Pseudopotential SCF and Cl investigation of the electronic structure of PdH,PdC and PdCO

The electronic structure of PdH, PdC and PdCO molecules has been investigated by means of pseudopotential multireference double-excitations configuration interaction method (PP MRD Cl). Three different optimized AO basis sets have been adopted for determination of bond lengths, dissociation energies, dipole moments and population analysis. The basis set optimized for the Pd³D)rd⁹5s¹) excited state is more suitable for describing the electronic redistribution occurring on Pd during formation of the band than that derived from the ¹S(4d¹⁰) ground state. Differences in the bonds of PdH, PdC and PdCO can be explained with these calculations. Correlation effects are crucial for understanding the chemical bonding.     

Restricted Hartree–Fock approximation. II. Computational aspects of the direct minimization procedure

A RHF energy minimization procedure based on the treatment outlined in Part I of this series of articles is presented. Test calculations performed on several closed- and open-shell systems show that the present procedure is definitely superior to the conventional SCF methods. In particular, the convergence of this procedure is ensured, the rate of convergency is high, and the computational cost of each cycle is low.     

Quantum molecular interpretation of absorption spectra of small alkali metal clusters

The ab initio CI study of excited states of alkali metal clusters accounts for spectroscopical patterns obtained from the photodepletion spectra of the neutral or cationic species, predicts the excitation energies and transition intensities in the complete agreement with the measured quantities and permits an assignment of the cluster structures. The calculated optical spectra for various clusters with 4 and 8 valence electrons are compared: Na₄, Li₄, LiNa₃; Na₈ and Na ₉ ⁺ . A molecular interpretation of the rich spectra of tetramers as well as of the dominant intense transitions located at ～2.5–2.7 eV in the case of Na₈ and Na ₉ ⁺ with the weak fine structure shifted to the red is given.     

DFT characterization of the reaction pathways for terminal- to μ-hydride isomerisation in synthetic models of the [FeFe]-hydrogenase active site

The mechanism of terminal- to μ-hydride isomerisation in models of synthetic complexes resembling the [FeFe]-hydrogenase active site has been elucidated by DFT calculations, revealing that Ray-Dutt reaction pathways are generally favoured, and providing some clues for the rational design of novel synthetic catalysts to produce H(2).     

Jahn-Teller transition in Al doped LiMn2O4 spinel

Al-doped lithium manganese spinels, with starting composition Li_1.02Al_xMn_1.98−xO₄ (0.00<x≤0.06), are investigated to determine the influence of the Al³⁺ doping on the Jahn-Teller (J-T) cooperative transition temperature T_J-T. X-ray powder diffraction (XRPD), nuclear magnetic resonance, electron paramagnetic resonance, conductivity and magnetic susceptibility data are put into relation with the tetrahedral and octahedral occupancy fraction of the spinel sites and with the homogeneous distribution of the Al³⁺ ions in the spinel phase. It is observed that Al³⁺ may distribute between the two cationic sublattices. The J-T distortion, associated with a drop of conductivity near room temperature in the undoped sample, is shifted towards lower temperature by very low substitution. However, for x>0.04 T_J-T it increases with increasing x, as clearly evidenced in low temperature XRPD observations. A charge distribution model in the cationic sublattice, for Al substitution, is proposed to explain this peculiar behavior.     

Probenecid and benzamide: cocrystal prepared by a green method and its physico-chemical and pharmaceutical characterization

Journal of Thermal Analysis and Calorimetry - The eco-friendly method of kneading was here used to synthesize a cocrystal of probenecid, an uricosuric drug used in treating gout and hyperuricemia,...     

Structure-property interplay of proton conducting membranes based on PBI5N, SiO2-Im and H3PO4 for high temperature fuel cells

Polybenzimidazoles (PBIs) are among the polymers of choice to prepare membranes for high temperature polymer fuel cells. Poly-2,2'(2,6-pyridine)-5,5'-bibenzimidazole (PBI5N), doped with H(3)PO(4), and acid-doped PBI5N containing 10 wt% of imidazole-functionalized silica membranes were studied with thermogravimetric analysis, differential scanning calorimetry, dynamic-mechanical analysis, infrared spectroscopy, and broadband electric spectroscopy to examine the structure-property relationships. Key results show that: (1) doped PBI5N membranes show thermal decomposition starting at 120 °C, while pristine PBI5N is stable up to 300 °C; (2) the presence of filler increases the acid uptake and decreases the crystallinity of PBI5N; (3) the addition of phosphoric acid reduces the mechanical properties of the membrane, while the addition of filler has the opposite effect; (4) acid-doped membranes have conductivity values on the order of 10(-2)-10(-3) S cm(-1); and (5) membranes exhibit a Vogel-Tamman-Fulcher (VTF) type proton conduction mechanism, where proton hopping is coupled with the segmental motion of the polymer chain. Infrared spectroscopy combined with DFT quantum mechanical calculations was used to assign the experimental spectrum of PBI5N.     

Direct minimization of the energy functional in the LCAO-MO density matrix formalism

The previously proposed method of direct energy minimization by a gradient approach is here applied to the separated electronic pair functions (strongly orthogonal geminals). The minimization of the energy is achieved by an alternate iterative procedure in which the geminal occupation coefficients are optimized making each geminal self-consistent in the field of the others and the orbital forms are directly optimized using a sequence of orthogonal transformations of an arbitrary orthonormal basis set. The method is tested on H₂, He₂, LiH, NH₃ and H₂O molecules.

Possible generalizations of the formalism to other geminal models are briefly presented.