The Spectroscopy of HMn(CO)(5): A CASSCF/MRCI and CASPT2 ab Initio Study

The electronic structure of the states of HMn(CO)(5) in the near-UV region was investigated by CASSCF/MRCI and CASPT2 ab initio methods with a basis set between double- and triple-zeta quality including very diffuse functions. On the basis of calculated vertical excitation energies and oscillator strengths, the following absorption band assignments could be made: (1) A(1)E (3d(pi) --> 3d(x)()()2(-)(y)()()2) and B(1)E (3d(pi) --> sigma(Mn-H)) transitions underlie the low-intensity shoulder at 229 nm; (2) the high-intensity B(1)A(1) (3d(pi) --> pi(CO,12e) +3d(xy)() --> pi(CO,3b)2) and weak C(1)E (3d(pi) --> pi(CO,3b)2) transitions underlie the central band at 214 nm; and (3) the very high intensity C(1)A(1) (sigma(Mn)(-)(H) --> sigma(Mn)(-)(H)) underlies mainly the band at 193 nm. This identifies the most likely photoinitiating states for the photochemistry of HMn(CO)(5). The a(3)A(1) was shown to have valence character, contrary to previous suggestions.