Theoretical study of the electronic structure of MCH(2)(+)(M=Fe,Co,Ni)

State of the art coupled cluster (CC) methods are applied to accurately characterize the ground state electronic structure and photoelectron spectra of transition metal carbene ions MCH(2) (+) (M=Fe, Co, and Ni). The geometries and energies of the lowest energy quartet, triplet, and doublet electronic states as well as several low-lying vertical excitation energies of FeCH(2) (+), CoCH(2) (+), and NiCH(2) (+) are reported. The excitation energies are computed using the equation-of-motion CC and for states of different symmetries, by the energy differences of single reference ground and excited states (Delta-CC). The latter employ several reference states; the unrestricted Hartree-Fock, restricted open shell Hartree-Fock, and unrestricted Kohn-Sham. We conclude that the (2)A(1) electronic ground state of NiCH(2) (+) is separated by about 30.0 kJ/mol from the next highest state, and the lowest (4)B(1) and (4)B(2) states of FeCH(2) (+) as well as the (3)A(2) and (3)A(1) states of CoCH(2) (+) are nearly degenerate. The presence of metal-pi*(MCH(2)) charge transfer states with significant oscillator strengths in the visible/near-UV energy domain of the theoretical spectra of FeCH(2) (+) and CoCH(2) (+) are at the origin of the photofragmentation of these compounds observed after irradiation between 310 and 360 nm.