Articles you may be interested inAb initio calculations of triplet excited states and potential-energy surfaces of vinyl chloride: Insights into spectroscopy and photodissociation dynamics Ab initio theoretical studies of potential energy surfaces in the photodissociation of the vinyl radical. I. Ã state dissociation

The photodissociation dynamics of H2Fe(CO)4 have been studied through wave packet propagations on CASSCF/CCI potentials, calculated for the electronic ground and excited states, as a function of two coordinates q a and q b corresponding to molecular hydrogen elimination and CO dissociation, respectively. The theoretical absorption spectrum is characterized by two bands at 272 nm (36 700 cm-1) and 246 nm (40 500 cm-1) which have been assigned to the a1A1 → a1B1 and a1A1 → b1A1 transitions, respectively. The first band corresponds to the experimental maximum observed around 270 nm. A semiquantitative mechanism has been proposed for the photodissociation of the title molecule:  (i) under irradiation at 254 nm, the a1B1 (3d yz → σg*) and b1A1 (3d x 2 - y 2 → σg*) excited states will be populated; (ii) from the b1A1 state, the molecule will dissociate in a total and ultrafast (less than 40 fs) reaction toward the formation of Fe(CO)4 and molecular hydrogen; (iii) after the initial a1A1 → a1B1 transition, the reactive system will evolve into two competitive channels, leading mainly to the elimination of H2 in a very short time scale (40 fs) and as second minor primary reaction (4%) toward the CO dissociation in 100 fs. The nonradiationless transitions to the low-lying triplet states occur in a picosecond time scale and have a rather low probability indicating the minor role of the triplet states at the early stage of the photodissociation.

The singlet valence excited states of an iron-porphyrin-pyrazine-carbonyl complex are investigated up to the Soret band (about 3 eV) using multi-state complete active space with perturbation at the second order (MS-CASPT2). This complex is a model for the active site of carboxy-hemoglobin/myoglobin. The spectrum of the excited states is rather dense, comprising states of different nature: d→π* transitions, d→d states, π→π* excitations of the porphyrin, and doubly excited states involving simultaneous intra-porphyrin π→π* and d→d transitions. Specific features of the MS-CASPT2 method are investigated. The effect of varying the number of roots in the state average calculation is quantified as well as the consequence of targeted modifications of the active space. The effect of inclusion of standard ionization potential-electron affinity (IPEA) shift in the perturbation treatment is also investigated.

A coherent control algorithm is applied to obtain complex-shaped infrared laser pulses for the selective vibrational excitation of carbon monoxide at the active site of carbonmonoxyhemoglobin, modeled by the six-coordinated iron-porphyrin-imidazole-CO complex. The influence of the distal histidine is taken into account by an additional imidazole molecule. Density-functional theory is employed to calculate a multidimensional ground-state potential energy surface, and the vibrational dynamics as well as the laser interaction is described by quantum wave-packet calculations. At each instant in time, the optimal electric field is calculated and used for the subsequent quantum dynamics. The results presented show that the control scheme is applicable to complex systems and that it yields laser pulses with complex time-frequency structures, which, nevertheless, have a clear physical interpretation.

Relaxation dynamics following photoexcitation of a calcium atom deposited on an icosahedral-like argon cluster Ar(n) (n approximately = 55) is investigated through theoretical simulations. Based on ab initio calculations of the CaAr molecule, a diatomics-in-molecules model is set up to efficiently describe the electronic excited states of the system. The excited state dynamics is studied using molecular dynamics with electronic transitions (Tully, J. C. J. Chem. Phys. 1990, 93, 1061). The signature of this dynamics in the time-resolved photoelectron spectra is investigated, to assess the possibility of detecting competing vibrational and electronic relaxations through pump-probe experiments. The vibrational relaxation, influenced by nonadiabatic transitions, can clearly be seen in the time-resolved photoelectron spectra. The details of the electronic relaxation, as well as the possible ejection of the chromophore, are found to be sensitive to the local environment of the calcium atom deposited on the argon cluster.

The traditional Polanyi rules for control of bimolecular reactions by selective investment of energy, e.g. preferentially translational, not vibrational energy for early-downhill reactions on attractive potential energy surfaces, are extended to ultrafast unimolecular reactions. Specifically, we consider photodissociations of the metal-hydrogen bond of HCo(C0)4( lE), occurring on a time scale of approximately 20 fs, much faster than competing intramolecular vibrational energy redistribution (IVR). Here the reaction path toward the products H + C O ( C O )~ is hindered by a barrier located in the H + Co(CO)4 exit valley of the potential energy surface.In order to overcome this barrier and, therefore, to increase reactivity, vibrational energy should be invested selectively into the bond to be broken, i.e. [H-Co], not into complementary "spectator" modes, e.g. [CO-Co].The required energetic preparation of reactants may be achieved by selective IR + UV two-photon excitations or by alternative techniques including frequency-selective UV single-photon photodissociation. The Polanyi rules for unimolecular reactions are demonstrated by fast Fourier transform (FFT) propagations of representative wave packets.

Spin-orbit coupled excited states in transition metal complexes: A configuration interaction treatment of HCo(CO)4A theoretical description of the ''fast'' ͑Ͻ50 ps͒ intersystem crossing processes occurring at critical geometries during the photodissociation of HCo͑CO͒ 4 is presented. The radiationless transitions are simulated by wave packet propagations along one-dimensional reaction coordinate on the spin-orbit coupled potential energy surfaces. The propagation are performed separately, either along the Co-H bond or along the Co-CO ax bond. This original approach has enabled us to understand the mechanism of desactivation of the initially populated singlet excited state in this molecule which should be considered as a model for other organometallics. We propose the following mechanism: ͑i͒ in a very short time scale ͑Ͻ20 fs͒ 40% of the system dissociates towards the primary products HϩCo͑CO͒ 4 ( 1 E), whereas the 1 E→ 3 A 1 intersystem crossing along the Co-H bond elongation occurs within 50 ps; ͑ii͒ the dissociation of an axial carbonyl ligand occurs in a larger time scale ͑200 fs͒ and only 2% of the system dissociates along the Co-CO ax elongation. The dominant process is the 1 E→ 3 A 1 intersystem crossing leading to HCo͑CO͒ 4 ͑ 3 A 1 ͒; ͑iii͒ as soon as the lowest triplet state is populated, the system dissociates either to HϩCo͑CO͒ 4 or to HCo͑CO͒ 3 ϩCO ax on the 3 A 1 potential energy surface; ͑iv͒ the intersystem crossing process may be described as a succession of elementary transitions occurring at critical geometries or crossing points between the singlet and the triplet potential energy surfaces; ͑v͒ the efficiency of the radiationless transition is governed by the overlap of the propagated wave packet with the critical region of the coupled potential energy surfaces.

We apply the mixed quantum/classical method based on the Bohmian formulation of quantum mechanics [E. Gindensperger, C. Meier, and J. A. Beswick, J. Chem. Phys. 113, 9369 (2000)] to the case of rotational diffractive surface scattering of a diatomic molecule. The rotation as well as the normal translational degree of freedom are treated classically while the two parallel degrees of freedom that account for the diffraction are treated quantum mechanically. The effects of treating some degrees of freedom classically are discussed in detail by comparing our novel approximate method to quantum wave packet results obtained by the multiconfiguration time-dependent Hartree method.