Combined Jahn-Teller and Pseudo-Jahn-Teller Effect in the CO3 Molecule: A Seven-State Six-Mode Problem.

A complicated problem of seven electronic states in four terms, (1)A1', (1)E'', 1(1)E', and 2(1)E', interacting with six vibrational modes, a1', a2'', e', and e', was solved to take into account the combined two-mode Jahn-Teller (JT) plus two-mode pseudo JT effects and rationalize the electronic structure of the CO3 molecule. The JT and first-order pseudo JT effects in the E'' state are separated from the rest of the problem by symmetry; they do not influence the ground state properties. In the remaining five-state five-mode problem including the ground state, (A1' + 1E' + 2E') ⊗ (a1' + e' + e'), the JT two-mode problem is reduced to the one-mode one by means of coordinate transformations. Several high-level ab initio calculations including all of the five states confirm the previously found coexistence of a central minimum of D3h symmetry and three equivalent minima with a distorted geometry of C2v symmetry in the ground state; the barrier between them is rather small, 0.2-0.3 eV, but large enough to distinguish them spectroscopically. Harmonic vibrational frequencies of the two configurations near the minima of the adiabatic potential energy surface are also evaluated. The calculations show how the distorted configurations are produced by the JT effect in one of the excited E states, similar to a previous finding in O3. Numerical data of ab initio calculations yield also the effective vibronic and primary force constants for all of the terms. An electronic structure problem of this complexity including a reduction of the two-mode problem to one mode with full interpretation of the origin of coexisting different geometries as due to the JTE in the excited state is presented here for the first time.