Alessandro Toniolo

Direct semiclassical simulation of photochemical processes with semiempirical wave functions

We describe a new method for the simulation of excited state dynamics, based on classical trajectories and surface hopping, with direct semiempirical calculation of the electronic wave functions and potential energy surfaces ~DTSH method!. Semiempirical self-consistent-field molecular orbitals ~SCF MO’s! are computed with geometry-dependent occupation numbers, in order to ensure correct homolytic dissociation, fragment orbital degeneracy, and partial optimization of the lowest virtuals. Electronic wave functions are of the MO active space configuration interaction ~CI! type, for which analytic energy gradients have been implemented. The time-dependent electronic wave function is propagated by means of a local diabatization algorithm which is inherently stable also in the case of surface crossings. The method is tested for the problem of excited ethylene nonadiabatic dynamics, and the results are compared with recent quantum mechanical calculations.

Molecular gradients for semiempirical CI wavefunctions with floating occupation molecular orbitals

Abstract Configuration Interaction based on floating occupation Hartree–Fock molecular orbitals has proved to be an useful tool in the study of photochemical reactions. In this paper we describe a method for the calculation of the molecular energy gradient of a CI type wavefunction built with orbitals obtained from a semiempirical Hartree–Fock calculation using floating occupation numbers with gaussian broadening. Our method is tested by computing potential energy curves for the acetone photodissociation reaction.

Efficient calculation of Franck–Condon factors and vibronic couplings in polyatomics

We present a technique for the calculation of Franck–Condon factors and other integrals between vibronic wave functions belonging to different electronic states. The technique is well suited for the determination of the nonadiabatic or spin‐orbit couplings related to radiationless decays in polyatomics. Rigorous or approximate partitions of the internal coordinate space are exploited to achieve better efficiency and/or to go beyond the harmonic approximation. The technique is tested by computing the Internal Conversion and InterSystem Crossing rates of (CH3)3CNO in its 1(n→π*) state.

Semiclassical simulation of photochemical reactions in condensed phase

We compare two strategies for the semiclassical simulation of photochemical reactions in condensed phase (solutes in liquids, impurities in solid matrices, adsorbates, or photoreactive units in biological environments). Both strategies are based on classical nuclear trajectories, with surface hopping to simulate nonadiabatic transitions. In the first approach we calculate electronic energies and couplings by ab initio methods for many nuclear geometries of the reactive system (the ‘solute’), and we fit the results by analytic functions of the internal coordinates. In the case of surface crossings it is mandatory to resort to a (quasi-)diabatic representation of the electronic states. A condensed state environment (the ‘solvent’) is described by ordinary molecular mechanics (MM), and the solute ‐ solvent interactions can be state-specific. The other strategy is direct, i.e. it involves the calculation of electronic energies and wavefunctions at each integration step of a nuclear trajectory. The method we have implemented is based on semiempirical configuration interaction calculations with floating occupation SCF orbitals; within this approach we have developed a hybrid quantum mechanics/molecular mechanics (QM/MM) procedure to represent the solvent, which is here briefly described for the first time. While the intra- and intermolecular potentials for the solvent molecules are of MM type, the QM/MM interaction is introduced in the semiempirical electronic hamiltonian, so that it influences in a state-specific way electronic energies and wavefunctions. Applications of both approaches to photoreaction dynamics are briefly described. q 2002 Elsevier Science B.V. All rights reserved.

An ab initio study of spectroscopy and predissociation of ClO

We have computed all the electronic states of ClO arising from the Cl(2P)+O(3P) dissociation limit and several of those connected with Cl(2P)+O(1D). Only two excited states have attractive potentials, A 2Π and 1 4Σ−. The A 2Π state undergoes a well known predissociation, because several as yet unknown potential curves cross the A 2Π one and are coupled to it by nonadiabatic and/or spin-orbit interactions. The calculation of the interaction matrix elements allows to explain the predissociation of A 2Π, due to transitions to the 3 2Π,  12Δ, 2 4Σ− and other less important states, all leading to the Cl(2P)+O(3P) dissociation.

Theoretical study of the photochemistry of Cl2O

This is a theoretical study of the photochemistry of Cl2O based on ab initio potential energy surfaces and trajectory surface-hopping calculations. We calculated quasidiabatic states and couplings for eight singlet states of Cl2O with a multireference perturbation configuration interaction (CI) technique. Analytical representations of the three-dimensional potential energy surfaces and electronic couplings were used for semiclassical simulations of the nonadiabatic dynamics of excited Cl2O up to 5.4 eV. The computational results allow us to relate the photodissociation mechanism to observable quantities such as the anisotropy of the recoil velocity and the translational energy distribution of the fragments.

Above threshold dissociation of LiNa+: monitoring an avoided crossing with femtosecond spectroscopy

Abstract Computer simulations of one- and two-color experiments in above threshold dissociation (ATD) are reported for the first heteronuclear alkali ion LiNa + . We focus on the 1 2 Σ + →1 2 Π→4,5 2 Σ + process, with dissociation to Li + +Na(3p) or Li(3s)+Na + . The product yields are determined by the presence of an avoided crossing between the 4 and 5 2 Σ + potential curves, according to the frequency and delay of the second laser pulse.

A theoretical study of spectroscopy and predissociation dynamics in nitrosoalkanes

We have computed ab initio transition energies, equilibrium geometries, force constants and potential energy curves for the dissociation of S0, T1, and S1 of two nitrosoalkanes, CH3NO and t-BuNO. A normal coordinate analysis has been performed for the three states, and the harmonic wave function for the C–N bond torsional coordinate has been replaced by hindered rotor eigenfunctions. The n→π* absorption spectra have been simulated by computing the appropriate Franck–Condon factors in order to assign the vibrational sub-bands. The predissociation lifetimes of several vibrational states of S1 have been evaluated by computing nonadiabatic and spin-orbit couplings, which determine the Internal Conversion and Intersystem Crossing rates. For t-BuNO the computed lifetimes (10–160 ns) are in the same range as those measured by Noble et al. [J. Chem. Phys. 85, 5763 (1986)]. The lifetimes of CH3NO, for which no experimental data are available, are longer (50–330 ns). Both the IC to S0 and the ISC to T1 are important.

A computational study of the excited states of bilirubin IX

We have determined the lowest excited states of bilirubin IX by TD-DFT calculations. The lowest pair of excited states, S(1) and S(2), turn out to be of charge-transfer (CT) nature. Although DFT based methods tend to underestimate the energy of CT states, the small oscillator strengths we have computed indicate that such states may actually exist in this spectral region, but would have escaped spectroscopic detection. The next pair of excited states, S(3) and S(4), account for the most prominent spectral feature of bilirubin. They can be accurately described by the exciton coupling model, as we show by a thorough analysis of wavefunctions and properties. This finding therefore supports the interpretation of bilirubin photoisomerization behaviour, based on the exciton coupling model.

Theoretical study of the photodissociation dynamics of ClOOCl

This is a thorough theoretical study of the photodissociation of ClOOCl. We present ab initio calculations of the potential energy curves and a modification of the semiempirical MNDO-d method, designed so as to reproduce the ab initio results as well as available experimental data. Simulations of the nonadiabatic photodissociation dynamics have been run with a direct semiclassical method, based on the semiempirical wavefunctions and potential energy surfaces. We have run three groups of trajectories, with randomly chosen initial conditions, such as to simulate excitation in three different regions of the absorption spectrum, around 460, 325 and 264 nm, respectively. We find that dissociation to 2Cl + O2 is the main photoreaction, and a small amount of ClO is formed at the highest excitation energies. The mechanism is mainly sequential at low energies, involving the short lived species ClOO, while at high energies the synchronous Cl–O bond breaking prevails. We compare the computed quantum yields, final fragment energies and anisotropy parameters with the corresponding experimental quantities, measured by Moore et al. (T. A. Moore, M. Okomura, J. W. Seale and T. K. Minton, J. Phys. Chem., 1999, 103, 1691. Ref. 1.) and we propose a partially new interpretation of their observations.