Pseudopotential study of lanthanum and lutetium dimers

Relativistic energy-consistent small-core lanthanide pseudopotentials of the Stuttgart–Bonn variety and extended valence basis sets have been used for the investigation of the dimers La₂ and Lu₂. It was found that the ground states for La₂ and Lu₂ are most likely ¹∑ _g ⁺ (σ _g ²π _u ⁴) and ³∑ _g ⁻ (4f ¹⁴4f ¹⁴σ _g ²σ _u ²π_u ²), respectively. The molecular constants including error bars were derived from multireference configuration interaction as well as coupled-cluster calculations, taking into account corrections for atomic spin–orbit splitting as well as possible basis set superposition errors. The theoretical values for La₂ (R _e=2.70±0.03 ?, D _e=2.31±0.13 eV, ω_e=186±13 cm⁻¹) show good agreement with the experimental binding energy (D _e=2.52±0.22 eV), but the experimental vibrational constant in an Ar matrix (ω_e=236±0.8 cm⁻¹) is significantly higher. For Lu₂ the theoretical values (R _e=3.07±0.03 ?, D _e=1.40±0.12 eV, ω_e=123±1 cm⁻¹) are in overall excellent agreement with experimental data (D _e=1.43±0.34 eV, ω_e=122± 1 cm⁻¹). The electronic structures of La₂ and Lu₂ are compared to those other lanthanide dimers and trends in the series are discussed. Received: 25 March 2002 / Accepted: 2 June 2002 / Published online: 21 August 2002     

Ab initio calculations for the potential curves and spin–orbit coupling of Mg2

 The ground state and several low-lying excited states of the Mg₂ dimer have been studied by means of a combination of the complete-active-space multiconfiguration self-consistent-field (CASSCF)/CAS multireference second-order perturbation theory (CASPT2) method and coupled-cluster with single and double excitations and perturbative contribution of connected triple excitations [CCSD(T)] scheme. Reasonably good agreement with experiment has been obtained for the CCSD(T) ground-state potential curve but the dissociation energy of the only experimentally known A¹Σ _u ⁺ excited state of Mg₂ is somewhat overestimated at the CASSCF/CASPT2 level. The spectroscopic constants D _e, R _e and ω_e deduced from the calculated potential curves for other states are also reported. In addition, some spin–orbit matrix elements between the excited singlet and triplet states of Mg₂ have been evaluated as a function of internuclear separation. Received: 10 May 2001 / Accepted: 15 August 2001 / Published online: 30 October 2001     

Experimental and Theoretical Studies on the Characteristics of Atmospheric Pressure Glow Discharge with Liquid Cathode

This study investigated the characteristics of an atmospheric pressure air-glow discharge with a liquid cathode. Distilled water was utilized as the cathode. The electric field strength, gas temperature as well as the emission intensity of some N₂(C³Π_u → B³Π_g) and OH (A²Σ⁺ → X²Π) bands were measured at a discharge current ranging from 15 to 50 mA. Based upon the data obtained, the reduced electric field strength, E/N, and effective vibrational temperatures for N₂(C³Π_u, X¹Σ _g ⁺ ) and OH (A²Σ⁺) were examined. The electron energy distribution function (EEDF) and some electron parameters (average energy, electron density and rate coefficients) were obtained based on a numerical solution of the Boltzmann kinetic equation. The result showed that the EEDF was not in equilibrium and the effective vibrational temperatures for N₂(C³Π_u, X¹Σ _g ⁺ ) were essentially higher than the gas temperatures.     

Electron diffraction study of molecular structure and inversion potential for 1,1-dimethylsilacyclobutane and 1,1,3,3-tetramethyl-1,3-disilacyclobutane

The geometrical parameters for 1,1-dimethylsilacyclobiitane and 1,1,3,3-tetramethyl-13-disilacyclobutune are detemiined by gas phase electron diffraction analysis using a dynamic model, which considers ring inversion as a large-amplitude motion. The structural and potential function parameters were refined with allowance for molecular geometry relaxation estimated by quantum chemical calculations with an HF/6-311G** basis. The potential function of 1,1-dim ethylsilacyclobutane is represented as V(ϕ) = V₀[(ϕ/ϕ_e)² − 1]² with V₀ = 1.3 ± 1.2 kcallmole and ϕ_e = 29.7±4.5°, where ϕ is the ring puckering angle. A more reliable estimate for the height of the barrier (V₀ = 0.56 kcallmole) was obtained by solving the one-dimensional quantum vibrational problem and by fitting the frequency of the 0 →2 transition to the experimental value. For 1,1,3,3-tetram ethyl-1,3-disilacyclobutane, the potential function is adequately represented bx V(ϕ) = Aϕ², where A = (4.9 ± 1.8) · 10⁻⁴ kcall(mole · deg²) with a minimum coiresponding to a planar ring conformation with ϕ = 0°. The calculated structural parameters are compared with the data for related compounds. Translated fromZhumal Struktumoi Khimii. Vol. 41, No. 2, pp. 269–284, March–April, 2000     

π-Aromatic B<Subscript>16</Subscript>H<Subscript>6</Subscript>: A Neutral Boron Hydride Analogue of Naphthalene

Systematic ab inito calculations are carried out in this work to investigate the geometrical and electronic structures of B₁₆H₆ neutral and its anion B₁₆H₆ ⁻. The quasi-planar C_2v B₁₆H₆(¹A₁) with 10 delocalized π-electrons proves to be the neutral boron hydride analogue of naphthalene (D_2h C₁₀H₈). This π-aromatic neutral may be obtained from the experimentally known π-antiaromatic C_2h B₁₆ (Sergeeva et al., J. Am. Chem. Soc. 130:7244, 2008) upon hydrogenation at the six corner positions and is expected to be undistinguishable from a perfect planar D_2h B₁₆H₆ in experiments. Detailed adaptive natural density partitioning (AdNDP) analyses clearly reveal the bonding pattern of B₁₆H₆ and the calculated nucleus independent chemical shifts (NICS) strongly support its global π-aromaticity. C_2v B₁₆H₆ ⁻(²B₂) anion with one extra electron appears to have a similar quasi-planar structure with even a less severe out-of-plane distortion. Ultraviolet (UV) absorption spectrum of B₁₆H₆ and photoelectron spectroscopy (PES) spectrum of B₁₆H₆ ⁻ are simulated to facilitate their spectroscopic characterizations.     

Assignment of the vibrational bands of the B1u ← A1g transition in solid benzene films

Low temperature absorption spectra of benzene films were observed in the ¹B_1u ← ¹A_1g transition region. The origins of the two progressions of the totally symmetric vibration v₂(a_1g) are assigned to the crystal-field-induced 0—0 transition and to the false origin 0 + v_1g(e_2g).     

Large-scale configuration interaction calculations on the π-electron states of benzene

Ab initio extensive configuration interaction calculations were carried out on the π-electron states of benzene. Among the three π → π^*(e_1g → e_2u) singlet states, ¹B_2u(S₁). ¹B_1u(S₂), and ¹E_1u(S₃), the π^* orbital was found to be velence-like in S₁ and S₂, but diffuse in S₃. All three corresponding triplet states, ³B_1u(T₁) and ³B_2u(T₃), were found to be valence-like. The valence-like ¹E_2g(S₄) and ³E_2g(T₄) states were found to have significant double-excitation character, and were estimated to lie somewhat above S₃ and T₃, respectively. No low-lying S₅ and T₅ states were found. Several low-lying Rydberg states were identified.     

Raman spectroscopy of the [TeX6]2− ions (X  Cl or Br) at resonance with their lowest 3T1u and 1T1u states: Evidence for tetragonal distortion in these excited states

The observation that the v₂(e_g) band is the most enchanced Raman band at resonance with the bands assigned to both the ³T_1u ← ¹A_1g and ¹T_1u ← ¹A_1g transitions of the [TeX₆]^2? ions indicates that the ions are tetragonally distorted in these excited states. The depolarisation ratio of 2v₂ band of [TeBr₆]^2? at resonance with the ¹T_1u ← ¹A_1g transition is found to be 0.18, in close agreement with that expected (3/14) for the first overtone of a doubly degenerate vibration coupled to a triply degenerate excite state.     

Electronic absorption spectra of charge-transfer complexes and effects of substituents in radical cations

The effects of X substituents on the energies of charge-transfer bandshv _CT in electronic absorption spectra of charge-transfer complexes of π-, n-, or σ-donors (DX) with π- or σ-acceptors (A) as well as on the ionization potentialsI _D of individual DX molecules are described by the equationhv _CT(l _D)=a +bσ₁ +cσ _R ⁺ +dσ_α. When DX and A are fixed, the inductive (bσ₁), resonance (cσ _R ⁺ ), and polarization (dσ_α) contributions tohv _CT andI _D are virtually identical. The electronic structure of the D^.+X donor component of the compact [A^.−, D^.+X] radical-ionic pair in a solution is similar to that of the radical cation generated upon photoionization of the individual DX molecule in the gaseous phase. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1002–1006, June, 2000.     

Ionization potentials of ferrocene and Koopmans' theorem. Anab initio LCAO-MO-SCF calculation

The first ionization potentials of ferrocene have been computed in the LCAO-MO-SCF scheme as the difference of the total energy for the neutral molecule and the positive ion. The corresponding sequence of ionization potentials is found to be IP.(e _2g ) _1g )1u e _1g ) in good agreement with the experimental assignment. However, this is different from the sequence of orbital energies for the neutral molecule which is found to bea _1g (3d)<e _2g (-Cp)a _2u (-Cp)e _2u (-Cp)<e _2g (3d)1g (-Cp)e _1u (-Cp).It is concluded that Koopmans' theorem is not valid for the ferrocene molecule. This is traced to the different extent of the electronic rearrangement which occurs upon ionization, depending on the nature (ligand or metal) of the orbital involved in the ionization process.

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Zusammenfassung Das erste Ionisierungspotential von Ferrocen ist im Rahmen des LCAO-MO-SCF-Verfahrens als Differenz der Gesamtenergie von Molekül zu Ion berechnet worden. Dabei ergibt sich in guter Übereinstimmung mit dem Experiment die Folge I.P.(e _2g )1g )e _1u )e _1g ). Sie ist allerdings durchaus von der Orbitalreihenfolge des neutralen Moleküls, diea _1g (3d)<e _2g (-Cp)a _2u (-Cp)e _2u (-Cp)<e _2g (3d)<e _1g (-Cp)e _1u(-Cp). ist, verschieden, woraus sich die Nichtgültigkeit des Koopmans-Theorems für Ferrocen ergibt, und zwar läßt sie sich auf den unterschiedlichen Umfang der Elektronenumordnung infolge Ionisation zurückverfolgen, je nachdem, aus welchem MO die Ionisierung stattfindet.

     

Dissociative Excitation of C2H2 in the Electron Cyclotron Resonance Plasma of Ar: Production of CH(A2Δ) Radicals and Formation of Hydrogenated Amorphous Carbon Films

The dissociative excitation reaction of C₂H₂ with the electron-cyclotron resonance plasma of Ar was investigated based on the electrostatic-probe measurements and on the optical emission spectroscopy of the CH(A²Δ–X²Π) transition. The density, n _e, and the temperature, T _e, of free electrons were controlled by adding H₂O molecules externally into the reaction region, and the dependence of the CH(A²Δ–X²Π) emission intensity on the addition of H₂O was observed to compare with the evaluated dependencies based on n _e and T _e. The mechanism of production of CH(A²Δ) was found, predominantly, to be the electron impact with the contribution of 10–20% of the electron-impact dissociation of C₂H radicals; the contribution of the ion–electron recombination was negligible. Hydrogenated amorphous carbon films were fabricated using the same reaction system. The atomic compositions, Raman spectra, and the hardness of films were discussed in terms of the variations of n _e and T _e upon the addition of H₂O molecules.     

Influence of the out-of-plane distortions of nuclear configurations of molecules with then-AO of the heteroatom directed along theC 2 symmetry axis on spin-orbit coupling between thenπ*- and ππ*-states

This paper studies changes in the matrix elements (U_mp) of spin-orbit coupling between the nπ^*- and ππ^*-states, which are induced by the (“chair,” “bath”) distortions of the nuclear configurations of molecules. The analysis is performed for acridine molecules in which the n-p_z atomic orbital (AO) of the heteroatom is directed along the C₂ symmetry axis. Earlier, for molecules with a planar nuclear configuration of C_2v symmetry and with the heteroatom lying on the C₂ axis, we established the dependence of U_mp on the symmetry of ππ^*-states [Γ(ππ^*)=A₁ or B₂]. The values of U_mp differ by more than one order of magnitude; this is in line with the difference between the interconversion rate constants (K_isc; two or three orders). In this work we have found that this contrast in U_mp (and, accordingly, in K_isc) is retained when the nuclear configuration of the acridine molecule is distorted to the “chair” (AC-A) configuration, although the individuality of both molecular orbital types (nσ-MO and π-MO) and states nπ^* and ππ^* is annihilated to a certain extent. For the “bath” (AC-B) conformation the difference in U_mp considerably diminishes. Reasons for the changes in the matrix elements of spin-orbit coupling and rate constants of the S-T conversion are analyzed. The available energy level diagram is critically analyzed, and a slightly different diagam as well as a scheme of nonradiating deactivation of acridine are suggested. L. Ya. Karpov Physicochemical Scientific Research Institute. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 2, pp. 292–297, March–April, 1995. Translated by L. Smolina     

New details in the polarized spectrum of naphthalene by means of linear dichroism studies in oriented polymer matrices

Linear dichroism (LD) spectra are presented for naphthalene oriented in stretched polyethylene and polypropylene matrices at 77 K and 296 K. From the calculated spectrum LD(λ)/A(λ), where A(λ) is the corrected absorbance spectrum of the sample by unpolarized light, orientational parameters are calculated and component spectra, 235–315 nm, are resolved corresponding to polarization parallel to the long (B_3u = x) and the short (B_2u = y) axes in the molecular plane (D_2h). The orientational parameters indicate different orientational mechanisms in polyethylene and polypropylene, but the resolving procedure yields mainly identical component spectra. It is suggested that the polarization (B_3u) predominating in the 245–275 nm region isdue to a B_1g vibronic perturbation of the ¹B_2u state.     

Evidence for charge exchange between N and N2(A Σu) in a low-temperature nitrogen plasma

The emission from the first negative system, N₂⁺(B ²Σ⁺_u)→N₂⁺(X ²Σ⁺_g)+hν, is studied in the flowing nitrogen afterglow of a DC arc plasma. Investigation of the spectrum shows overpopulation of the vibrational levels 6 and 7 of the excited molecular ion, N₂⁺(B ²Σ⁺_u). Selective excitation of these levels is explained by a charge exchange reaction between atomic ions in the ground state and metastable molecules in the N₂(A ³Σ⁺_u) state. The emitted intensity of the first negative system is shown to be linear with electron density n_e for n_e>2×10¹⁶ m⁻³, a higher-order dependence exists below this value. This is consistent with population of N₂⁺(B ²Σ⁺_u) by atomic ions, N⁺.     

Multiple-layer heterometallic MnII–AgI coordination polymers constructed from [Ag 2I (CN)3]? and [AgI(CN)2]? units

Two multiple-layer heterometallic Mn^II–Ag^I coordination polymers, {Mn^II(ampyz)(H₂O)[Ag₂^I(CN)₃][Ag^I(CN)₂]·ampyz} _n (1) and {[Mn^II(benzim)₂[Ag^I(CN)₂]₂][(benzim)Ag^I(CN)]·H₂O} _n (2) where ampyz = 2-aminopyrazine and benzim = benzimidazole, have been prepared and structurally characterized. Compound 1 reveals a multiple-layer two-dimensional network with strong hexanuclear argentophilic interactions leading to an infinite three-dimensional framework. Compound 2 has an unprecedented double-layer two-dimensional squared grid-type network with (4,4) topology through Ag^I···Ag^I and π–π interactions between two adjacent squared layers. These double-layer networks of 2 are linked to others by π–π interactions, leading to a three-dimensional framework.     

Conductance of the hydroxide ion intert-butyl alcohol-water and 1,4-dioxane-water mixtures

Limiting molar conductances λ_o of potassium hydroxide in 2 to 25 mol%tert-butyl alcohol (TBA)-water mixtures were determined at 25°C as a function of pressure up to 196 MPa. λ_o’s of KOH in (2.5 to 15 mol%) 1,4-dioxane-water mixtures at 25°C and 1 atm were also determined. The excess conductance λ _o ^e of the OH^- ion estimated as [λ _o ^e (OH^-) = λ_o(KOH) - λ_o(KCl)] decreased with an increase in the TBA or dioxane content, as did the excess proton conductance λ _o ^e (H⁺) [λ _o ^e (H⁺) = λ_O(HC1) - λ_o(KCl)]. Although λ _o ^e (OH^-) is smaller than λ _o ^e (H⁺) at all solvent compositions studied, the rate of decrease in λ _o ^e with organic content is larger for the OH^- ion than for the H₃O⁺ ion in both solvent mixtures except in the water-rich region of TBA-water mixtures. λ _o ^e (OH^-) increases with pressure more strongly in TBA-water mixtures than in pure water, and the rate of increase in λ _o ^e (OH^-) with pressure has a maximum at 5 mol% of TBA. These results are discussed in terms of the difference in stability of hydrogen bonds between the OH^- or the H₃O⁺ ion and water molecules and the increase in repulsive forces due to the orientation [H-O O-H] of water molecules in the mixtures.     

Hydrothermal syntheses and crystal structures of two CuI coordination polymers: [CuI(TATP)(CN)] n and [CuI(bpy)(SCN)] n (TATP = 1,4,8,9-tetranitrogen-trisphene, bpy = 2,2′-bipyridine)

Two new Cu^I coordination polymers, [Cu^I(TATP) (CN)] _n (1) and [Cu^I(bpy)(SCN)] _n (2) (TATP = 1,4,8,9-tetranitrogen-trisphene, bpy = 2,2′-bipyiridine), have been synthesized under hydrothermal conditions and structurally characterized by elemental analysis, IR, and X-ray crystallography. In 1 and 2, the metal centers are linked by bridging CN⁻/SCN⁻ to form one-dimensional chains in the crystals and are stabilized by interchain π–π stacking interaction.     

Self-assembly luminescent complexes with [Ag2 (μ-dppm)2 (MeCN)2](SbF6)2 and tptz as components

[Ag₂(μ-dppm)₂(tptz)(MeCN)](SbF₆)₂·2H₂O·2MeCN (1), [Ag₂(μ-dppm)₂(tptz)₂](SbF₆)₂·1.75H₂O (2) and [Ag₂(μ-dppm)(tptz)₂](SbF₆)₂· 2MeCN (3) were synthesized by self-assembly with metal diphosphine [Ag₂(μ-dppm)₂ (MeCN)₂](SbF₆) ₂ and tptz as components in different molar ratios [(dppm = bis(diphenylphosphino)methane and tptz = 2,4,6-tris(2′-pyridyl)-1,3,5-triazine)] and characterized by IR spectra, elemental analysis, ¹H NMR spectra, ³¹P NMR spectra and Visible–Ultraviolet spectra. Structures of all the complexes were determined by X-ray analysis. π − π interactions were found in complex (3). Further studies show that all the complexes were of well luminescent properties both in solution and solid state.     

Characterizing the Intramolecular H-bond and Secondary Structure in Methylated GlyGlyH<Superscript>+</Superscript> with H<Subscript>2</Subscript> Predissociation Spectroscopy

We report vibrational predissociation spectra of the four protonated dipeptides derived from glycine and sarcosine, GlyGlyH⁺•(H₂)_1,2, GlySarH⁺•(D₂)₂, SarGlyH⁺•(H₂)₂, and SarSarH⁺•(D₂)₂, generated in a cryogenic ion trap. Sharp bands were recovered by monitoring photoevaporation of the weakly bound H₂ (D₂) molecules in a linear action regime throughout the 700–4200 cm^–1 range using a table-top laser system. The spectral patterns were analyzed in the context of the low energy structures obtained from electronic structure calculations. These results indicate that all four species are protonated on the N-terminus, and feature an intramolecular H-bond involving the amino group. The large blue-shift in the H-bonded N–H fundamental upon incorporation of a methyl group at the N-terminus indicates that this modification significantly lowers the strength of the intramolecular H-bond. Methylation at the amide nitrogen, on the other hand, induces a significant rotation (~110^o) about the peptide backbone.