The Jahn-Teller and related effects in the cyclopentadienyl radical.: I.: The ab initio calculation of spectroscopically observable parameters

Ab initio calculations are performed for the (X) over tilde E-2(1)" and (A) over tilde (2)A(2)" states of the cyclopentadienyl radical. An important goal of these calculations is to guide the analysis of the experimentally observed (A) over tilde (2)A(2)"- (X) over tilde E-2(1)" electronic spectrum. Vibrational frequencies for both the (X) over tilde and (A) over tilde state are reported. Large changes in frequency between the states for out-of-plane vibrations are found, leading to the expectation that overtones of these modes will appear strongly in the spectrum. Additionally, spectroscopically obtainable parameters describing the Jahn-Teller effect are calculated for the (X) over tilde state. Using all this information the (X) over tilde-(A) over tilde electronic spectrum is predicted for both C5H5 and C5D5. (C) 2001 American Institute of Physics.