Nonadiabatic Effects in the Photodissociation and Electronic Spectroscopy of HMn(CO)₃(dab):  Quantum Wave Packet Dynamics Based onab InitioPotentials

Abstract

The photochemistry of many transition metal complexes is governed by a multitude of electronically excited states, coupled by various mechanisms. For the transition metal complex HMn(CO) 3 (dab) (dab ) 1,4-diaza-1,3-butadiene) the photoreactivity (cleavage of the Mn-H bond) and electronic absorption spectra are characterized on the basis of quantum mechanical first-principles calculations. In a first step, the A′ ground (singlet) and the three lowest electronically excited (triplet) potential curves along the Mn-H bond distance are computed using the CASSCF/CCI method. Two of the excited states are found to be bound and are of the metal-to-ligand charge transfer type, whereas the third, ligand-to-ligand charge transfer state is repulsive. In the relevant energy region, two avoided crossings are observed, indicative for strong nonadiabatic couplings. In a second step, the UV/vis photochemistry of the complex is investigated by means of nuclear wave packet dynamics. We solve the nonadiabatically coupled, time-dependent Schrödinger equation in a diabatic representation for different initial conditions to determine both photodissociation yields and electronic absorption spectra. In particular, the effect of the nonadiabatic couplings on the electronic absorption spectrum and on the photoreactivity is investigated.

Femtosecond Spectroscopic Study of MLCT Excited-State Dynamics of Cr(CO)4(bpy): Excitation-Energy-Dependent Branching between CO Dissociation and Relaxation

“…Recent theoretical results, which indicate that quantum yields of photodissociation are higher when starting from a vibrationally excited ground state, could suggest possible explanation. 37,38 In conclusion, all the events which determine the quantum yield of the CO photodissociation from Cr(CO) 4 (bpy) were found to occur in a less than 400 fs after the MLCT excitation, i.e., with a rate constant larger than 2.5 × 10 12 s -1 . The probability of the dissociation of the axial Cr-CO bond is highly dependent on the excitation energy.…”

Excited states dynamics of polydiacetylenes: An ab initio and femtosecond spectroscopic investigation of the change from the acetylenic to the butatrienic structure

“…In order to describe correctly the thermalization process responsible for such a long time scale, further theoretical studies should include dissipative effects not taken into account in the present work. 55 …”

“…The crossing point at d CϭC ϭ1.411 Å has been taken as reference for the Lorentzian, which gives the parameters useful to determine approximate kinetic coupling terms in nonadiabatic situations. 55 The potential coupling terms used in Eq. ͑1͒ and obtained after a transformation of these kinetic couplings are drawn as function of the ␦ coordinate in Fig.…”

Photodissociation and Electronic Spectroscopy of [Re(H)(CO)3(H-dab)] (H-dab=1,4-diaza-1,3-butadiene): Quantum Wavepacket Dynamics Based on Ab Initio Potentials

“…The spin‐orbit coupling between the singlet and the triplet states were evaluated with a restricted full CI scheme and an effective one‐electron SO operator operating on the metal center in the L–S coupling scheme 26. Since only the first derivative part of the kinetic coupling is needed in our approach to the non‐adiabatic effects, the kinetic coupling around avoided crossings was approximated by Lorentzians in order to avoid fastidious numerical differentiation of the electronic CASSCF wavefunctions 27. A few DFT (B3LYP)28 optimizations were performed for the electronic ground state by the use of either the same ECP and associated basis sets or the LANL2DZ with D95 on first‐row atoms29 and Los Alamos ECP plus Double‐ ξ on the Re atom30 with the Gaussian 98 quantum chemistry software 31…”

“…The potential coupling V $_{\rm kk\prime }^{d}$ is given either by the spin‐orbit coupling elements of the SOC‐CI matrix (singlet–triplet interactions) or by the transformation of the estimated kinetic couplings around avoided crossings (singlet–singlet and triplet–triplet interactions). The adiabatic/diabatic transformation is performed according to the scheme described in reference 27.…”