The V state of ethylene: valence bond theory takes up the challenge

The ground state and first singlet excited state of ethylene, so-called N and V states, respectively, are studied by means of modern valence bond methods. It is found that extremely compact wave functions, made of three VB structures for the N state and four structures for the V state, provide an N → V transition energy of 8.01 eV, in good agreement with experiment (7.88 eV for the N → V transition energy estimated from experiments). Further improvement to 7.96/7.93 eV is achieved at the variational and diffusion Monte Carlo (MC) levels, respectively, VMC/DMC, using a Jastrow factor coupled with the same compact VB wave function. Furthermore, the measure of the spatial extension of the V state wave function, 19.14 a ₀ ² , is in the range of accepted values obtained by large-scale state-of-the-art molecular orbital-based methods. The σ response to the fluctuations of the π electrons in the V state, known to be a crucial feature of the V state, is taken into account using the breathing orbital valence bond method, which allows the VB structures to have different sets of orbitals. Further valence bond calculations in a larger space of configurations, involving explicit participation of the σ response, with 9 VB structures for the N state and 14 for the V state, confirm the results of the minimal structure set, yielding an N → V transition energy of 7.97 eV and a spatial extension of 19.16 a ₀ ² for the V state. Both types of valence bond calculations show that the V state of ethylene is not fully ionic as usually assumed, but involving also a symmetry-adapted combination of VB structures each with asymmetric covalent π bonds. The latter VB structures have cumulated weights of 18–26 % and stabilize the V state by about 0.9 eV. It is further shown that these latter VB structures, rather than the commonly considered zwitterionic ones, are the ones responsible for the spatial extension of the V state, known to be ca. 50 % larger than the V state.     

Synthesis of novel pyrene discotics for potential electronic applications

Novel pyrene discotics, 6,7,15,16-tetrakis(alkylthio)quinoxalino[2′,3′:9,10]phenanthro[4,5-abc]phenazines, TQPP-[SR]₄, were synthesized efficiently. The HOMO and LUMO energy levels of TQPP-[SR]₄ were estimated to be 5.57 eV and 2.97 eV, respectively. The average saturation hole mobility of TQPP-[C₁₂H₂₅]₄ was ∼10⁻³ cm² V⁻¹ s⁻¹.     

Gas phase ion molecule chemistry of tetracarbonyl (η5-cyclopentadienyl) vanadium by Fourier transform ion cyclotron resonance

The kinetics of the reactions between the CpV(CO)₄ molecule and its fragment cations and anions have been examined using Fourier transform ion cyclotron resonance (FTICR) techniques. With 25 eV electron impact ionization the fragment cations V⁺ and CpV(CO)_n=0–4⁺ react principally by charge exchange or by condensation with the parent neutral molecule. Rate constants for these pathways have been determined along with kinetic evidence for the existence of excited state cations. Some of the product cations show unexpected stability despite their large formal electron deficiency. Exchange of carbonyl ligands was also observed. Under 2.5 eV electron impact, only two anions are produced: CpV(CO)_n=2,3⁻, both of which are unreactive with the parent neutral.     

The Conductance- and Capacitance-Frequency Characteristics of the Rectifying Junctions Formed by Sublimation of Organic Pyronine-B on p-Type Silicon

The rectifying junction characteristics of the organic compound pyronine-B film on a p-type Si substrate has been studied. The pyronine-B has been sublimed on the top of p-Si surface. The barrier height and ideality factor values of 0.79±0.04 and 1.13±0.06 eV for this structure have been obtained from the forward bias current-voltage (I-V) characteristics. From the low capacitance-frequency (C-f) characteristics as well as conductance-frequency (G-f) characteristics, the energy distribution of the interface states and their relaxation time have been determined in the energy range of (0.53−E_v)-(0.79−E_v) eV taking into account the forward bias I-V data. The interface state density N_ss ranges from 4.93×10¹⁰ cm⁻² eV⁻¹ in (0.79−E_v) eV to 3.67×10¹³ cm⁻² eV⁻¹ in (0.53−E_v) eV. Furthermore, the relaxation ranges from 3.80×10⁻³ s in (0.53−E_v) eV to 4.21×10⁻⁴ s in (0.79−E_v) eV. It has been seen that the interface state density has an exponential rise with bias from the midgap towards the top of the valence band. The relaxation time shows a slow exponential rise with bias from the top of the valence band towards the midgap.     

Perturbations of molecular extravalence excitations by rare-gas fluids

In this paper we report the results of an experimental study of the vacuum ultraviolet absorption spectra of molecular impurity states of methyl iodide in Ar (density range ? = 0–1.4 g cm^?3) and in Kr (? = 0–2.3 g cm^?3), of carbon disulphide in Ar (? = 0–1.4 g cm^?3) and of formaldehyde in Ar (? = 0–1.25 g cm^?3). The experimental results provide new information regarding medium perturbations of intravalenc transitions, of the lowest extravalence transitions and of transitions to mixed valence—Rydberg configurations, which serve as a diagnostic tool to distinguish between different types of electronic excitations. All the lowest extravalence molecular excitations exhibit appreciable blue spectral shifts at moderate and at high fluid densities, intravalence transitions are practically insensitive to medium effects, while excitations to mixed valence—Rydberg configurations are characterized by a moderate blue spectral shift. New information has been obtained concerning the energetics of molecular ionization processes in a dense fluid. The high n = 2–5 Rydberg states of CH₃l exhibit a large red shift at moderate (? = 0–0.5 cm^?3) Ar densities. The ionization potential E_g and the effective Rydberg constant G for CH₃I in Ar was found to decrease from G = 13.6 eV and E_g = 9.55 eV at ? = 0 and E_g = 9.08 eV and constant G for CH₃l in Ar was found to decrease from G = 13.6 eV and E_g = 9.55eV at ? = 0 and E_g = 9.08 eV and G ≈ 7.15 eV at ? = 0.5 g cm^?3. Experimental evidence was obtained for the identification of n = 2 molecular Wannier impurity states of CH₃I and of CH₂O in liquid Ar. These spectroscopic data result in E_g ≈ 8.6 eV for CH₃I in liquid Ar and E_g ≈ 10.2 eV for CH₂O in liquid Ar.     

Studies on the V2O5-TiO2 system part 1. Thermoelectric power and electrical conductivity

Summary The thermoelectric power and the resistivity of V₂O₅- TiO₂ mixtures over ranges of composition and temperature from 20° to 500° in air have been measured, The mixtures were obtained by coprecipitation of aqueous solutions of NH₄VO₃ and TiCl₄. calcined during 90 h at 550° and then sintered. Resistances were measured by the four points method.At the VO_5/2 35–100% M range, the activation energies for conductivity change from 0,36 to 0.62eV, while for the thermoelectric power they change from 0,18 to 0.24eV. It can be assumed from these values that the conduction mechanism over this concentration range is due to the hopping of small polarons, arising principally, from the presence of V⁴⁺ ions.In the samples with a high TiO₂ content, the activation energies for conductivity were 0.82 and 0.36eV, for the different samples.From the variation of thermoelectric power with temperature, it can be assumed that the ionization energy of the donors centers lies at 0.83eV under the conducting band. A mechanism for band conduction is inferred from the results, being the V⁴⁺ donating centers and the V⁵⁺ receptive centers.Author to whom all correspondence should be addressed     

Designing and tuning properties of a three-dimensional porous quaternary chalcogenide built on a bimetallic tetrahedral cluster [M4Sn3S13] (M=Zn/Sn)

A multifunctional three-dimensional quaternary chalcogenide [Na₅Zn_3.5Sn_3.5S₁₃]·6H₂O has been synthesized by solvothermal reactions. [Na₅Zn_3.5Sn_3.5S₁₃]·6H₂O represents an interesting example of metal chalcogenides that combines semiconductivity, porosity, and light emission in a single structure. It crystallizes in the cubic space group Fm-3c, a=17.8630(3) Å, V=5699.85(17) Å³, Z=8. The compound decomposes at ∼450 °C. A band gap of 2.9 eV is estimated from the optical diffuse reflectance data. A strong photoluminescence peak is observed at 2.43 eV in Mn doped samples. The electronic and optical properties of this compound can be systematically tuned by substitution of metal and chalcogen elements.     

Crystal growth,structural and electrical properties of rubidium and cesium vanadium oxide bronzes

Crystals of the following compounds were grown by cathodic reduction of CsV⁵⁺O or RbV⁵⁺O metls: Cs_0.3V₂O₅ (A), Cs₂V₅O₁₃ (B), CsV₂O₅ (C), Rb_0.4V₂O₅ (D), Rb_0.37V₂O_∼4.8 (E) (a new orthorhombic compound) and Rb_2−xV_3+2xO_8+2x (F). The crystal symmetry and cell parameters of the Rb compounds (which were known for F only) were determined, as well as those of Rb_0.3V₂O₅, which has the structure of A. Magnetic susceptibility and ESR measurements confirm the intermediate valence in E. A, C, and E are semiconductors with activation energies in the range 0.07–0.2 eV. Cs_0.3V₂O₅ (A), in which V⁴⁺ and V⁵⁺ do not occupy distinct crystallographic sites, has the highest electrical conductivity.     

Thermochemistry of molecular FeO,FeO+ and FeO

The gaseous oxides FeO and FeO₂ were identified by mass spectrometry as components of the effusion beam from a cell containing, initially, solid Fe₂O₃. From studies of gaseous equilibria involving these species, the dissociation energiesD₀⁰(FeO) = 96.8 ± 3 kcal (4.20 ± 0.13 eV) andD₀⁰(FeO₂) = 199.0 ± 5 kcal (8.64 ± 0.22 eV) were derived. The ionization potential of FeO was found to be 8.71 ± 0.10 eV, leading toD₀⁰(Fe⁺-O) = 78.2 ± 4.6 kcal (3.39 ± 0.20 eV).     

Study of singlet-triplet coupling in glyoxal by level anticrossing spectroscopy. VI. Vibrational density and statistics of matrix elements versus energy

By laser excitation of the rotationless level (J = 0) of ten vibrational levels of the S₁ (A_u) state (0⁰, 7², 5¹, 8¹, 6¹7¹, 4¹, 8¹7², 2¹, 8¹4¹ and 2¹7²) of supersonic jet cooled glyoxal, we have obtained S₁-T₁ anticrossing spectra using the homogeneous, high magnetic field (0–8 T) of a Bitter coil. As explained previously, V_st is readily obtained from the width of an anticrossing. As triplet vibrational energy increases from 2776 (0⁰ of S₁) to 4636 cm⁻¹ (2¹7² of S₁), the number of anticrossings increases from 38 (0⁰) to 871 (2¹7²). The anticrossing density is related to the vibrational density of T₁. The V_st histrograms obtained for each vibrational level are very similar: p(V_st) ∝ V^−1−α_st with 0.4 ⩽ α ⩽ 0.7. The more significant and surprising result is that <V_st > is independent of vibrational energy, even though the corresponding vibrational overlaps predicted a decrease in <V_st >, of at least two orders of magnitude between 0⁰ and 2¹7². From V_st statistics we determine ϱ<V_st > and ϱ<V²_st > which are the dominant factors for ISC (intersystem crossing). We predict that strong S-T mixing should occur above 6900 ± 500 cm⁻¹.     

Evaluation of the equivalence point in potentiometric titrations with application to traces of chloride

A mathematical model applicable to the determination of the equivalence point (V_eq) is described. The regression equation is (1 + V/V₀)³E = Σ³_i= _0AiVⁱ, where E is the e.m.f. corresponding to volume V of titrant added, V₀ is the initial volume of titrand, and A_i are the regression coefficients. On the basis of A_i values obtained by the least-squares method, the algorithm for V_eq and the criterion of correctness of results obtained from measurements are presented. The method is applied to titrations of chloride with silver nitrate solutions.     

Vanadiumoxo–aroylhydrazone complexes: Synthesis,structure and DFT calculations

The aroylhydrazone Schiff base ligands (E)-N’-(2-hydroxybenzylidene)benzohydrazide = H₂L¹, (E)-N’-(2-hydroxy-3-methoxybenzylidene)benzohydrazide = H₂L² and = (E)-N’-(5-bromo-2-hydroxybenzylidene)benzohydrazide = H₂L³ gave the vanadium(V)oxo-aroylhydrazone complexes [V^VOL¹(OCH₃)(OHCH₃] (1), [V^VOL²(OCH₃)(OHCH₃]·CH₃OH (2) and [V^VOL³(OCH₃)(OHCH₃] (3) on reaction with vanadium(IV) oxide acetylacetonate. The complexes were characterized by spectroscopic methods in the solid state (IR) and in solution (UV–Vis, ¹H NMR). Single crystal X-ray analysis was performed with 3. In methanol solution six-coordinated V^VOL³(OCH₃)(OHCH₃) was formed. V^IV was oxidized to V^v by aerial oxygen in the synthesis. In the VO₅N coordination sphere the alcohol oxygen lies trans to the oxo oxygen. The general V–O bond length order is oxo < methoxylato < phenoxidic < enolato < alcoholic. The complexes are mononuclear, but intermolecular O–H?N hydrogen bonding affords a zigzag chain. DFT calculations on complex 3 reproduced the geometric parameters, IR and UV–Vis spectroscopic data well in a reasonable range.     

Dyes and Redox Couples with Matched Energy Levels: Elimination of the Dye‐Regeneration Energy Loss in Dye‐Sensitized Solar Cells

In dye‐sensitized solar cells (DSSCs), a significant dye‐regeneration force (ΔG_reg⁰≥0.5 eV) is usually required for effective dye regeneration, which results in a major energy loss and limits the energy‐conversion efficiency of state‐of‐art DSSCs. We demonstrate that when dye molecules and redox couples that possess similar conjugated ligands are used, efficient dye regeneration occurs with zero or close‐to‐zero driving force. By using Ru(dcbpy)(bpy)₂²⁺ as the dye and Ru(bpy)₂(MeIm)₂^3+//2+ as the redox couple, a short‐circuit current (J_sc) of 4 mA cm^?2 and an open‐circuit voltage (V_oc) of 0.9 V were obtained with a ΔG_reg⁰ of 0.07 eV. The same was observed for the N3 dye and Ru(bpy)₂(SCN)₂^1+/0 (ΔG_reg⁰=0.0 eV), which produced an J_sc of 2.5 mA cm^?2 and V_oc of 0.6 V. Charge recombination occurs at pinholes, limiting the performance of the cells. This proof‐of‐concept study demonstrates that high V_oc values can be attained by significantly curtailing the dye‐regeneration force.     

Syntheses, structures, optical properties, and theoretical calculations of Cs2Bi2ZnS5, Cs2Bi2CdS5, and Cs2Bi2MnS5

Three new compounds, Cs₂Bi₂ZnS₅, Cs₂Bi₂CdS₅, and Cs₂Bi₂MnS₅, have been synthesized from the respective elements and a reactive flux Cs₂S₃ at 973 K. The compounds are isostructural and crystallize in a new structure type in space group Pnma of the orthorhombic system with four formula units in cells of dimensions at 153 K of a=15.763(3), b=4.0965(9), c=18.197(4) Å, V=1175.0(4) Å³ for Cs₂Bi₂ZnS₅; a=15.817(2), b=4.1782(6), c=18.473(3)  Å, V=1220.8(3)  ų for Cs₂Bi₂CdS₅; and a=15.830(2), b=4.1515(5), c=18.372(2) Å, V=1207.4(2) Å³ for Cs₂Bi₂MnS₅. The structure is composed of two-dimensional _∞²[Bi₂MS₅²⁻] (M=Zn, Cd, Mn) layers that stack perpendicular to the [100] axis and are separated by Cs⁺ cations. The layers consist of edge-sharing _∞¹[Bi₂S₆⁶⁻] and _∞¹[MS₃⁴⁻] chains built from BiS₆ octahedral and MS₄ tetrahedral units. Two crystallographically unique Cs atoms are coordinated to S atoms in octahedral and monocapped trigonal prismatic environments. The structure of Cs₂Bi₂MS₅, is related to that of Na₂ZrCu₂S₄ and those of the AMM′Q₃ materials (A=alkali metal, M=rare-earth or Group 4 element, M′= Group 11 or 12 element, Q=chalcogen). First-principles theoretical calculations indicate that Cs₂Bi₂ZnS₅ and Cs₂Bi₂CdS₅ are semiconductors with indirect band gaps of 1.85 and 1.75 eV, respectively. The experimental band gap for Cs₂Bi₂CdS₅ is ≈1.7 eV, as derived from its optical absorption spectrum.     

Formation of WF−6 and its dissociative products by collisional ionization

The formation of negative ions in electron transfer reactions between hyperthermal alkali atoms (Na, K) and WF₆ has been studied in the energy range 0–30 eV c.m. Relative cross sections and translational energy thresholds for ion pair formation have been measured, from which the following electron affinities (EA) and bond dissociation energies (D) have been derived: EA(WF₆) = 3.7 eV, EA(WF₅) = 1.25 eV, D(WF₅—F) = 5.1 eV, D)WF₅—F^?) = 5.4 eV, D(WF^?₅—F) = 7.6 eV. Several ion molecule reactions are discussed which result in formation of secondary fragmentation ions and WF^?₇.     

Electrochemical properties and state of paramagnetic centers in copper-modified complex vanadium and titanium oxides

Complex vanadium and titanium oxides modified by copper ions are studied by the electrochemical and ESR methods. Oxides Cu _x V_2?y Ti _y O_5?δ·nH₂O (0<y<1.33) have a layered structure and oxides Cu _x Ti_1?y V _y O_5+δ·nH₂O (0<y<0.25), an anatase structure. The intercalation of cations Cu²⁺ into the hydrates leads to oxidation of V⁴⁺. According to ESR data, V⁴⁺ exists in the oxides in the form of VO²⁺ and an octahedral surround of oxygen (V⁴⁺?O₆), respectively. The electroreduction of ions of d-elements and chemisorbed oxygen in the oxides is analyzed. The intercalation of cations Cu²⁺ alters the content of V⁴⁺ and the chemisorption ability of the oxides. Possible reasons for this phenomenon are discussed.     

Quaternary selenostannates Na2−xGa2−xSn1+xSe6 and AGaSnSe4 (A=K, Rb, and Cs) through rapid cooling of melts. Kinetics versus thermodynamics in the polymorphism of AGaSnSe4

The quaternary alkali-metal gallium selenostannates, Na_2−xGa_2−xSn_1+xSe₆ and AGaSnSe₄ (A=K, Rb, and Cs), were synthesized by reacting alkali-metal selenide, Ga, Sn, and Se with a flame melting-rapid cooling method. Na_2−xGa_2−xSn_1+xSe₆ crystallizes in the non-centrosymmetric space group C2 with cell constants a=13.308(3) Å, b=7.594(2) Å, c=13.842(3) Å, β=118.730(4)°, V=1226.7(5) Å³. α-KGaSnSe₄ crystallizes in the tetragonal space group I4/mcm with a=8.186(5) Å and c=6.403(5) Å, V=429.1(5) Å³. β-KGaSnSe₄ crystallizes in the space group P2₁/c with cell constants a=7.490(2) Å, b=12.578(3) Å, c=18.306(5) Å, β=98.653(5)°, V=1705.0(8) Å³. The unit cell of isostructural RbGaSnSe₄ is a=7.567(2) Å, b=12.656(3) Å, c=18.277(4) Å, β=95.924(4)°, V=1741.1(7) Å³. CsGaSnSe₄ crystallizes in the orthorhombic space group Pmcn with a=7.679(2) Å, b=12.655(3) Å, c=18.278(5) Å, V=1776.1(8) Å³. The structure of Na_2−xGa_2−xSn_1+xSe₆ consists of a polar three-dimensional network of trimeric (Sn,Ga)₃Se₉ units with Na atoms located in tunnels. The AGaSnSe₄ possess layered structures. The compounds show nearly the same Raman spectral features, except for Na_2−xGa_2−xSn_1+xSe₆. Optical band gaps, determined from UV-Vis spectroscopy, range from 1.50 eV in Na_2−xGa_2−xSn_1+xSe₆ to 1.97 eV in CsGaSnSe₄. Cooling of the melts of KGaSnSe₄ and RbGaSnSe₄ produces only kinetically stable products. The thermodynamically stable product is accessible under extended annealing, which leads to the so-called γ-form (BaGa₂S₄-type) of these compounds.     

Low energy,variable angle electron-impact excitation of 1,3,5-hexatriene

A mixture of cis and trans 1,3,5-hexatriene has been studied by electron impact at incident electron energies of 20 eV, 40 eV, 50 eV, and 70 eV, at scattering angles from 0° to 80°, and with effective energy resolutions in the range from 0.05 eV to 0.15 eV. Singlet → triplet transitions with maximum intensities at 2.61 eV and 4.11 eV are observed. The lowest energy spin-allowed excitation which can be detected is the electric dipole-allowed $\text{X}$¹ A_g → 1 ¹B_u transition (in the notation appropriate for the trans isomer). No evidence has been found for a spin-allowed but symmetry-forbidden $\text{X}$¹ A_g → 2 ¹A_g excitation in the vicinity of 4.4 eV transition energy. Many other spin-allowed excitations are observed in the 6–11 eV energy-loss region, and the correlation between these features and those observed in high resolution ultraviolet absorption spectra and other electron-impact spectra is discussed.     

CEPA calculations of potential energy surfaces for open-shell systems

Potential energy surfaces for the collision of He atoms with NH radicals in the electronically excitedA ³ Π state have been calculated using quantum chemical ab initio methods. The NH distance was kept fixed to its equilibrium value, the range of the HN-He distancesR and HNHe angles γ was chosen to be 4.0≦R≦8.0a ₀, 0≦γ≦180°. The doubly degenerate NH(A ³ Π) state is split upon approach of He into two components, 1³ A′ and 2³ A″, which remain degenerate only for collinear geometries. The resulting two potential surfacesV _A′ andV _A″ are essentially repulsive with shallow van der Waals minima at large distances. An expansion of the sum and the difference potential,V _A′ +V _A″ andV _A′ ?V _A″, respectively, in terms of Legendre polynomials shows that the anisotropic componentsV ₁₀(R) andV ₂₂(R) which mainly govern rotational transitions and Λ-doublet mixing are of the same size. It is therefore expected that these processes as well as fine-structure transitions are similarly probable in NH(A ³ Π)-He collisions. This is in accord with recent experiments of Kaes and Stuhl.     

Time-dependent perturbation: a recursive generation of the transition amplitudes between bound states

We present a new expansion of the solution to the time-dependent Schrödinger equation it ∂U/∂t = {H₀ + V(t)}U. A complete set of eigenvectors of H₀ spanning the Hilbert space in which H₀ and V operate is partitioned in two subsets. Transition amplitudes from the first subspace to the second one are calculated by building an adequate series of intermediate representations with respect to the couplings which produce the transition from the model space into its orthogonal complement. These expansions yield a coupling matrix series V⁽ⁿ⁾ for which an iterative solution V⁽ⁿ⁾ → V⁽ⁿ⁺¹⁾ is derived. This solution leads to a recursive numerical treatment of the calculation of transition amplitudes. A simple example, concerning a harmonic oscillator under an intense laser field, is considered using a Fourier analysis of the perturbation.