Vibrational motion of the radical cation C3+ in its degenerate electronic ground state

The cation C-3(+) plays an important role in astrophysical reactions but so far it has not been characterised spectroscopically. Theory has established a complicated degenerate electronic ground state with a conical intersection of D-3h structure and three local minima of D-infinity h structure. The small isomerisation energy is not well determined with estimates ranging from 3 to 7 kcal mol(-1). In this paper the first attempt is made to generate a reliable global potential energy surface. The MR-CI method is used to cope with a very strong multi-configurational electronic structure. Including Davidson corrections and complete basis extrapolation, we predict a Jahn-Teller stabilisation energy of 1334 cm(-1), a pseudorotation barrier of 376 cm(-1), a well depth of the linear minima of 289 cm(-1) and an electronic isomerisation energy of 2238 cm(-1) (6.4 kcal mol(-1)). An analytical form of the potential surface is used in variational calculations of low-energy vibrational motions which are based on hyperspherical coordinates. Vibronic coupling is treated in a diabatic framework for the Jahn-Teller region as well as the near-linear region. We find a unique mixture of vibrational states which are either located around the three cyclic minima or the three linear minima, or they are global in the sense that they fill the energy valleys between these minima. Assignments are proposed in terms of local vibrator quantum numbers. Zero point energies in the cyclic global and linear local minima are 1626 cm(-1) and 1195 cm(-1), respectively, i.e. the isomerisation energy is reduced by vibrational effects to 1668 cm(-1) (4.75 kcal mol(-1)).