Floating spherical gaussian orbital open-shell calculations on the four-electron H4 system

Floating spherical Gaussian orbital (FSGO ) open-shell calculations have been made to determine the potential energy surface of planar square and rectangular arrangements of the four-electron system H₄. This surface is discussed in relation to the bimolecular isotope exchange reaction H2+D2-→ 2HD. The changes in energy and geometry accompanying the coplanar approach of two hydrogen molecules interacting chemically have also been investigated. Calculations on the electronic energies of planar T-shaped and kite arrangements of H4 of various sizes show that it is unlikely that these configurations can serve as transition states for the exchange reaction. However, the energy curve for linear configurations of H4 (H? H? H … H), calculated as a function of the H₃ … H distance with the symmetric linear H₃ (H-H-H) unit fixed at the internuclear distance of 1.9080 a.u., is found to have a deep minimum (?1.9176 a.u.) at an r(H₃ … H) distance of 1.5846 a.u. The overall results suggest that the following mechanism for the exchange reaction, H₂+H₂→H₂+H+H→H₃+H→H+H₂+ H→H₂+H₂ could be advantageous as it requires a barrier height of 0.1604 a.u. which is significantly lower than that calculated from the saddle point energy (0.1950 a.u.). However, the problem of reconciling this with the experimental activation energy of 0.0685 a.u. still remains.     

Velocity selection and mass spectrometric detection of an H2S molecular beam and a collisional study of its interactions with rare gases

Total integral cross section experiments, for collisions of rotationally hot H₂S projectile molecules by He and Xe targets, have been performed in the thermal energy range by using a molecular beam apparatus, which operates under high resolution conditions and exploits a quadrupole mass spectrometer as a detector. Information on the radial dependence of the intermolecular interaction, averaged over all relative orientations of colliding partners, is obtained. This study completes the investigation of H₂S–rare gas family [see D. Cappelletti, A.F.A. Vilela, P.R.P. Barreto, R. Gargano, F. Pirani, V. Aquilanti, J. Chem. Phys. 125 (2006) 133111, for Ne, Ar and Kr] and permits us to discover similarities and differences with the analogous H₂O–Rg series of systems. In particular, the analysis of the scattering data suggests that while the complexes of H₂S–rare gases from He to Kr are essentially bound by nearly isotropic noncovalent interactions of van der Waals type, the H₂S–Xe system exhibits a weak, but measurable, additional component which emerges at intermediate intermolecular distance (in proximity of the potential well), and provides a bond stabilization. This observation is of possible interest for the investigation of the phenomenology of the hydrogen bond formation.     

Vibration-to-rotation and vibration-to-vibration energy transfer between diatomic molecules

A classical model for VR and VV energy transfer between two dissimilar diatomic molecules is proposed. For the exponential repulsive potential, in the limit as the rotation of the molecules goes to zero, the present model reduces to the previous results for VT and VV energy transfer. For the hydrogen halides, whose relaxation is generally accepted as being governed by VR energy transfer, the Pr's predicted by the present theory agree reasonably well with the experimental data. In particular, the positive and negative temperature dependence of the VV Pr's for N₂HI and N₂DI are predicted.     

What kind of potential does a H-bond proton feel? An electrostatic study

A study was made by electrostatic molecular potentials from CNDO/2 wavefunctions of the potential curves in the hydrogen bonding region of (H₂O)₂ and (OH)^p2?₂ simulating the proton transfer in (H₅O₂)⁺ and (H₃O₂)^? respectively. A picture of the bridge potential could also be obtained from simple superposition of the potentials of the partner systems. The double potential well was found to vanish at on O…O distance of 2.4 A A     

Quantum-chemical studies of dimethylformamide 1 : 1 complexes with phosphoric acid

The structures of two phosphoric acid conformations, dimethylformamide (DMFA), four protonated DMFA forms, and nine DMFA-H₃PO₄ complexes in which the proton acceptor is a oxygen or nitrogen atom of the DMFA molecule are optimized by DFT/B3LYP using the 6-31++G(d, p) basis set. The structural changes in DMFA that occur upon its protonation are discussed. The stabilization energy and transferred charge values upon the formation of a hydrogen bond are calculated for all of the studied complexes by means of NBO analysis. The potential energy surface is scanned to study the possibility of proton transfer.     

Mechanism of electron excitation energy degradation in solutions

The inductive-resonant mechanism of electronic energy degradation is proposed and proved for rare earth ions, transition metal ions and simple molecules (NO^?₂) in solutions. The interaction of two oscillators is considered, that is the vibronic interaction corresponding to the radiation spectrum of an excited ion or molecular and the vibrational spectrum corresponding to the excitation of high frequency vibrations of the solvent. The calculation of the energy degradation rare constant (k_degr.) by Förster's formula is shown to give k_degr. values of the same order as the experiment. Such a treatment can quantitatively explain all experimental regularities of the degradation process, for instance, the dependence of k_degr. on the distance from the electronic excitation centre to the nearest high frequency vibration gravity centre. It is shown that the suggested mechanism corretly explains the deuteration effect, the dependence of k_degr. on ΔE (energy gap) and the variation of this dependence for differenct classes of compounds. The possibility of proving the validity of the suggested model for the case of complex organic molecules is discussed.     

A Monte Carlo error estimator for the expansion of rigid-rotor potential energy surfaces

The computation of rotational energy transfer in nonreactive molecular collisions requires expanding the orientation dependence of the interaction potential over an appropriate complete set of orthonormal functions. We show that the use of random grids for the sampling of the angular geometries combined with the Monte Carlo theorem allows to estimate the mean accuracy on each expansion term determined by a least squares fit. The interest of our approach is illustrated by an application to the H₂O–H₂ system, of great astrophysical interest.     

Perturbing the O–H…O Hydrogen Bond in 1-oxo-3-hydroxy-2-propene

Ab initio MP2/aug’-cc-pVTZ calculations have been carried out to identify and characterize equilibrium structures and transition structures on the 1-oxo-3-hydroxy-2-propene: Lewis acid potential energy surfaces, with the acids LiH, LiF, BeH₂, and BeF₂. Two equilibrium structures, one with the acid interacting with the C=O group and the other with the interaction occurring at the O–H group, exist on all surfaces. These structures are separated by transition structures that present the barriers to the interconversion of the two equilibrium structures. The structures with the acid interacting at the C=O group have the greater binding energies. Since the barriers to convert the structures with interaction occurring at the O–H group are small, only the isomers with interaction occurring at the C=O group could be experimentally observed, even at low temperatures. Charge-transfer energies were computed for equilibrium structures, and EOM-CCSD spin–spin coupling constants ^2hJ(O–O), ^1hJ(H–O), and ¹J(O–H) were computed for equilibrium and transition structures. These coupling constants exhibit a second-order dependence on the corresponding distances, with very high correlation coefficients.     

Ba2In2O4(OH)2: Proton sites, disorder and vibrational properties

The structure of Ba₂In₂O₄(OH)₂ is analysed by the explicit full optimization of a large number of possible proton arrangements using periodic density functional theory. It is shown that the experimental assignments in which protons appear to be located at high symmetry positions with unphysical bond lengths do not correspond to minima on the potential energy hypersurface. The apparent sites are averages of a number of possible proton locations involving a set of possible local structural environments in which the internuclear separations are more realistic. Such problems with structural refinements are common where profile refinement programs place the atoms at the average position due to dynamic and/or static disorder. Thus while the calculations support a previous neutron diffraction analysis of the structure in that the average structure contains two different proton sites, they also reveal substantial information about the local environments of the protons. In all optimizations, the protons moved from the average positions suggested in the neutron diffraction study with calculated O–H and OHO distances consistent with those observed in other oxides. The energies of different proton distributions vary significantly so the protons are not randomly distributed. We also present an analysis of the vibrational properties of the O–H bonds. Since the strength of the hydrogen bonds is closely related to the local structural environments of the protons, a range of vibrational frequencies is obtained providing a prediction of the vibrational spectra. In O–HO linkages, O–H stretching modes soften with increasing HO hydrogen bond strength, while the in-plane and out-of-plane bending or libration modes stiffen. Together, our results show how modern theoretical methods can provide a clearer understanding of the structure and dynamics of a complex inorganic material.     

Classical trajectory study of orientation and alignment effects for charge exchange processes in H+- Na*(3p m) collisions

The orientation and alignment effects for charge exchange in H⁺ + Na*(3p) collisions are studied using the classical trajectory Monte-Carlo method in the energy range from 1 to 8 keV. For Na*(3p _-1) → H*(2s, 2p _±1) transitions a large orientation effect is predicted by the probability functions, in very good agreement with semiclassical calculations. Angular differential cross sections are also calculated and interpreted using the impact parameter dependence of the proton deflection angle. They predict left-right asymmetry in agreement with semiclassical calculations or experimental results, but slightly smaller. Another geometry, not experimentally realized, is considered, where the proton velocity is parallel to the quantization axis of the p _±1 oriented states. Charge exchange from different aligned states with respect to the direction of the projectile velocity is also investigated, but the alignment effects are not as well described as the orientation effects. Total cross sections from oriented or aligned states with cylindrical symmetry around the projectile velocity direction are calculated and allow the hypothesis of velocity matching to be tested.     

Thermodynamics of solid-state connected ion-sensitive membrane electrodes: The silver-Silver chloride system: Part II. Standard potentials (ε0′+εj) between 5 and 90°C of the 2nd kind silver-Silver chloride reference electrode with 3.5 M and sat'd KCl measured by means of membrane electrodes

Standard potentials (ε_M^0′+ε_j)_T of solid-state connected silver-silver chloride membrane electrodes (“chloride ion-sensitive electrodes”) in saturated and in 3.5 M (25°C) aqueous KCl solution (Ag|AgCl|KCl(sat'd or 3.5 M)||…) were measured between 5 and 90°C. The cells with transference comprised a Pt,H₂ electrode half-cell with NBS (DIN) phosphate buffer solution D. Identical thermodynamic behaviour of the membrane electrodes and respective 2nd kind silver-silver chloride electrodes and the small differences ΔE⁰ of their standard potentials E_M⁰ and E⁰, both reported in Part I of this work [1], allowed the application of the membrane electrode data to determine also standard potentials (ε^0′+ε_j)_T of the respective silver-silver chloride reference electrode of the second kind with “fixed potential” (Ag|AgCl(sat'd), KCl(sat'd or 3.5 M)6…). The results are discussed and compared with literature data.     

Core level photoemission spectroscopy and chemical bonding in Sr2Ta2O7

Electronic parameters of constituent element core levels of strontium pyrotantalate (Sr₂Ta₂O₇) were measured with X-ray photoelectron spectroscopy (XPS). The Sr₂Ta₂O₇ powder sample was synthesized using standard solid state method. The valence electron transfer on the formation of the Sr–O and Ta–O bonds was characterized by the binding energy differences between the O 1s and cation core levels, Δ(O–Sr) = BE(O 1s) − BE(Sr 3d_5/2) and Δ(O–Ta) = BE(O 1s) − BE(Ta 4f_7/2). The chemical bonding effects were considered on the basis of our XPS results for Sr₂Ta₂O₇ and earlier published structural and XPS data for other Sr- and Ta-containing oxide compounds. The new data point for Sr₂Ta₂O₇ is consistent with the previously derived relationship for a set of Sr-bearing oxides. The binding energy difference Δ(O–Sr) was found to decrease with increasing bond distance L(Sr–O).     

A pseudopotential study of the hydrogen bond in H2O·H2S,H2S·H2S and H2O·H2Se systems

Intermolecular potential energy curves for the hydrogen bonded systems H₂O·H₂S, H₂O·H₂Se and H₂S·H₂S were calculated with nonempirical pseudopotentials using optimized-in-molecules basis sets augmented by polarization functions. The H₂O·H₂O interaction energy curve has been also considered as a test case. The present results for H₂O·H₂S and H₂S·H₂S indicate much weaker intermolecular interactions than those found in previous ab initio calculations. The H₂O·H₂Se interaction was found to be quite similar to H₂O·H₂S.This work was partly supported by the Polish Academy of Sciences within the Project PAN-09, 7.1.1.1On leave from Quantum Chemistry Laboratory, Dept. of Chemistry, University of Warsaw, Pasteura 1, 02-093. Warsaw, Poland     

Magnetostructural correlation for the Gd complexes with bridging oxygen

The dependence between the energy of magnetic exchange interaction J _Gd-Gd′ and the distance between gadolinium atoms in the binuclear structure D _Gd...Gd′ was constructed on the basis of our and literature experimental data for 34 gadolinium complexes with the [Gd₂O₂] core. The character of the established dependence is similar to the Bethe-Slater curve. The obtained correlation indicates the possibility to predict the character of interaction in the Gd complexes built of binuclear fragments with the [Gd₂O₂] core depending on the single parameter, namely, distance D _Gd...Gd′. The validity of this correlation was tested using as examples four binuclear gadolinium complexes obtained by the authors.     

X-ray photoelectron spectra and electronic structure of rare-earth orthovanadates

Photoelectron spectra of 4d and valence states in RVO₄ (R = Y, Nd, Eu, Gd, Tb, Dy, Yb) have been investigated. The experimental spectra are interpreted using the results of the Xα discrete variational method calculations for orthovanadates. Transformations of electronic structure and covalency in the RVO₄ series are discussed. It is shown that lanthanide 4f orbitals significantly mix with the O 2pAO's and hybridize with the rare-earths 5pAO's. The 5p levels spin-orbital splitting in orthovanadates has been evaluated.     

Uncovering an oxide ion substitution for the OH− + CH3F reaction

Theoretical investigations on chemical reactions allow us to understand the dynamics of the possible pathways and identify new unexpected routes. Here, we develop a global analytical potential energy surface (PES) for the OH⁻ + CH₃F reaction in order to perform high-level dynamics simulations. Besides bimolecular nucleophilic substitution (S_N2) and proton abstraction, our quasi-classical trajectory computations reveal a novel oxide ion substitution leading to the HF + CH₃O⁻ products. This exothermic reaction pathway occurs via the CH₃OH⋯F⁻ deep potential well of the S_N2 product channel as a result of a proton abstraction from the hydroxyl group by the fluoride ion. The present detailed dynamics study of the OH⁻ + CH₃F reaction focusing on the surprising oxide ion substitution demonstrates how incomplete our knowledge is of fundamental chemical reactions.

Reaction dynamics simulations on a high-level ab initio analytical potential energy surface reveal a novel oxide ion substitution channel for the OH⁻ + CH₃F reaction.     

Comparison of Proton Acceptor and Proton Donor Properties of H2O and H2O2 in Organic Crystals of Drug-like Compounds: Peroxosolvates vs. Crystallohydrates

Two new peroxosolvates of drug-like compounds were synthesized and studied by a combination of X-ray crystallographic, Raman spectroscopic methods, and periodic DFT computations. The enthalpies of H-bonds formed by hydrogen peroxide (H₂O₂) as a donor and an acceptor of protons were compared with the enthalpies of analogous H-bonds formed by water (H₂O) in isomorphic (isostructural) hydrates. The enthalpies of H-bonds formed by H₂O₂ as a proton donor turned out to be higher than the values of the corresponding H-bonds formed by H₂O. In the case of H₂O₂ as a proton acceptor in H-bonds, the ratio appeared reversed. The neutral O∙∙∙H-O/O∙∙∙H-N bonds formed by the lone electron pair of the oxygen atom of water were the strongest H-bonds in the considered crystals. In the paper, it was found out that the low-frequency Raman spectra of isomorphous crystalline hydrate and peroxosolvate of N-(5-Nitro-2-furfurylidene)-1-aminohydantoin are similar. As for the isostructural hydrate and peroxosolvate of the salt of protonated 2-amino-nicotinic acid and maleic acid monoanion, the Raman spectra are different.     

Complex hydrogen bonded cations-V further asymmetric ions

Solutions have been prepared containing the complex cations (¹BH²B)⁺, where the differing bases may be pyridine, 4-Me pyridine, NMe imidazole, thiazole, isooxazole, trimethylamine or NMe piperidine. The infrared spectra usually show a double maximum in the v_s band, the intensity of the lower component decreasing as the difference in the pKa's of the bases increases. In the D-bridged analogues only a single v_sband occurs. Other features of the spectra indicate that the bridging proton potential is asymmetric. The lower frequency component of the v_s band of the H-bonded system is attributed to Fermi resonance, not proton tunnelling. The proton potential in these and the related symmetric complex cations is reviewed.     

The role of surface oxides in formic acid oxidation on Au

Compared to Pt or Pd electrodes, Au is a poor catalyst for the direct anodic oxidation of HCOOH, but the formation of Au surface oxides in acidic solutions is accompanied by a fast oxidation of HCOOH. This fast reaction is not simply a secondary reaction of Au surface oxides since those oxides are kinetically stable in HCOOH solutions. They do oxidize HCOOH only via a slow and purely electrochemical process which occurs on free Au sites and is “driven” by oxide reduction. The fast HCOOH oxidation is due to a highly reactive intermediate which is able either to form stable Au oxides Au_nO_m or to react with HCOOH. Our results are consistent with the model that by the charge transfer step a reactive non-equilibrium {Au…O> species is formed which converts to stable equilibrium oxides Au_nO_m after migration and rearrangement steps. Pre-equilibrium <Au…O> oxidizes HCOOH and this oxidation is of lower order with respect to <Au…O> compared with the formation of Au_nO_m.     

Well-defined silica supported aluminum hydride: another step towards the utopian single site dream?

Reaction of triisobutylaluminum with SBA15₇₀₀ at room temperature occurs by two parallel pathways involving either silanol or siloxane bridges. It leads to the formation of a well-defined bipodal [(SiO)₂Al–CH₂CH(CH₃)₂] 1a, silicon isobutyl [Si–CH₂CH(CH₃)₂] 1b and a silicon hydride [Si–H] 1c. Their structural identity was characterized by FT-IR and advanced solid-state NMR spectroscopies (¹H, ¹³C, ²⁹Si, ²⁷Al and 2D multiple quantum), elemental and gas phase analysis, and DFT calculations. The reaction involves the formation of a highly reactive monopodal intermediate: [SiO–Al–[CH₂CH(CH₃)₂]₂], with evolution of isobutane. This intermediate undergoes two parallel routes: transfer of either one isobutyl fragment or of one hydride to an adjacent silicon atom. Both processes occur by opening of a strained siloxane bridge, Si–O–Si but with two different mechanisms, showing that the reality of “single site” catalyst may be an utopia: DFT calculations indicate that isobutyl transfer occurs via a simple metathesis between the Al-isobutyl and O–Si bonds, while hydride transfer occurs via a two steps mechanism, the first one is a β-H elimination to Al with elimination of isobutene, whereas the second is a metathesis step between the formed Al–H bond and a O–Si bond. Thermal treatment of 1a (at 250 °C) under high vacuum (10^–5 mbar) generates Al–H through a β-H elimination of isobutyl fragment. These supported well-defined Al–H which are highly stable with time, are tetra, penta and octa coordinated as demonstrated by IR and ²⁷Al–¹H J-HMQC NMR spectroscopy. All these observations indicate that surfaces atoms around the site of grafting play a considerable role in the reactivity of a single site system.