Leonardo Bernasconi

Time dependent density functional theory study of charge-transfer and intramolecular electronic excitations in acetone–water systems

A recently introduced formulation of time dependent linear response density functional theory within the plane-wave pseudopotential framework [J. Hutter, J. Chem. Phys. 118, 3928 (2003)] is applied to the study of solvent shift and intensity enhancement effects of the 1A2 n→π* electronic transition in acetone, treating solute and solvent at the same level of theory. We propose a suitable formalism for computing transition intensities based on the modern theory of polarization, which is applicable to condensed-phase and finite systems alike. The gain in intensity brought about by thermal fluctuations is studied in molecular acetone at room temperature, and in gas-phase (CH3)2CO⋅(H2O)2 at 25 K. The latter system is characterized by the appearance of relatively intense features in the low-energy region of the spectrum, attributable to spurious solvent→solute charge-transfer excitations created by deficiencies in the DFT methodology. The n→π* transition can be partially isolated from the charge-transfer bands...

Density functional calculation of the electronic absorption spectrum of Cu+ and Ag+ aqua ions

The UV absorption of aqueous Cu+ and Ag+ has been studied using Time Dependent Density Functional Theory (TDDFT) response techniques. The TDDFT electronic spectrum was computed from finite temperature dynamical trajectories in solution generated using the Density Functional Theory (DFT) based Ab Initio Molecular Dynamics (AIMD) method. The absorption of the two ions is shown to arise from similar excitation mechanisms, namely transitions from d orbitals localized on the metal center to a rather delocalized state originating from hybridization of the metal s orbital to the conduction band edge of the solvent. The ions differ in the way the spectral profile builds up as a consequence of solvent thermal motion. The Cu+ absorption is widely modulated, both in transition energies and intensities by fluctuations in the coordination environment which is characterized by the formation of strong coordination bonds to two water molecules in an approximately linear geometry. Though, on average, absorption intensities are typical of symmetry forbidden transitions of metal ions in the solid state, occasionally very short (<100 fs) bursts in intensity are observed, associated with anomalous Cu-H interactions. Absorption by the Ag+ complex is in comparison relatively stable in time, and can be interpreted in terms of the energy splitting of the metal 4d manifold in an average crystal field corresponding to a fourfold coordination in a distorted tetrahedral arrangement. Whereas the spectral profile of the Ag+ aqua ion is in good agreement with experiment, the overall position of the band is underestimated by 2 eV in the BLYP approximation to DFT. The discrepancy with experiment is reduced to 1.3 eV when a hybrid functional (PBE0) is used. The remaining inaccuracy of TDDFT in this situation is related to the delocalized character of the target state in d-->s transitions.

Parallel implementation of the ab initio CRYSTAL program: electronic structure calculations for periodic systems

CRYSTAL is an ab initio electronic structure program, based on the linear combination of atomic orbitals, for periodic systems. This paper concerns the ability of CRYSTAL to exploit massively parallel computer hardware. A brief review of the theory, numerical implementations and parallel solutions will be given and some of the functionalities and capabilities highlighted. Some features that are unique to CRYSTAL will be described and development plans outlined.

O2 activation in a dinuclear Fe(II)/EDTA complex: spin surface crossing as a route to highly reactive Fe(IV)oxo species.

We study the cleavage of O2 in gas phase [(EDTAH)Fe(O2)Fe(EDTAH)]2-, a proposed intermediate in the aqueous Fe(II)-to-Fe(III) autoxidation reaction in the presence of atmospheric dioxygen and EDTA ligand. The role of the exchange coupling between the locally high-spin Fe centers in the O-O dissociation is investigated. Using results from broken symmetry (BS) density functional theory (DFT) calculations, we show that the system can be modeled as two high-spin (HS) S = 5/2 Fe(III) d5 centers coupled through a bridging peroxo O2(2-) ligand, consistent with hypotheses advanced in the literature. We show that in this electronic configuration the O-O cleavage reaction is forbidden by (spin) symmetry. Dissociation of the O2(2-) group to the product ground state may only take place if the system is allowed to undergo a transition to a state of lower spin multiplicity (S = 4) as the O-O bond is stretched. We show that the exchange coupling between the two Fe ions in [(EDTAH)Fe(O2)Fe(EDTAH)]2- plays only a minor role in defining the chemistry of O2 activation in this system. The peroxo/oxo interconversion involves a state outside the Heisenberg spin ladder of the initial S = 5 state. In this S = 4 state, the dinuclear complex evolves to two oxo complexes, [EDTAH x Fe(IV)O]-, with an overall energy barrier of only approximately 86 kJ mol(-1). According to recent theoretical work, the latter species are exceptionally strong oxidants, making them ideal candidate catalysts for organic oxidations (including C-H bond hydroxylation). We highlight the (spin) symmetry forbidden nature of the reaction on the S = 5 surface and its symmetry allowed character in the electronic configuration with S = 4.

Generation of Ferryl Species through Dioxygen Activation in Iron/EDTA Systems: A Computational Study

The ferryl species (oxidoiron(IV), FeO(2+)) is a ubiquitous, highly oxidative intermediate in oxidation catalysis. We study theoretically its abiotic generation, in the form of the singularly active complex of FeO(2+) with the EDTAH(n)(-4+n), n = 0-4 ligands, from O(2) and Fe(2+)-EDTA complexes. The calculations are for the gas phase using generalized gradient corrected (BLYP and OPBE) Density Functional Theory (DFT). We examine the effects of ligand protonation on the coordination geometry and electronic structure of the chelated Fe(2+) ion, on its affinity to bind dioxygen, and on the generation of dinuclear Fe/EDTA/O(2) complexes, whose formation has been hypothesized on the basis of kinetic measurements of Fe(II)/Fe(III) autoxidation reactions in aqueous solution. We also consider the homolytic cleavage of the O-O bond within one such complex, [Fe x EDTAH x O(2) x EDTAH x Fe](2-), and we show that this reaction leads to a pair of Fe(IV)O/EDTA systems with an energetic barrier comparable to those computed for model systems of active sites of enzymes involved in dioxygen activation, such as methane monooxygenase. Our study supports the recently advanced hypothesis that high valent iron compounds capable of oxidizing organic substrates may be produced as a byproduct of the Fe(II)/Fe(III) autoxidation in aqueous Fe/EDTA/O(2) solutions at ambient conditions. We also identify the origin of the enhanced O(2) activation ability in the monoprotonated [Fe x EDTAH](-) complex, compared to other ligand protonation states, which has been observed in kinetic measurements.

A Frontier Orbital Study with ab Initio Molecular Dynamics of the Effects of Solvation on Chemical Reactivity: Solvent-Induced Orbital Control in FeO-Activated Hydroxylation Reactions

Solvation effects on chemical reactivity are often rationalized using electrostatic considerations: the reduced stabilization of the transition state results in higher reaction barriers and lower reactivity in solution. We demonstrate that the effect of solvation on the relative energies of the frontier orbitals is equally important and may even reverse the trend expected from purely electrostatic arguments. We consider the H abstraction reaction from methane by quintet [EDTAH(n)·FeO]((n-2)+), (n = 0-4) complexes in the gas phase and in aqueous solution, which we examine using ab initio thermodynamic integration. The variation of the charge of the complex with the protonation of the EDTA ligand reveals that the free energy barrier in gas phase increases with the negative charge, varying from 16 kJ mol(-1) for [EDTAH4·FeO](2+) to 57 kJ mol(-1) for [EDTAHn·FeO](2-). In aqueous solution, the barrier for the +2 complex (38 kJ mol(-1)) is higher than in gas phase, as predicted by purely electrostatic arguments. For the negative complexes, however, the barrier is lower than in gas phase (e.g., 45 kJ mol(-1) for the -2 complex). We explain this increase in reactivity in terms of a stabilization of the virtual 3σ* orbital of FeO(2+), which acts as the dominant electron acceptor in the H-atom transfer from CH4. This stabilization originates from the dielectric screening caused by the reorientation of the water dipoles in the first solvation shell of the charged solute, which stabilizes the acceptor orbital energy for the -2 complex sufficiently to outweigh the unfavorable electrostatic destabilization of the transition-state relative to the reactants in solution.

Statistical average of model orbital potentials for extended systems: Calculation of the optical absorption spectrum of liquid water

Time-dependent density functional theory (TD-DFT) calculations of the electronic response of molecular and bulk liquid water based on a very accurate orbital-dependent ground-state exchange-correlation potential, the statistical average of (model) orbital potentials (SAOP), and on the adiabatic local density approximation (ALDA) for the exchange-correlation kernel are described. The quality of the calculated excitation energies, both in the molecule and in the liquid, is assessed by comparison to hybrid TD-DFT calculations and experimental data. A combination of classical molecular dynamics simulations and TD-DFT calculations sampling several disordered configurations of a small liquid sample is then used to simulate the optical absorption spectrum in the region of 0–15 eV. The resulting room-temperature absorption profile is discussed in connection with previous TD-DFT calculations as well as with results from Green’s function theory and experiment.

Optical properties of alkali halide crystals from all-electron hybrid TD-DFT calculations.

We present a study of the electronic and optical properties of a series of alkali halide crystals AX, with A = Li, Na, K, Rb and X = F, Cl, Br based on a recent implementation of hybrid-exchange time-dependent density functional theory (TD-DFT) (TD-B3LYP) in the all-electron Gaussian basis set code CRYSTAL. We examine, in particular, the impact of basis set size and quality on the prediction of the optical gap and exciton binding energy. The formation of bound excitons by photoexcitation is observed in all the studied systems and this is shown to be correlated to specific features of the Hartree-Fock exchange component of the TD-DFT response kernel. All computed optical gaps and exciton binding energies are however markedly below estimated experimental and, where available, 2-particle Greens function (GW-Bethe-Salpeter equation, GW-BSE) values. We attribute this reduced exciton binding to the incorrect asymptotics of the B3LYP exchange correlation ground state functional and of the TD-B3LYP response kernel, which lead to a large underestimation of the Coulomb interaction between the excited electron and hole wavefunctions. Considering LiF as an example, we correlate the asymptotic behaviour of the TD-B3LYP kernel to the fraction of Fock exchange admixed in the ground state functional cHF and show that there exists one value of cHF (∼0.32) that reproduces at least semi-quantitatively the optical gap of this material.

Titanium as a Potential Addition for High-Capacity Hydrogen Storage Medium

We study the adsorption of hydrogen molecules on a titanium atom supported by a benzene molecule using generalized gradient corrected Density Functional Theory (DFT). This simple system is found to bear important analogies with titanium adsorption sites in (8, 0) titanium-coated single-walled carbon nanotubes (SWNTs) (T. Yildirim and S. Ciraci, 2005) In particular, we show that up to four H2 molecules can coordinate to the metal ion center, with adsorption patterns similar to those observed in Ti-SWNTs and no more than one molecule dissociating in the process. We analyze in detail the orbital interactions responsible for Ti-benzene binding and for the electron transfer responsible for the H2 dissociation. We find the latter to involve a transition from a triplet to a singlet ground state as the hydrogen molecule approaches the adsorption site, similar to what has been observed in Ti-SWNTs. The total Ti-H2-binding energy for the first dissociative addition is somewhat inferior (~0.4 eV) to the value estimated for adsorption on Ti-SWNTs. We analyze in detail the orbital interactions responsible for the H2 binding.

Simulating the pyrolysis of polyazides: a mechanistic case study of the [[P(N3)6]- anion.

Pyrolysis of the homoleptic azido complex [P(N(3))(6)](-) was simulated using density functional theory based molecular dynamics and analyzed further using electronic-structure calculations in atom-centered basis sets to calculate the geometries and electronic structures. Simulations at 600 and 1200 K predict a thermally induced and, on the simulation time scale, irreversible dissociation of an azido anion. The ligand loss is accompanied by a barrierless (free-energy) transition of the geometry of the complex coordination sphere from octahedral to trigonal bipyramidal. [P(N(3))(5)] is fluxional and engages in pseudorotation via a Berry mechanism.