Quantum chemical characterization of the ground and lowest excited singlet and triplet states of CH3CF and CF3CF and their photochemical isomerization and dissociation pathways

The (a) over tilde((3)A") <-- <(X)over tilde>((1)A') and (A) over tilde((1)A") <--<(X)over tilde>((1)A') excitation energies of CH3CF and CF3CF have been computed by a range of quantum chemical methods, including coupled cluster with single, double and perturbative triple excitations (CCSD( T)), equations of motion coupled cluster (EOM-CCSD), complete active space second-order perturbation (CASPT2), multireference configuration interaction (MRCI) and time-dependent density functional methods. The equilibrium geometries and vibrational frequencies were determined by density functional (B3LYP) and complete active space self-consistent field (CASSCF) methods. The recommended singlet - triplet (s/t) and singlet - singlet (s/s) excitation energies (based on CCSD(T) and EOM-CCSD computations, respectively using cc-pVTZ basis sets with corrections for core - valence correlation, basis expansion to cc-pVQZ as well as scalar relativistic effects and including zero point corrections) are: 21.2 (s/t) and 57.2 kcal mol(-1) (s/s) for CH3CF and 15.9 (s/t) and 52.8 kcal mol(-1) (s/s) for CF3CF. According to CASPT2 computations on the (A) over tilde surface, in both molecules the barriers to dissociation at similar to17 kcal mol(-1) are similar to2 kcal mol(-1) lower than the barriers to linearity of the CCF backbones, namely C-3v structures, where the (X) over tilde and (A) over tilde states become degenerate. The barriers to isomerization to the thermodynamically more stable fluoroethylene molecules on the singlet ground state surfaces, computed at the CCSD( T)/cc-pVTZ level of theory, are 16.9 and 37.5 kcal mol(-1) for CH3CF and CF3CF, respectively, while on the triplet surface they are considerably higher at 47.4 and 53.8 kcal mol(-1). As the barriers on the singlet surface are well below the (A) over tilde <-- <(X)over tilde> excitation energies, both molecules have the potential to isomerize on excitation. Moreover, in the case of CF3CF, excitation could result in photodissociation to singlet CF2 carbenes.