Ab initio molecular orbital study of the isomerization reaction surfaces of C3 and C3−

The potential-energy surfaces of C3 and C3− in the ground and some low-lying excited states are investigated by the CASSCF and MRD-CI methods to clarify the features of the isomerizations between the linear and bent structures. In addition, the asymmetric linear structures corresponding to the antisymmetric stretching mode in the à 1Πu state are examined. It is shown that the ã 3Πu state of C3 is isomerized into the equilateral triangle structure via an asymmetric transition state. Also, the central barrier height (215 cm−1) calculated for the double-minimum potential of à 1Πu agrees reasonably well with the experimental data (284.3 cm−1).