A CI non-empirical pseudopotential calculation of the vertical valence excited states of the iodine molecule

The non-empirical atomic pseudopotential proposed by Durand and Barthelat has been used, together with the CIPSI algorithm for large scale CI, to calculate the vertical transition energies of the iodine molecule, in a valence extended (double-zeta + d) basis set. All the valence excited states were considered. The mixing of configurations is very important especially for the Σ⁺_g, Π_g and Π_u symmetries. The experimentally known transition energies are calculated within a 1 eV error, despite the lack of diffuse orbitals and spin-orbit interaction. Some qualitative Mulliken's estimates are discussed. A new ³Σ⁺_g state from the 10 σ_u → 11 σ_u single excitation is predicted in the 9 eV region.     

A systematic search for the valence structures of the tetradecahydrotetradecaborate dianion

A systematic search has been used to find all the allowed valence structures of the D_6d bicapped hexagonal antiprismatic BH₁₄²⁻closo-boron hydride dianion. From the number of allowed valence structures (72), a valence state index (VSI) equal to 0.0773 can be deduced for the tetradecahydrotetradecaborate dianion, that is compared to those of the parent B_nH_n²⁻ derivatives (n = 6–12).     

Valence and structural transitions in the mixed Ru-Ir perovskites Ba2PrRu1−xIrxO6

The valence state of Pr in the B-site ordered double perovskites Ba₂PrRu_1−xIr_xO₆ is shown to be sensitive to both the precise Ru:Ir content and temperature. Pr L_III XANES measurements show that at room temperature the Pr is trivalent in the Ru-rich compounds with x<0.25. At higher Ir contents the Pr is tetravalent. High-resolution powder synchrotron X-ray and neutron diffraction methods have been used to study the composition and temperature dependence of the crystal structures of these oxides. The Ru and Ir are statistically distributed on one of the two available B-sites. The oxides undergo an apparently first-order monoclinic P2₁/n to tetragonal P4/mnc phase transition in response to the change on the Pr and Ru/Ir valence. High temperatures and Ru contents favor the lower symmetry monoclinic structure, that is arises from the presence of the larger Pr^III cations. The variations in the observed metal-oxygen bond distances are consistent with a simultaneous change in the valence of the Ru/Ir accompanying the Pr^III-Pr^IV valence transition.     

An Ab initio study on the conformations of protonated,neutral, and deprotonated amidine

Ab initio SCF molecular orbital calculations have been performed to ascertain the conformational preferences of protonated, neutral, and deprotonated amidine [HC(?NH)NH₂], using the 3-21G split valence basis set. The states of eight stable species, eight transition states, and four higher-order saddle points have been determined by complete geometry optimization utilizing analytic energy gradient techniques. Protonation at the amidine ?NH is preferred over the –NH₂ site by 37.1 kcal/mol. Neutral amidine has rotational barriers of 9.6 and 11.7 kcal/mol for the HN?CN cis and trans isomers, respectively, while all the stable HC(NH₂)₂⁺ and HC(NH)₂^? species possess torsional barriers larger than 23 kcal/mol. There is, however, essentially free C—N single-bond rotation in HC(?NH)NH₃⁺, the calculated barriers being 0.7 and 1.8 kcal/mol for the cis and trans HN?CN isomers, respectively.     

Researches by the method of low-temperature adiabatic calorimetry and quantum chemical computation of vibrational states

The temperature dependence of heat capacity of a natural zinc silicate, hemimorphite Zn₄Si₂O₇(OH)₂·H₂O, over the temperature range 5–320 K has been investigated by the method of low-temperature adiabatic calorimetry. On the basis of the experimental data on heat capacity over the whole temperature interval, its thermodynamic functions C _p (T), S(T) and H(T) ? H(0) have been calculated. The existence of a phase transition in the area of 90–105 K determined on the basis of vibrational spectra has been confirmed, and changes of entropy ΔS _tr. and enthalpy ΔH _tr. of the phase transition have been calculated. Hemimorphite heat capacity has also been determined by the calculation methods according to the valence force field model in LADY program. The values of force constants of valence bonds and angles have been calculated by semi-empirical method PM5. The calculated IR and Raman spectra concordant with the experimental spectra have been obtained. The heat capacity values calculated according to the found vibrational states satisfactorily agree with those experimentally obtained with an accuracy of ±1.7% in the area of 120–200 K, and not more than ±0.8% for the interval of 200–300 K. This fact testifies that the calculation of thermodynamic characteristics is correct.     

Theoretical study of the electronic spectrum of diimide by AB initio methods

A series of ab initio calculations is reported for the ground and low-lying valence and Rydberg states of diimide N₂H₂. Symmetric bending potential curves for both the cis and trans forms of this system have been obtained at the SCF level of treatment. In addition Cl calculations have been carried out for the trans-diimide ground state equilibrium nuclear conformation, using a configuration selection procedure described elsewhere; an associated energy extrapolation scheme is also employed which enables the effective solution of secular equations with orders of up to 40000. The ensuing Cl wavefunctions are interpreted in the discussion and the corresponding calculated energy differences between the various electronic states are compared with experimental transition energy results for both diimide and for related systems such as trans-azomethane. A more detailed analysis of the observed absorption bands in the ¹B_g-X¹A_g transition in N₂H₂ is also given, making use of calculated potential curve data as well as the pertinent Cl vertical energy difference. The dipole-forbiddenness of the excitation process is thereupon concluded to result in a distinct non-verticality for this electronic band system, causing its absorption maximum to occur at a position some 0.6 eV to the blue of the so-called vertical transition, i.e., that for which maximum vibrational overlap is obtained.     

Electronic structure of poly-diacetylene: Calculations on a model system

Calculations of electronic structure have been performed for a molecule that models poly-diacetylene. The self-consistent field, X_α method was used. It is concluded that poly-diacetylene has a nearly-free-electron-like valence band and is best described as a wide band gap semiconductor. The π → π^* transition of the model molecule is in numerical agreement with the size of the band gap observed in most poly-diacetylene systems.     

Some clues about the interphase reaction between ZnO and MnO2 oxides

Raman spectroscopy is used to evidence both the nature of the interphase reaction between ZnO and MnO₂ particles and its kinetic evolution. Zn cations migrate from the ZnO grains during oxygen vacancies formation process and diffuse into the MnO₂ particles leading to an interphase region with an intermediate valence Mn⁺³-O-Mn⁺⁴. Large amounts of desorbed Zn cations promote the formation of ZnMn₂O₄ structure, in addition to the intermediate valence state. The system evolves towards complete formation of the spinel phase at higher thermal treatment times. The reactivity of the ZnO plays an important role in the formation of this interphase. Low-reactivity ZnO powder, in which the oxygen vacancies are previously produced, shows a stabilization of the intermediate valence state with very limited formation of the spinel phase. A clear correlation between the amount of the intermediate state interphase and the magnetic properties has been established.     

Transition metal bonding functions and their applications in catalytic adsorptions and reactions: Part III. Effect of metal valence band and periodic trend of CO σ-π bonding functions on transition metals

Our model of metal valence band and our new concept of σ-π coordination are further discussed and confirmed in this paper.The infrared stretching frequencies of C-O decrease in the order 2056, 1886 and 1786 cm⁻¹ in Ni(CO)₄, Co(CO)₄⁻¹ and Fe(CO)₄⁻², which parallels the increase in d electron back-donation functions (B metal bonding functions) from 1.539, 2.121 to 2.895 on Ni, Co and Fe metals, respectively. On the other hand, the M-C bond orders increase from 1.33, 1.89 to 2.16 for Ni(CO)₄, Co(CO)₄⁻¹ and Fe(CO)₄⁻², which parallel the increase in A(CO5σ-Mσ)-B(CO2π-Mπ) metal bonding functions from 24.61, 30.01 to 33.19, respectively. They are in agreement with our new concept of σ-π coordination proposed in the previous paper. This new concept has also been used to analyze the mechanism of the formation of Ni(CO)₄, Co(CO)₄⁻¹ and Fe(CO)₄⁻², and to explain why they can automotively hybridize each other despite the energy differences between 3d and 4s, 4p, which are very large.The effects of metal valence bands have been accounted for on all transition metals (d¹ to d⁸), and it is demonstrated that d orbitals increase from the Vd band upward to the Vs band, and s orbitals from the Vs band downward to the Vd band, which is equivalent to a change in orbital potential, and would modify their orbital overlap integrals with the adsorbate M.O.s and the A, B metal bonding functions significantly. The effective potentials and the percentage s, d functions of Vs, Vd and d_occ bands are the most important factors for determining the effect of the metal valence band. The effects of promoter and support are also altered by changes in the above factors. For Group VIII metals, the valence band provides various s and d orbitals at various potentials, in which a certain number of s and d orbitals can match better with CO adsorbate M.O.s, which explains why CO adsorbed species on Group VIII metals are all stable and adsorption rates are all relatively rapid.The periodic trends of metal A, B, AB and D_c bonding functions depend on the structures of the metal valence band, i.e. the potential levels and s, d percentage functions of Vs, Vd and d_occ bands. For 4d and 5d metals, the potential levels of the Vs band are high, which cannot form a strong CO 5σ-M σ bond, but the potential levels of Vd band are higher and the width of the d band is wider than those of 3d metal, so their B bonding functions are larger, and they can be used to activate saturated and unsaturated hydrocarbons. In contrast, for 3d metals, the potentials of the Vs band are lower, which favour formation of strong CO 5σ-M σ and M-C bonds, i.e. their A and D_c bonding functions are larger, which can promote coke formation. While ABD_cD_o can be used to characterize CO dissociation, B/A can be used to characterize C-C formation.The characteristics of various metal bonding functions on each transition metal are useful for designing catalyst composition. A typical example has been illustrated, using the possibility to select non-noble metals instead of noble metals in hydrocarbon reactions.     

The 3400 Å band system of sulphur dioxide

The contour of the band of SO₂ at 29937 cm^?1 has been shown to be of type-c, and an approximate excited state structure derived as r_SO = 1.50 Å, valence angle = 112°. For a number of reasons it is proposed that the principal bands between 3150 and 3400 Å correspond to vibronically induced B₁-A₁ transitions of a ¹A₂-¹A₁ forbidden electronic transition rather than to an allowed ¹B₁-¹A₁ transition.     

Quantum chemical studies on electrophilic addition

The geometries of the 2-chloroethyl and ethylenechloronium cations, two possible intermediates in the electrophilic addition of chlorine to ethylene, have been fully optimized using ab initio molecular orbital calculations employing the split valence shell 4-31G basis set.These geometries were then used to compute more accurate wave functions using Dunning's double-zeta basis set. The bridged chloronium ion was found to be more stable by 9.35 kcal/mole, the opposite order of stability from the C₂H₄F⁺ ions. Interconversion of the two C₂H₄Cl⁺ cations was computed to have a barrier of 6.25 kcal/mole.The activation energy for this chlorination reaction, using the ethylenechloronium cation and a chlorine anion at infinite separation as the model for the activated complex, was computed to be 128.7 kcal/mole, showing that this is not a feasible gas phase reaction.     

X‐Ray Diffraction and Mössbauer Spectroscopy Studies of Pressure‐Induced Phase Transitions in a Mixed‐Valence Trinuclear Iron Complex

The mixed‐valence complex Fe₃O(cyanoacetate)₆(H₂O)₃ ( 1 ) has been studied by single‐crystal X‐ray diffraction analysis at pressures up to 5.3(1) GPa and by (synchrotron) Mössbauer spectroscopy at pressures up to 8(1) GPa. Crystal structure refinements were possible up to 4.0(1) GPa. In this pressure range, 1 undergoes two pressure‐induced phase transitions. The first phase transition at around 3 GPa is isosymmetric and involves a 60° rotation of 50 % of the cyanoacetate ligands. The second phase transition at around 4 GPa reduces the symmetry from rhombohedral to triclinic. Mössbauer spectra show that the complex becomes partially valence‐trapped after the second phase transition. This sluggish pressure‐induced valence‐trapping is in contrast to the very abrupt valence‐trapping observed when compound 1 is cooled from 130 to 120 K at ambient pressure.     

Analysis of chemical bonding in C2 using Dyson orbitals

Multireference configuration interaction wave functions with single and double excitations were calculated for the ¹Σ⁺_g ground state of the C₂ molecule and the excited states of C⁺₂ with symmetries ²Σ⁺_g, ²Σ^-_u, ²Π_u, and ²Π_g. The corresponding σ_g, σ_u, π_u, and π_g valence Dyson orbitals were calculated. Most of the density due to the valence electrons is accounted for by three σ_g, one σ_u, and one degenerate pair of π_u Dyson orbitals. Electron correlation plays an important role in the bond strength of C₂ by increasing the occupation of the σ_g valence orbitals and decreasing the occupation of the σ_u and π_u valence orbitals. © 1996 John Wiley & Sons, Inc.     

Determination of the radiative lifetimes of the b1Σ+ and a1Δ states in NH by ab initio methods

Probabilities for the spin-forbidden transitions from the b¹Σ⁺ and a¹Δ states to the X³Σ^? ground state of NH have been evaluated by a first-order perturbation expansion into S-eigenfunctions Nine ³Π and ¹Π, five ¹Σ⁺ and three ³Σ^? states have been calculated by the MRD CI method at the experimental equilibrium distance of the X³Σ^? state (1.0362 Å) which cover a vertical spectral region of = 100000 cm^?1. The expansion terms of the perturbation sum are spin-orbit coupling coefficients obtained by using the Breit-Pauli one- and two-electron spin-orbit operator. The radiative lifetime of b¹Σ⁺ has been determined in the Franck-Condon approximation to be 72 ms from ab initio data and 97 ms if experimental excitation energies for the low-lying valence states are employed. Recent experiments give a somewhat shorter lifetime for the corresponding 0-0 transition of 53 ms. The lifetime is governed by the transition to the ³Σ^?_±1 level of the non-rotating molecule, borrowing its intensity mainly from the A³Π → X³Σ^? dipole transition. The second possible transition to the Ω = 0 level of the ground state is found to be weak. A similar relation of μ₁/μ₀ is expected for all the hydrogen containing isovalent molecules such as PH and AsH. The radiative lifetime of the a¹Δ state has been calculated to be = 1.7 s. Recent matrix experiments predict a gas-phase lifetime of at least 3 s. Further experimental and theoretical investigations are in progress to clarify this unusual finding that the experimentally determined lifetime is longer than that calculated theoretically.     

Quantum chemical study of the geometries and stabilities of the two valence-tautomers of C2H2F+

Non-empirical SCF-MO molecular wavefunctions were computed for the two limiting structures of C₂H₂F⁺ with full geometry optimization using double-zeta quality atomic orbital basis sets. The bridged structure (fluorenium ion) was found to be an energy maximum (transition state) about 31 kcal/mole higher than the open structure (fluoro-vinyl cation). The latter, contrary to the unsubstituted vinyl cation, is slightly (4.5 °) bent away from fluorine at the electron deficient centre.     

A non-local pseudopotential in the FSGO model: Study of some organometallic systems

A non-local core pseudopotential has been used in the framework of floating spherical Gaussian orbital (FSGO ) model to study the equilibrium geometries and valence electronic structures of some organolithium and organoberyllium systems. The calculated equilibrium geometries, heats of hydrogenation, average electric polarizabilities, and magnetic susceptibilities are in good agreement with the results of the all-electron FSGO model calculations. Valence electron wave functions obtained here have been used to predict the valence electron Compton profiles (CP ) and electron momentum distributions (EMD ) of the systems studied. A good correlation has been shown among the peak height of the CP (J(0)), valence electron energy (E_v), and number of valence electrons (N_v).     

A valence bond study of the oxygen molecule

Ab initio valence bond calculations are performed for the three lowest states of the oxygen molecule (³Σ⁻_g, ¹Δ_g, and ¹Σ⁺_g). One objective of the present study was to make a contribution to previous valence bond discussions about the oxygen “double” bond. Further, we study the origin of a small barrier in the potential energy surface of the ground state. Two compact models are employed to maintain the clear picture that can be offered by the valence bond method. The first model has only the Rumer structures that are essential for bonding and a proper dissociation. The second model, in addition, has structures which represent excited atoms. These prove to be important for the dissociation energies. For both models, the orbitals are fully optimized. The spectroscopic data obtained are significantly better than are the (few) valence bond results on O₂ that have been published and have the quality of multiconfiguration self-consistent field calculations in which the same valence space is used. The “hump” in the potential energy surface of the ground state is shown to arise from a spin recoupling. The free atoms correspond to a spin coupling that is incapable of describing the formation of bonds. Only at short distances, an alternative spin coupling provides bonding and the repulsive curve is converted into an attractive one. Our results on this subject support a valence bond explanation previously given by McWeeny [R. McWeeny, Int. J. Quantum Chem. Symp. 24 , 733 (1990)]. © 1996 John Wiley & Sons, Inc.     

Nature of the chemical bonding in D3h [MH3M]+ cations (M = Be,Mg)

Motivated by the particularly short metal-metal distance that has been predicted for the D_3h [BeH₃Be]⁺ cation, comparable to those anticipated for triple bonds, we investigate the nature of the bonding interactions in the D_3h [MH₃M]⁺ cations (M = Be, Mg). CCSD(T)/cc-pVQZ calculations are used to determine optimized geometries for all of the various species, including those “capped” by He or Ne atoms (as proxies for an inert gas matrix). The primary tools that are then used to investigate the nature of the chemical bonding are spin-coupled generalized valence bond calculations and the analysis of localized natural orbitals and of domain-averaged Fermi holes. The various results for all of the systems considered indicate the presence of highly polar three-center two-electron M─H─M bonding character instead of any significant direct metal-metal bonding.     

All-valence-electron CI treatment of the electronic spectrum of trans-butadiene

An all-valence-electron CI treatment is reported for the low-lying valence and Rydberg states of butadiene. All singly- and doubly-excited configurations relative to a series of the leading terms in a given CI expansion are taken into account, with resulting secular equation orders of as high as 150 000. The agreement between calculated and experimental transition energies is invariably better than 0.2 eV where comparison is possible, with all low-lying valence triplet and Rydberg singlet excited states being unambiguously assigned. The valence-shell excitation to the 2 ¹A_g species is concluded to correspond to the 7.06 eV band system, while the forbidden singlet—singlet transition reported by McDiarmid is assigned as x₂ → 3s. The possibility of an avoided crossing between Rydberg valence ¹B_u excited states having a determining influence on the appearance of the broad intense V₁—N absorption is also discussed.