Alexandre B. Rocha

Ab Initio Simulation of Changes in Geometry, Electronic Structure, and Gibbs Free Energy Caused by Dehydration of Hydrotalcites Containing Cl− and CO32− Counteranions

This ab initio study was performed to better understand the correlation between intercalated water molecules and layered double hydroxides (LDH), as well as the changes that occur by the dehydration process of Zn-Al hydrotalcite-like compounds containing Cl⁻ and CO₃²⁻ counterions. We have verified that the strong interaction among intercalated water molecules, cointercalated anions, and OH groups from hydroxyl layers is reflected in the thermal stability of these compounds. The Zn(2/3)Al(1/3)(OH)₂Cl(1/3)·2/3H₂O hydrotalcite loses all the intercalated water molecules around 125 °C, while the Zn(2/3)Al(1/3)(OH)₂(CO₃)(1/6)·4/6H₂O compound dehydrates at about 175 °C. These values are in good agreement with experimental data. The interlayer interactions were discussed on the basis of electron density difference analyses. Our calculation shows that the electron density in the interlayer region decreases during the dehydration process, inducing the migration of the Cl⁻ anion and the displacement of the hydroxyl layer from adjacent layers. Changes in these compound structures occur to recover part of the hydrogen bonds broken due to the removal of water molecules. It was observed that the chloride ion had initially a lower Löwdin charge (Cl(-0.43)), which has increased its absolute value (Cl(-0.58)) after the water molecules removal, while the charges on carbonate ions remain invariant, leading to the conclusion that the Cl⁻ anion can be more influenced by the amount of water molecules in the interlayer space than the CO₃²⁻ anion in hydrotalcite-like compounds.

Vibronic coupling for H2CO and CO2

Abstract We calculated the optical oscillator strength (OOS) for the sum of the vibronic excitations related to the valence electronic A 1 →A 2 state in H 2 CO and the inner-shell C 1s (2σ g →5σ g ) state of the CO 2 molecule. Electronic transition moments are calculated along the normal coordinates using configuration interaction wave functions. A quantum mechanical sum rule is used, which provides a final expression for the OOS that depends only on the ground-state vibrational function. The present results are in excellent agreement with the experiment. In particular, the calculation for C 1s (2σ g →5σ g ) vibronic coupling is reported for the first time.

Mixed-oxide formation during preparation of alumina-supported zirconia: an EXAFS and DFT study

Alumina-supported zirconia catalysts, containing from 2.1 to 20.1 wt% ZrO2 and prepared by multiple incipient wetness impregnation using benzene solutions of zirconium acetylacetonate, were characterised using the EXAFS technique. The results indicated that the supported phase consists of Zr4+ species that do not have Zr as second neighbours, hexacoordinated to oxide anions, suggesting that these ions occupy octahedral positions in the defective spinel structure of the γ-alumina support. In order to test this hypothesis, an idealised unit cell was constructed with the formula ZrAl4O8, with spinel structure. Atomic positions in this cell were optimised using theoretical calculations based on DFT. The smallest energies corresponded to structures where the Zr4+ ions occupy octahedral sites in the spinel structure. Simulations of the EXAFS spectra using this structure provided excellent agreement between the theoretical and experimental spectra.

Potential curves for inner-shell states of CO calculated at multiconfigurational self-consistent field level

A general strategy to calculate potential curves at multiconfigurational self-consistent field (MCSCF) level for inner-shell states is reported in this paper. Convergence is commonly very tough for inner-shell states, especially at this level of calculation, due to the problem of variational collapse of the inner-shell wave function to the ground or to a low-lying excited state. The present method allows to avoid this drawback by a sequence of constrained optimization in the orbital mixing step. The specific states studied are that resulting from transitions X (1)Σ(+) → (C 1s(-1) π(∗)) (1,3)Π of CO. Accurate values are achieved for transition energies and vibrational splittings. A comparison is made with other approach, i.e., inner-shell CI based on a MCSCF wave function optimized for ground or low-lying excited states. This last approach is shown to fail in describing the whole potential curve.

Generalized oscillator strengths for C 1s excitation of acetylene and ethylene

Abstract The generalized oscillator strength profiles for discrete C 1s excited states of C 2 H 2 and C 2 H 4 have been derived from angle-dependent inelastic electron scattering cross-sections measured with 1300 eV final electron energy. The measured GOS profiles for the strong C 1s→π* transition in each species are compared to theoretical calculations computed within the first Born approximation, using ab-initio generalized multi structural wave functions. These wave functions include relaxation, correlation and hole localization effects. Theory predicts large quadrupole contributions to the π* GOS of each species, analogous to those previously reported for computed GOS profiles for O 1s→π* excitation of CO 2 . We find good agreement between experiment and theory as to the shape of the π* GOS but, when the relative GOS extracted from the experimental data is normalized to the optical oscillator strength at K 2 =0, the magnitude is in better agreement with the GOS computed for only the dipole channel than for the sum of the dipole and quadrupole channels.

Intensity of the n→π∗ symmetry-forbidden electronic transition in acetone by direct vibronic coupling mechanism

Abstract Absolute absorption intensities were calculated for the symmetry dipole forbidden n → π ∗ transition in acetone. An analysis of the distribution per normal modes is performed and the results are compared with a recent calculation. Vibronic coupling mechanism is taken into account in a way that is different from the traditional Herzberg–Teller perturbation approach. In the present method the electronic transition moment is directly expanded in power series of the vibration normal coordinates. This approach was recently used for the equivalent n → π ∗ transition in formaldehyde presenting an excellent agreement with the experimental results.

Core level (S 2p) excitation and fragmentation of the dimethyl sulfide and dimethyldisulfide molecules

Electronic excitation and ionic dissociation of dimethylsulfide (DMS) and dimethyldisulfide (DMDS) have been studied around the S 2p edge using synchrotron radiation and time-of-flight mass spectrometry techniques. Mass spectra were obtained for both molecules, below, on and above the well defined resonances observed in the S 2p photoabsorption spectrum and centered at approximately 166 eV photon energy. Ab initio IS-CASSCF calculations were performed for a better understanding of the photoabsorption spectra. Similar calculations were also performed for the H(2)S molecule, in order to establish a bench mark. For both molecules, a higher fragmentation degree is observed with increasing photon energy. In the DMDS case, selective fragmentation was observed in the formation of the [CH(n)S](+) ions at the first S 2p resonance (corresponding to excitation to a σ*SS state) and in the formation of the [S(2)](+) and [S](+) ions at the third S 2p resonance (corresponding to excitation to a σ*CS state). Previously unreported doubly charged ([S](2+), [CH(3)](2+)) are observed for DMS and DMDS.

Forbidden transitions in benzene

We have computed the optical oscillator strengths for the symmetry-forbidden transitions 1 1 B2uXand 1 1 B1uXof benzene through vibronic coupling. Electronic transition dipole moments were calculated at the complete active space self consistent field level along the normal coordinates. Optical oscillator strengths for the sum of the total vibronic excitations are compared with available theoretical and experimental results. q 2002 Elsevier Science B.V. All rights reserved.

Theoretical investigations on valence vibronic transitions

This article reviews previously employed methods to study several valence electronic transitions, optically forbidden or not, enhancing intensity through vibronic coupling. Electronic transition dipole moments were calculated using several ab initio methods including electron correlation. In this method the square of the electronic transition dipole moments are directly calculated along the normal coordinates of vibration and then expanded with a polynomial function. Afterwards, analytical vibrational integration using harmonic wave functions, of the square of the transition moments function, allows us to obtain partial (i.e. for each vibrational mode) and total optical oscillator strengths (OOS), for the vibronic transition of interest. We illustrate the accuracy of the method through valence transitions of benzene (C6H6), formaldehyde (H2CO), acetone (C3H6O) and formic acid (HCOOH).

Inner-shell excitations of water molecule

Abstract Theoretical results for the generalized oscillator strength (GOS) and optical oscillator strength (OOS) have been obtained for the excitations from the ground state X1Σ+ to the inner-shell (O1s) 1A1(1a1→4a1), 1B2(1a1→2b2), 1B1(1a1→2b1) electronic states within the first Born approximation (FBA). The target electronic wave functions were determined using the configuration-interaction method, with a Hartree-Fock basis for the occupied molecular orbitals, and improved virtual orbitals for the virtual space. Relaxation and correlation effects were explicitly taken into account. We present, as well, results for the generalized oscillator strength of the first valence A1B1 excited state, in connection with the discussion of the inner-shell problem. The results were compared with the available experimental and theoretical data.