Thermodynamic functions for the cyclopentadienyl radical: The effect of Jahn-Teller distortion

We report CASSCF/cc-PVDZ calculations of the potential surface for the title Jahn–Teller molecule. Energies are calculated along all normal coordinates that can be affected by Jahn–Teller distortion. Both the E′2 in-plane bend and stretch modes are found to be involved, but the bend contribution is very small and its contribution to the entropy is negligible. The in-plane stretch modes strongly dominate, affording a maximum stabilization energy of 4.73 kcal/mol. Harmonic frequencies are calculated for the other modes in the D5h configuration, and these are scaled in accordance with a comparison of experimental frequencies with similar calculations on cyclopentadiene. Energy levels for nuclear motion in the Jahn–Teller modes are then determined using the calculated surface, and these are combined with the scaled harmonic frequencies to obtain complete thermodynamic functions for the radical. The results are then fit to both the NASA polynomial form and a compact power series in inverse temperature for modeling applications. Because of changes in degeneracy and symmetry number, the entropy is actually lower than previous estimates. Nonetheless, the effect of the pseudorotation on the entropy is noticeable over the entire span of temperatures. This work is evidently the first to present both energy levels and thermodynamic functions for a Jahn–Teller molecule derived from a complete and consistent theoretical treatment. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 834–845, 2001