Ab Initio Calculations on 1,3,5-Trimethylenebenzene and Its Negative Ion Provide Predictions about the Photoelectron Spectrum of the Ion

The results of ab initio calculations on 1,3,5-trimethylenebenzene ( 1 ) and its negative ion ( 1 - ) are presented. Geometries were optimized at the CASSCF/6-31G* and CASSCF/6-31+G* levels. Single-point calculations were performed using second-order perturbation and multireference configuration interaction (MR-CI) methods, in order to include the effects of dynamic correlation between the σ and π electrons. The ground state of 1 is predicted to be the high spin 4 A 1 ‘‘ state, which has D 3 h symmetry. The lowest energy excited state is 2 E‘‘ in D 3 h symmetry and is thus subject to first-order Jahn−Teller distortions to geometries of lower symmetry. The C 2 v geometries of the two Jahn−Teller distorted components of 2 E‘‘, 2 B 1 and 2 A 2, have been optimized and are found to have nearly the same energies. After correction for zero-point energy differences, the adiabatic energy separation between the lowest C 2 v doublet and the D 3 h quartet ground state of 1 is computed to be 14 ± 1 kcal/mol. The ground state of 1 - is predicted to be 3 E‘ in D 3 h symmetry. Molecular distortion to C 2 v symmetry stabilizes the 3 B 2 component of 3 E‘ via not only first- but also second-order Jahn−Teller effects. Consequently, the 3 B 2 component of 3 E‘, at its optimized geometry, is calculated to be lower in energy by ca. 1.5 kcal/mol than the 3 A 1 component at its optimized geometry. The lowest singlet excited state of 1 - is 1 A 1 ‘ in D 3 h symmetry, which is predicted to undergo second-order Jahn−Teller distortions, at least at the CASSCF level of theory. The resulting state, 1 A 1 in C 2 v symmetry, is calculated to be 4−6 kcal/mol above the 3 B 2 ground state but slightly below a 3 A 2 ‘ excited state of D 3 h symmetry. In contrast to CASSCF and MR-CI, UB3LYP/6-31+G* calculations predict 3 A 2 ‘ to be the ground state of 1 -, slightly below either component of the Jahn−Teller distorted 3 E‘ state. The UB3LYP calculations afford an estimate of 21−22 kcal/mol for the electron affinity of 1 and provide vibrational frequencies for the 4 A 1 ‘‘ state. On the basis of computational results for 1 and 1 -, the most important features of the photoelectron spectrum of 1 - are predicted.