• bondpseudorotation;
  • Jahn–Teller effect;
  • pseudo-Jahn–Teller effect;
  • cyopentadienyl cation;
  • deformation coordinates


Multireference averaged quadratic coupled cluster (MRAQCC) (4,5)/cc-pVTZ calculations predict that bond pseudorotation (BPR) in the first excited singlet state of the cyclopentadienyl cation (CPC) proceeds with a barrier of just 0.35 kcal/mol, where five dienylic forms present the minima and five allylic forms the transition states of the pseudorotation process. Vibrational and entropic corrections revert the order of stabilities and lead to a Δ G(298) of just 0.05 kcal/mol indicating that BPR is unhindered at room temperature. The description of the CPC ring in terms of curvilinear deformation coordinates (seven for C5, seven for X5, and three coupling coordinates) make it possible to explore both the six-dimensional (6D) Jahn-Teller and the 8D pseudo-Jahn-Teller space and assess the importance of Jahn–Teller and pseudo-Jahn–Teller deformations of the CPC ring. The latter dominate the ring deformations along the BPR path. The only somewhat larger Jahn–Teller contribution results from a Emath image-symmetrical CCH bending motion. For the perhalogenated CPCs, the dominance of the pseudo-Jahn–Teller effect increases, however, the total deformation of the D5h-symmetrical ring decreases and thereby also the stabilization of the 1A1 forms along the BPR path. This leads to a reduction of the BPR barriers to just 0.14 kcal/mol for C5Imath image. For all pentahalogeno CPCs, the dienylic form is more stable both at the energy and free energy level. The use of curvilinear deformation coordinates facilitates the understanding of the electronic features of cyclic (pseudo-) Jahn–Teller systems. © 2012 Wiley Periodicals, Inc.