Multireference CI Study of Excitation Energies and Potential Energy Surfaces of CH3F

Vertical excitation energies for the eight singlet-excited electronic states 1 1 E (2e → 3s), 2 1 E (2e → 3p a1), 3 1 E (2e → 3p e ), 2 1 A 1 (2e → 3p e ), 1 1 A 2 (2e → 3p e ), 3 1 A 1 (2p a1 → 3s), 4 1 A 1 (2p a1 → 3p a1 ), and 4 1 E (2p a1 → 3p e ) of CH 3 F were investigated using the SA-MCSCF, MR-CISD, and MRCISD+Q approaches. Our results mostly confirm the experimental assignments but suggest some modifications for the main contribution to the maximum observed in the range from 12.5−14 eV. The dissociation channels for the production of fluorine atoms have been characterized. Potential energy curves for the dissociation of the CF bond under C 3 v symmetry restrictions were computed for all states mentioned above leading to the ground-state dissociation channel CH 3 (X̃ 2 A 2 ‘ ‘) + F( 2 P), the excited-state channels CH 3 (3s 2 A 1 ‘) + F( 2 P) and CH 3 (3p 2 A 2 ‘ ‘) + F( 2 P) and also to the ionic limit CH 3 + ( 1 A 1 ‘) + F - ( 1 S). All curves except the one for the ionic state show repulsive behavior. The search for a global minimum for the ionic state led to the structure H 2 CH + F - in the 3 1 A‘ state. It is strongly bound by 5.67 eV with respect to the ionic dissociation limit of F - + CH 3 + .