Adsorbate binding energies for ammonia on cobalt and nickel clusters

Equilibrium reactions of ammonia with cobalt and nickel clusters are analyzed to determine cluster-adsorbate binding energies. The temperature dependence of reaction equilibrium constantsK _eq are measured and ?ΔH ⁰ values obtained from plots of lnK _eq vs 1/T. We find that binding energies generally decrease with increasing ammonia coverage, and that for a given number of NH₃ molecules binding energies increase with increasing cluster size. The pattern of binding energies is found to be consistent with proposed geometrical structures for Co₁₉ and for clusters in the 55-atom size range. Cluster-ammonia binding energies are generally somewhat higher than for bulk metal surfaces, an expected result considering the character of the cluster surface and the nature of the NH₃-metal interaction.     

Structure and conformation in molecular peroxides

Molecular structures and energies have been calculated in the MINDO approximation for fifteen neutral and anionic peroxides: fully optimized torsional potential functions have been calculated for twelve of these, and torsional potential functions, subject to constrained optimizations, for a further two peroxides. Bond dissociation energies D(R₁O—OR₂) were also calculated. Equilibrium structures and energies were calculated for the polyoxo species H₂O_n, HO_nF, F₂O_n, HO_n and FO_n (n ? 4), and a complete set of bond dissociation energies derived for H₂O_n, HO_nF, and F₂O_n.     

Intramolecular H-transfer in CH2OO and cis-HO3

Intramolecular H-transfer reactions in cis-HO₃ and CH₂OO were studied at the MP2 and B3LYP levels of theory. Activation energies (E _a) and Gibbs free energies of activation (?G ^#) of the H-transfer reactions were calculated. The activation energies of the H-transfer in cis-HO₃ and CH₂OO were about 110 and 130 kJ/mol, respectively. Catalytic effects of some protic molecules including HCOOH, CH₃OH, NH₃, CH₃NH₂, HOCl, H₂O₂, and H₂O, on the activation energies and transition state structures were studied. The more stable transition state structures were obtained in the presence of the protic molecules. Our calculations showed that the protic molecules decrease the activation energies of the H-transfer reactions about 50 kJ/mol.     

Non-expanded dispersion energies and damping functions for Ar2 and Li2

The non-expanded second-order dispersion energies and damping functions associated with the long-range dispersion energies varying as R⁻⁶, R⁻⁸and R⁻¹⁰ have been calculated for Ar₂ and Li₂ with the time-dependent Hartree-Fock method, using extended Gaussian basis sets. These results are used to discuss the difficulties associated with ab initio computations of these quantities.     

The X-ray photoelectron spectra of inorganic molecules: VI. Compounds containing rhenium-oxygen and rhenium-halogen bonds: rhenium 4f,chlorine 2p and oxygen 1s binding energies and their co

Rhenium 4f, chlorine 2p and oxygen 1s binding energies have been recorded for a series of coordination complexes of rhenium(V) and rhenium(VII) containing rhenium-oxygen bonds. Related studies on the rhenium(III) acetates Re₂(OAc)₄X₂(X = Cl, Br) and on several adducts of the rhenium chlorides (Re 4f and Cl 2p binding energies only) are also reported. Chemical shift data are related where possible to the molecular structures of the complexes. Correlations between rhenium 4f binding energies and oxidation state are of diagnostic value only in distinguishing the high oxidation state species (rhenium(V) and (VII)). Differences in rhenium 4f_{$\text{7}\text{2}$} and oxygen 1s binding energies [δ(O1s, Re 4f_{$\text{7}\text{2}$})] may be useful in probing the nature of the rhenium-oxygen bonds.     

Photoelectron emission spectroscopy of inorganic cations in aqueous solution

Threshold energies (6.1 <E_t ? 8.6 eV) are determined for photoelectron emission by 16 inorganic cations in aqueous solution E_t values are correlated with gas-phase ionization potentials, solvation and reorganization free energies, standard reduction potentials and ligand field stabilization energies (five transition metals). Dielectric saturation is shown to drastically lower threshold energies.     

Non empirical LCAO SCF MO investigations of electronic reorganizations accompanying core ionizations

Calculations have been carried out on an extensive series of molecules for both the neutral species and core ionized states. Substituent effects on C_1s , N_1s , O_1s , and F_1s levels have been investigated and where available comparison has been drawn with experiment. Comparison with Koopmans' theorem has allowed a relatively detailed study of change in relaxation energies as a function of substituent effect on a given core level. Whilst for C_1s levels the computed shifts in core binding energies are approximately linearly related to differences in relaxation energies for the N_1s , O_1s , and F_1s levels, the relative electronegativity of the substituent can invert this correlation. The empirical correction of Koopmans' theorem for differences in relaxation energies at different sites has been investigated for large molecules. The results compare well with direct hole state calculations.     

Diatomics-in-molecules study of the geometric and electronic structure of Xen+ clusters

Dissociation energies as well as electronic and geometric structure of singly charged xenon cluster cations, Xe_n⁺ (n=3–35), are calculated using the extended diatomics-in-molecules method (including the spin–orbit coupling and the most important ionic and neutral three-body interactions) and the state-of-the-art ab initio diatomic curves for Xe₂⁺ due to Paidarová and Gadéa [Chem. Phys. 274 (2001) 1]. Cluster growth of Xe_n⁺ and size dependence of the positive charge delocalization are discussed. The calculated dissociation energies are used to estimate the evaporation energies for Xe_n⁺→Xe_n−1⁺+Xe and to study the stability of the Xe_n⁺ clusters. The results obtained are compared with available experimental and theoretical data.     

Broken orbital symmetry and the description of valence hole states in the tetrahedral [CrO4]2− anion

The localization of ligand-based valence holes in the tetrahedral complex ion [CrO₄]^2? in a crystalline environment is studied by SCF calculations on the hole states, with progressively less restrictions on the spatial symmetry of the molecular orbitals. The final wavefunctions are obtained by constructing, from the symmetry broken SCF solutions, wavefunctions that exhibit again the proper transformation properties under the operations of T _d . The crystal environment of the [CrO₄]^2? anion is represented by a point charge model. In contrast with the situation for core hole states, the projection afterwards into T _d symmetry is important. The final ionization energies, which are obtained from projected C _3v adapted SCF solutions, are reduced considerably (?3 eV) with respect to the T _d ΔSCF results, but the ordering of the states has not changed essentially. The calculated ionization energies compare favourably with results of XPS experiments on Na₂CrO₄. The evaluation of the energies of projected symmetry broken SCF solutions requires the calculation of hamiltonian matrix elements between determinantal wavefunctions built from mutually non-orthogonal orbital sets. An efficient method for the calculation of such matrix elements is presented.     

A new approach to the relationship between bond energy and electronegativity

The familiar equation whereby Pauling related heteronuclear bond energies D_A–B to the electronegativity difference Δχ (=∣χ_A − χ_B∣) and the homonuclear bond energies D_A–A and D_B–B has been the subject of critical scrutiny for at least half a century. A modification to this equation that combines the concepts of electronegativity and hardness/softness can be rewritten in terms of two quantities x and y, both having absolute significance. Both homo- and heteronuclear bond energies can be rationalised from these new equations. The quantities x are linearly related to Pauling electronegativities, while y appears to be a measure of an atom’s intrinsic bonding potential, related to size and availability of valence orbitals.     

X-ray photoelectron spectra of inorganic molecules: XXX. Organometallic derivatives of tungsten(O) and tungsten(II)

The W 4f binding energies of various carbonyl-containing complexes of tungsten are reported. Comparisons between W^II complexes containing the [(η³-allyl)W(CO)₂] moiety and neutral derivatives of the type W(CO)_6-xL_x are made. The absence of a clear-cut correlation between the W 4f binding energies and the value of x in W(CO)_6-xL_x is discussed and these results compared with those obtained for the analogous molybdenum complexes.     

Next-nearest neighbour contributions to the XPS binding energies and XANES absorption energies of P and As in transition-metal arsenide phosphides MAs1−yPy having the MnP-type structure

X-ray photoelectron spectroscopic (XPS) and X-ray absorption near-edge spectroscopic (XANES) measurements have been made for several series of metal arsenide phosphides MAs_1−yP_y (M=Co, Fe, Cr) that adopt the MnP-type structure. The P and As XPS binding energies (BEs) and XANES absorption energies of the metal arsenide phosphides do not follow the trend observed for the simple binary phosphides or arsenides, a deviation that arises from the combination of nearest and next-nearest neighbour contributions acting on the P or As photoemission or absorption site. The P 2p_3/2 BEs and K-edge absorption energies are lower in MAs_1−yP_y than in MP because the P atoms are more negatively charged and because the P photoemission or absorption site is screened to a greater extent by less positively charged nearest-neighbour M atoms and more negatively charged next-nearest neighbour P atoms. The As L₃- and K-edge absorption energies are higher in MAs_1−yP_y than in MAs primarily because the As atoms are less negatively charged. The M charge has been evaluated from analysis of the M 2p XPS spectra and the M L- and K-edge XANES spectra.     

Second order charge overlap effects and damping functions for isotropic atomic and molecular interactions

Second order charge overlap effects and the related dispersion energy damping functions have been evaluated for the H-(1s)-H(1s) interaction through partial wave indices l_a+l_b such that (l_a+l_b) ? 13. These results are a substantial improvement on, or addition to, previous literature results and are important since they can be used, following several available scaling approaches, to construct damping functions for other interactions. They also indicate that the “spherical” energies are not an insignificant part of the total second order Coulomb energy until R becomes reasonably large. The various approaches for evaluating the non-expanded second order Coulomb energy are compared and the difficulties associated with the accurate determination of these energies, and the related damping functions, for general interactions are discussed in some detail.     

Rare earth niobate systems and the phosphor GdNbO4:Bi

An investigation to determine the compounds present in the Ln₂O₃Nb₂O₅ systems (Ln = La, Gd, and Y) and attempts to activate such compounds with Ti, In, Sb, Bi, Eu, and Tb revealed that only the orthoniobate (LnNbO₄) compound was a suitable host lattice and LnNbO₄:Bi under 2537 Å excitation produced the best phosphors. GdNbO₄:Bi is a brighter phosphor than the La and Y analogs, emitting at slightly higher energies. The position of the GdNbO₄:Bi peak emission at 4500 Å is independent of the activator concentration whereas peak emissions for LaNbO₄:Bi and YNbO₄:Bi move to lower energies with increasing Bi concentration.     

Correlations with and prediction of activation energies of gas permeation and diffusion in glassy polymers

Three types of novel correlations for activation energies of gas permeation E_P and diffusion E_D in amorphous glassy polymers are considered and their application for prediction of the E_P and E_D values for different gases are examined. The first one is based on application of the group contribution method. Combined consideration of the equation of free volume and Arrhenius equation results in the correlation of E_P and E_D with free volume V_f and fractional free volume (FFV). At last, the correlations between E_P and the permeability coefficient at a certain reference temperature P(T_ref), as well as E_D versus D(T_ref), are based on the fulfilment of the so-called compensation effect between activation energies and preexponential factors in activated processes. Examples of applicability of the correlations considered and recommendations for their use in prediction of the E_P and E_D values are given for transport of various gases in glassy polymers and separately in amorphous glassy polyimides.     

Heats of formation for Fe(CO)n(n=1–4)

The dissociation energies of Fe(CO)_n (n=2–4) are computed using correlation consistent basis sets and the CCSD(T) approach. The dissociation energies are extrapolated to the CBS limit and are corrected for core–valence (CV), scalar relativistic, spin–orbit, zero-point, and thermal effects. Our iron carbonyl bond strengths agree with experiment within the respective error bars. We use our dissociations energies at 298 K to obtain the heats of formation of Fe(CO)_n (n=1–4).     

Hydration of the ions Al3+ and Cu2+. An ab initio SCF study

Ab initio calculations are reported for the systems Al(H₂O)_n³⁺ and Cu(H₂O)_n²⁺ with n up to 7. The calculated binding energies increase monotically up to n = 6, with equal binding energies for n = 6 and 7 for the Al³⁺ cation. An estimate of the enthalpy of hydration of Al³⁺ is given, based on model calculations with one or two water molecules from the second solvation shell. An SN1 (dissociative) mechanism for the exchange of the water molecules from the first hydration shell of Al³⁺ appears energetically favorable if the leaving molecule remains in the second hydration shell.     

An electrochemical method for determination of the standard Gibbs energy of anion transfer between water and n-octanol

The transfer of the ions Cl⁻, Br⁻, I⁻, ClO₄⁻, SCN⁻, NO₃⁻, BF₄⁻, and (C₆H₅)₄B⁻ across the water|n-octanol (W|OC) liquid interface was studied and the standard Gibbs energies of ion transfer were determined. The ion transfer was achieved by oxidation of decamethylferrocene dissolved in a droplet of n-octanol that was attached to a graphite electrode immersed in the aqueous solutions of the respective alkali salts of the anions. The electrode reaction can be described by the equation: dmfc_(OC)+X_(W)⁻⇄dmfc⁺_(OC)+X⁻_(OC)+e⁻, where X⁻ is the transferred anion. Square-wave voltammetry at this three-phase arrangement was utilised to determine the formal potential of the decamethylferrocene/decamethylferrocenium (dmfc/dmfc⁺) couple under the condition of ion transfer across the water|n-octanol interface. For calibration the standard Gibbs energies of ion transfer have been extrapolated to octanol from the series of known data for methanol, ethanol, n-propanol, and n-butanol. All these data are consistent and the experimental dependence of the formal potentials on the standard Gibbs energies is as predicted by theory. The validity of data is further supported by calculations of Gibbs energies of ion transfer using the Born theory. Until now it was not possible to perform electrochemical measurements at the water|n-octanol interface because in the conventional four-electrode cells this interface cannot be polarised. With the new method it is now for the first time possible to determine the Gibbs energies of transfer of ions across the water|n-octanol interface. These values are of very wide use for assessing the lipophilicity of compounds in chemistry, medicine, and pharmacology.     

Theoretical study on the isomerization and tautomerism in barbituric acid

Isomerization and tautomerism of 16 isomers of barbituric acid (BA) were studied at the MP2 and B3LYP levels of theory. Activation energies (E _a), imaginary frequencies (υ), and Gibbs free energies (ΔG ^#) of the amine-imine and keto-enol tautomerisms and O–H internal rotations were calculated. The activation energies of amine-imine tautomerisms were in the range of 110–200 kJ/mol and for keto-enol tautomerisms were larger than 200 kJ/mol. The calculated activation energies of internal O–H rotations were smaller than 60 kJ/mol. Effect of micro-hydration on the transition state structures and activation energies of the tautomerisms were also investigated. Water molecule catalyzed the tautomerisms and decreased the activation energies of both the amine-imine and keto-enol tautomerisms about 100–120 kJ/mol.     

Internal excitation effects in the photodissociation of size selected ethylene and methanol dimers

The infrared photodissociation spectra of C₂H₄ and CH₃OH dimers are measured for different internal excitation energies ΔE. The dimers are size-selected in a scattering process with He and the internal energy is varied by using (1) different collision energies, (2) different scattering angles and (3) by measuring the complete energy loss spectra with and without laser radiation by time-of-flight analysis of the scattered clusters. For (CH₃OH)₂ the width Γ of the spectrum increases strongly with ΔE, while the cross section at the maximum is constant, a normal behaviour for an inhomogeneously broadened system. For (C₂H₄)₂ Γ is nearly constant after a sort of threshold and the cross section increases with increasing ΔE. These results are explained by hot band excitation and the coupling to the darkv ₁₀-mode.