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 g oal of these calculations is to guide the analysis of the experimentally ob served (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 sta te are reported. Large changes in frequency between the states for out-of-p lane vibrations are found, leading to the expectation that overtones of the se modes will appear strongly in the spectrum. Additionally, spectroscopica lly 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.