Spin-orbit absorption spectroscopy of transition metal hydrides: A TD-DFT and MS-CASPT2 study of HM(CO)5 (M = Mn, Re)

The spinorbit (SO) effects on the absorption spectra of HMn(CO)5 and HRe(CO)5 have been studied by means of ab initio multistate complete active space perturbation second order (MS-CASPT2) and time-dependent density functional theory (DFT) calculations. For both molecules and in contrast to the experimental spectra, the spin-free theoretical absorption spectra differ significantly, especially in the region of the intense band observed at 47,000 cm-1. The main features of the lowest and highest parts of the absorption spectra are well reproduced making a reasonable assignment of the bands observed at 34,500 and 51,300 cm-1 for HMn(CO)5 and 37,000 and 50,630 cm-1 for HRe(CO)5. Whereas, the lowest band of HMn(CO)5 has a significant metal-centered (MC) character, the band centered around 37,000 cm-1 in HRe(CO)5 corresponds to nearly pure metal-to-ligand-charge-transfer (MLCT) states. The intense bands observed above 48,000 cm-1 in both complexes have mixed character with MLCT, sigma bond to ligand charge transfer, sigma bond to sigma bond charge transfer, and metal to sigma bond charge transfer contributions, which may vary as a function of the level of calculation (DFT, MS-CASPT2, SO). This is attributed to the ability of the method at describing electronic correlation effects in various types of excited states in a balanced way. The SO absorption spectra are more realistic for both molecules with a red shift of the start of the absorption and a broadening of the bands due to a large mixing between the singlettriplet states and an increase of the density of electronic states contributing to the absorption. This red shift is more important in HMn(CO)5 than in HRe(CO)5 due to the MC contribution to the lowest states in first-row transition metal complexes. Our calculations point to the presence of an absorption of intermediate intensity, not observed experimentally, around 40,00042,000 cm-1, mostly of MLCT character. (C) 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012