 1,3 π-interactions and antiaromatic destabilization energies. The antiaromatic destabilization of the singlet cyclopentadienyl cation is larger than that of cyclobutadiene. The MMP2 heats of formation of various cyclopentadienyl cations agree with experimental and ab initio data. Two Jahn–Teller distorted isomeric structures with almost identical energies are found for the parent singlet cyclopentadienyl cation. Dynamic and nondynamic correlation calculations employing the CASSCF(4,5)/6-31G*//MP2(full)/6-31G* and CASPT2(4,5)/6-31G*//MP2(full)/6-31G* levels of theory clearly show the energetic favorability of an allylic structure for the parent singlet cyclopentadienyl cation. Aklylated cyclopentadienyl cations prefer one of the two possible isomeric cyclopentadienyl cation forms. The substitution pattern determines the preferred cyclopentadienyl structure. As a consequence of its strong antiaromatic destabilization the singlet t-butylcyclopentadienyl cation is predicted to rearrange to a homoallylic 2-(1-methyl-2,4-cyclopentadienyl)-2-propyl cation. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1402–1420, 1998