Isabelle Baraille

Calculation of IR frequencies and intensities in electrical and mechanical anharmonicity approximations: Application to small water clusters

We present a method for automatic computation of infrared (IR) intensities using parallel variational multiple window configuration interaction wave functions (P_VMWCI(2) algorithm). Inclusion of both mechanical and electrical anharmonic effects permits fundamental vibrational frequencies, including combinations and overtones, to be assigned. We use these developments to interpret the near-IR (NIR) and mid-IR (MIR) spectra of individual water clusters (H2O)(n) (n=1-4). Cyclic and linear systems are studied to provide equivalent reference theoretical data to investigate the structure of water as a function of density using NIR and MIR experimental spectra. Various density functional theory methods for generating the potential energy surface have been compared to reference results obtained at the CCSD(T) level [X. Huang et al., J. Chem. Phys. 128, 034312 (2008)]. For cyclic clusters, the IR intensities and frequencies obtained using B1LYP/cc-pVTZ are found to be in very good agreement with the available experimental values and of the same orders of magnitude as the reference theoretical values. These data are completed by the vibrational study of linear systems.

Calculation of non-fundamental IR frequencies and intensities at the anharmonic level. I. The overtone, combination and difference bands of diazomethane, H2CN2

Abstract The experimental assignment of IR non-fundamental bands can be assisted by calculation of both frequencies and intensities, as shown in this work on diazomethane. The ab initio B3LYP method is used to obtain the anharmonic force fields up to the fourth order. The anharmonic vibrational wave functions have been calculated using a variation–perturbation algorithm. The dipole moment expansion needed in the evaluation of absolute intensities is limited to the first derivatives. The results, including those for overtone, combination and difference bands disagree with some experimental attributions and complement the available experimental data.

On the cluster composition of supercritical water combining molecular modeling and vibrational spectroscopic data

The present study is aimed at a detailed analysis of supercritical water structure based on the combination of experimental vibrational spectra as well as molecular modeling calculations of isolated water clusters. We propose an equilibrium cluster composition model where supercritical water is considered as an ideal mixture of small water clusters (n=1-3) at the chemical equilibrium and the vibrational spectra are expected to result from the superposition of the spectra of the individual clusters, Thus, it was possible to extract from the decomposition of the midinfrared spectra the evolution of the partition of clusters in supercritical water as a function of density. The cluster composition predicted by this model was found to be quantitatively consistent with the near infrared and Raman spectra of supercritical water analyzed using the same procedure. We emphasize that such methodology could be applied to determine the portion of cluster in water in a wider thermodynamic range as well as in more complex aqueous supercritical solutions.

A new route for local probing of inner interactions within a layered double hydroxide/benzene derivative hybrid material.

This paper presents the preparation and characterization of hybrid hydrotalcite-type layered double hydroxides (Zn1-xAlx(OH)2HBSx.nH2O, with x=0.33) where HBS is the 4-phenol sulfonate, with a detailed analysis of the grafting process of this organic entity onto the host lattice. As a set of the usual techniques (XRD, TG-DT/MS, FTIR and 27Al MAS NMR) was used to characterize the hybrid materials, this work focuses on a joint study by X-ray photoelectron spectroscopy and some quantum-calculation modeling in order to highlight the nature of the interactions between the organic and the mineral sub-systems. For the as-prepared hybrid material, the main results lead to a quasi-vertical orientation of the organic molecules within the mineral sheets via H-bond stabilization. By heating the hybrid material up to 200 degrees C, the structure shrinks with the condensation of the organics; the different theoretical modeling done gives an energy-stable situation when a direct attachment of the HBS sulfonate group sets up with the mineral layers, in agreement with the recorded XPS experimental data.

First principles calculations of solid–solid interfaces: an application to conversion materials for lithium-ion batteries

Using periodic density functional theory approaches, the thermodynamic stability of solid–solid interfaces generated during the conversion reaction of copper oxide which is a promising electrode material is investigated. Previous experimental results showed that conversion reactions generate a huge proportion of solid–solid interfaces among Cu2O–Cu, Li2O–Cu and Cu2O–Li2O. Interface grand potentials as a function of the voltage against Li|Li+ were computed in order to determine the chemical composition of the most stable interfaces. Then a structural model of the electrode material is proposed, based on the works of adhesion of the most stable systems identified in the first step.

An ab initio Hartree-Fock study of electronic and structural properties of MgH2

Abstract A periodic ab initio Hartree—Fock method has been used to evaluate a number of electronic and structural properties of MgH2. The calculated quantities are the equilibrium lattice parameters, the cohesive energy, the elastic constants, the energy-band structure, the density of states, the electronic charge distribution and the Compton profiles. For the two former properties correlation effects are taken into account and improve the agreement with the experimental data. The strongly but not fully ionic character of MgH2 is confirmed by the present study.

Comparison between Hartree-Fock and Kohn-Sham electronic and structural properties for hexagonal-close-packed magnesium

The properties of hcp magnesium are investigated using the density functional method with the linear combination of atomic orbitals as implemented in the CRYSTAL95 code. The lattice equilibrium parameters and the binding energy have been calculated at the Hartree-Fock level, at the hybrid Hartree-Fock density functional level, and at the Kohn-Sham density functional level using local and non-local exchange and correlation potentials. The electronic properties (band structures, topologies of the Fermi surface, and densities of states) and the elastic constants are computed for each type of functional, and compared to experimental data.

Adaptive vibrational configuration interaction (A-VCI): A posteriori error estimation to efficiently compute anharmonic IR spectra.

A new variational algorithm called adaptive vibrational configuration interaction (A-VCI) intended for the resolution of the vibrational Schrödinger equation was developed. The main advantage of this approach is to efficiently reduce the dimension of the active space generated into the configuration interaction (CI) process. Here, we assume that the Hamiltonian writes as a sum of products of operators. This adaptive algorithm was developed with the use of three correlated conditions, i.e., a suitable starting space, a criterion for convergence, and a procedure to expand the approximate space. The velocity of the algorithm was increased with the use of a posteriori error estimator (residue) to select the most relevant direction to increase the space. Two examples have been selected for benchmark. In the case of H2CO, we mainly study the performance of A-VCI algorithm: comparison with the variation-perturbation method, choice of the initial space, and residual contributions. For CH3CN, we compare the A-VCI results with a computed reference spectrum using the same potential energy surface and for an active space reduced by about 90%.

Experimental (XPS/STM) and theoretical (FLAPW) studies of model systems M1/4TiS2 (M=Fe, Co, Ni): influence of the inserted metal

Abstract The effects of metal insertion (iron, cobalt and nickel) into 1T-CdI2-type TiS2 layered crystals, expressed as MxTiS2, have been studied by X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM) and band structure calculations (FLAPW method). The stoichiometry x=1/4 was chosen because of specific crystallographic features of the compounds studied. We focused our interest on the role played by chalcogen atoms. S 2p core spectra are found to depend strongly on their chemical surroundings (Ti or Ti and M) and on the guest metal. We imaged the top sulfur plane (001) for Fe1/4TiS2, Co1/4TiS2 and Ni1/4TiS2 and note that the results also depend on the compound considered. Theoretical calculations have been carried out in order to improve our knowledge of the electronic structure of M1/4TiS2 compounds and attempts are made to rationalize the experimental data.

Ab initio calculation of the electronic spectrum and ionization potentials of hydrazine

Abstract Ab initio calculations for electronic states of hydrazine are reported. The VUV spectrum is analyzed. The first transitions are all Rydberg transitions and the first valence states are only expected above 10 eV. The first five vertical ionization potentials are reported. The structural properties of the N 2 H 4 + ion conformers are discussed.