Photochemistry of methyltrioxorhenium revisited: A DFT/TD-DFT and CASSCF/MS-CASPT2 theoretical study

The electronic structure of (CH₃)ReO₃ was reanalyzed by means of DFT calculations (ADF and Gaussian03 programs). TD-DFT calculations were carried out, using the DFT-optimized structure and several functionals, to assign the electronic excitations and interpret the dissociative behavior under irradiation. The agreement between calculated and experimental wavelengths was very good and could not be improved when using highly correlated methods (CASSCF/MS-CASPT2). The lowest energy transition at 260 nm was assigned to a LMCT from p O to Re d (π* Re-O), correcting the earlier empirical assignment. The second transition, experimentally observed at 240 nm, is assigned to a charge transfer from C and O to Re in TD-DFT, but the weight of the C participation drops significantly in the CASSCF/MS-CASPT2 approach. (C₂H₅)ReO₃ exhibits a similar behavior. On the other hand, for (C₆H₅)-ReO₃ and {C ₆H₃(CH₃)₃}ReO₃ the strong low-energy absorption results from a LMCT from the phenyl π* to d Re (π* Re-O), reproducing the experimental trends.