METAL-TO-LIGAND CHARGE-TRANSFER PHOTOCHEMISTRY - POTENTIAL-ENERGY CURVES FOR THE PHOTODISSOCIATION OF HMN(CO)(3)(DAB)

The photodissociation of HMn(CO)(3)(dab) has been studied through contracted configuration interaction (CCI) calculations on top of CASSCF wave functions for the metal-hydrogen bond homolysis and the photodissociation of an axial carbonyl ligand. The corresponding potential energy curves (PEC) connect the ground and excited states of the reactant to the ground and excited states of the primary products. The calculations were carried out under C-s symmetry constraint with a basis set that is at least of double-zeta quality. The multinference CCI calculations that correlate the 3d electrons and the two electrons of the Mn-H bond were based on CASSCF wave functions with eight electrons in ten active orbitals (3d(yz), 3d(x -y)(2), 3d(xz), Mn-sigma-H, (sigma)*Mn-H, 3d(xy), 4d(yz), 4d(x -y)(2), 4d(xz), pi*) optimized for the required state. From the shape of the PEC it appears that the interaction between the two lowest a(3)A'(d(xz) --> pi*) and b(3)A'(d(x -y)(2) --> pi*) metal-to-ligand charge transfer (MLCT) excited states and the C(3)A' (sigma --> pi*) ligand-to-ligand charge transfer (LLCT) dissociative state plays a key role in the homolysis mechanism. It is proposed that excitation of HMn(CO)(3)(dab) at low energy (500 nm) will bring the molecule in the lowest (1)MLCT states (calculated at 18 030 and 21 710 cm(-1)). From these singlet states, after (1)MLCT --> (3)MLCT fast intersystem crossing, the system may either get trapped in the potential well of the long lifetime triplet excited states and provide emission or be able to overcome the energy barriers yielding a small fraction of the primary products H + Mn(CO)(3)(dab) in their ground states. Direct dissociation of the CO loss primary products with a rather low quantum yield is not excluded from the shape of the lowest MLCT PEC along the Mn-COax elongation. Excitation of HMn(CO)3(dab) at high energy (around 300 nm) will bring the system in one of the singlet states corresponding to d --> d or sigma --> pi* excitations (calculated between 28 980 and 37 980 cm(-1)). After fast intersystem crossing to the C(3)A'(sigma --> pi*) excited state calculated at 31 509 cm(-1), the system may either dissociate after two successive internal conversions (at 2.02 and 2.11 Angstrom) along the a(3)A' PEC to the primary products H + Mn(CO)(3)(dab) in their ground states or get trapped in the potential well of the a,b(3)A' MLCT PEC and provide emission.