Energies of optical transitions in metal and semiconductor nanotubes

The energies of low-energy optical transitions were calculated, using the linearized attached cylindrical waves (LACW) method, as a function of inverse diameter d ^?1 for (n, n) metal nanotubes with n ranging from 3 to 12 and for (n, 0) semiconductor nanotubes with n ranging from 10 to 25. The calculations show that E ₁₁ in the metal nanotubes is higher than in the semiconductor nanotubes. Significant violations of the E ₁₁ ～ d ^?1 relationship are observed. For metal nanotubes, the situation is more complex because of the close energies of the π-π*-and σ-π* vertical transitions and because of the intersection of these characteristics in the range of 0.7 nm^?1 < d ^?1 < 1.0 nm^?1 (n = 8, 9, 10). For the semiconductor nanotubes, the E ₁₁ versus d ^?1 relationship is not linear, rather, it is oscillating; the E ₁₁(d ^?1) function alternates between two curves that refer to the (n, 0) nanotubes for which division of n by 3 gives 1 or 2 in the residue. On one hand, these features hamper the use of optical measurements in structure determination for the nanotubes; on the other, new criteria for nanotube classification appear.     

Ab-initio study of localization of an excess electron in sodium halide clusters

The equilibrium geometries of Na _n F _n and Na _n F _n?1 are optimized forn=2, 3, 4 at the SCF level. The Na _n F _n molecules appear as formed by Na⁺ and F^? ions. The paper studies the localization of the excess electron in the Na _n F _n?1 molecules. Na _n F _n?1 is obtained by removing from Na _n F _n a fluorine atom, and the excess electron takes the place of this lacking center, and allows a weak bond between the neighbouring Na atoms. Secondary minimum geometries, for which the excess electron has no defined place, are also studied.     

Photoelectron spectroscopy of silicon and germanium-fluorine binary cluster anions (SinF m − , GenF m − )

Electronic properties of silicon-fluorine and germanium-fluorine cluster anions (Si_nF _m ^? n = 1–9, m = 1–3, Ge_nF _m ^? ; n =1–9, m = 1–3) were investigated by photoelectron spectroscopy using a magnetic-bottle type electron spectrometer. The binary cluster anions were generated by a laser vaporization of a silicon/germanium rod in an He carrier gas mixed with a small amount of SiF₄ or F₂ gas. Comparison between photoelectron spectra of Si_nF^?/Ge_nF^? and Sin /Gen (n = 4–9) gives the insight that the doped F atom can remove one electron from the corresponding Sin _n ^? /Ge _n ^? cluster without any serious rearrangement of Sin/Gen framework, because only the first peak of Si _n ^? /Ge _n ^? , corresponding singly occupied molecular orbital (SOMO), disappears and other successive spectral features are unchanged with the F atom doping     

Theoretical study of H2 elimination from [B n H n + 1]− monoanions (n = 6–9, 11)

Transition states of elementary reactions of H₂ molecule elimination from [B _n H _{n + 1}]^? anions (n = 6–9, 11) in which nucleophilic/electrophilic vacancies form at boron atoms have been localized by the density functional theory method (in the B3LYP/6-311++G** approximation). For a series of [B _n H _{n + 1}]^? anions (n = 6–12), the activation barriers to H2 elimination have been compared to consider the possibility of substitution for exopolyhedral hydrogen atoms by the mechanism with the first rate-limiting stage of formation of [B _n H _{n ? 1}]^? (n = 6–12) intermediates with a vacant “bare” vertex of the boron cluster. For the [B _n H _n ]^2?, [B _n H _{n + 1}]^?, and [B _n H _{n ? 1}]^? anions (n = 6–12), the electronic chemical potential μ and Pearson hardness η have been evaluated since these characteristics make it possible to assess the propensity of different reagents to react with each other in terms of the empirical HSAB principle (soft with soft and hard with hard). The application of this principle is exemplified by the interaction of the [B₁₀H₉]^? and [B₁₂H₁₁]^? anions with acetonitrile CH₃CN, furan C₄H₄O, and 18-crown-6.     

Gas-phase solvation of protonated aliphatic amines: methyl,ethyl, n-propyl,and iso-propylamine

The thermodynamic quantities K_{n?1 n}, ΔG⁰_{n?1, n} and ΔS⁰_n?1, n for the gas phase equilibrium reactions RNH⁺₃(RNH₂)_n?1 + RNH₂ = RNH⁺₃(RNH₂)_n, where n ? 3 and R indicates an alkyl group (CH₃, C₂H₅, n-C₃H₇ and iso-C₃H₇), have been determined.     

Experimental and theoretical studies of the valence-shell dipole excitation spectrum and absolute photoabsorption cross section of hydrogen fluoride

Absolute cross sectional measurements are reported of the valence-shell dipole excitation spectrum of HF obtained from suitably calibrated high impact energy, small momentum transfer, electron energy-loss scattering intensities. Detailed assignments are provided of all prominent features observed on the basis of concomitant single- and coupled-channel RPAE calculations. The measured spectrum, obtained at an energy resolution of = 0.06 eV (fwhm) in the = 9 to 21 eV interval, includes a dissociative feature centered at = 10.35 eV assigned as X¹Σ⁺ → (1π^?14σ)A¹Π, as well as numerous strong, sharp bands in the = 13 to 16 eV excitation energy region. These bands are attributed on basis of the present calculations to Rydberg (1π^?1npπ)-valence (3σ^?14σ) mixing in X¹Σ⁺ → ¹Σ⁺ excitation symmetry, which gives rise to a long conventional progression, and to strong 1π → nsσ, moderate 1π → ndσ, and weak 1π → npσ Rydberg series in X¹Σ⁺ → ¹Π excitation symmetry. A weaker 1π → ndπ Rydberg series also contributes to the spectrum in X¹Σ⁺ → ¹Σ⁺ symmetry. The calculated and measured excitation energies and f numbers, particularly for the X¹Σ → (1π^?14σ)A¹Π, → (1π^?13pπ)B¹Σ⁺, → (1π^?13sσ)C¹Π, and → (3σ^?14σ)D¹Σ⁺ transitions, are in good quantitative accord, suggesting that the overall nature of the HF spectrum is generally clarified on basis of the present studies. Finally, tentative assignments are provided of weak features observed above the 1π^?1 ionization threshold. As in previously reported joint experimental and theoretical studies of the valence-shell spectrum of F₂, high-resolution optical VUV measurements and calculated potential energy curves aid in the assignment and clarification of the HF spectrum.     

Photoelectron spectroscopy of negative ions solvated in clusters

We have measured the photoelectron spectra of Cl^?, Br^? and I^? complexed with water molecule and of I^?·(CO₂) _n clusters wheren=1?7. The significance of these measurements to the understanding of solvation phenomena is discussed.     

Comparative study of the gas-phase bond strengths of CO2 and N2O with the halide ions

Thermodynamic data, ΔH _{n-1, n} ^o and ΔS _{n-1, n} ^o, for clustering reactions of halide ions X^?(X = F, Cl, Br, and I) with N₂Owere measured with a pulsed electron beam high-pressure mass spectrometer. In contrast to the fact that CO₂ forms a covalent bond with the fluoride ion to yield the fluoroformate ion, FCO₂ ^?, the interaction between F^? and N₂O is mainly electrostatic. It was found that the cluster ions F^? (N₂O)_n complete the first shell at n = 6, thus forming an octahedral structure. The difference between F—CO₂ ^? and F^? ... N₂O is discussed in terms of Coulombic, exchange, and charge-transfer interactions. The X^? (N₂O)₂ clusters (X = Cl, Br and I) are found to be of C_2h symmetry, while F^? (N₂O)₂ is of a twisted form and is slightly asymmetric due to a slight participation of covalency (charge transfer) in the core ion F^? ... N₂O.     

Ab-initio study of Na n F n and Na n F n−1 clusters: atypical structures

Alkali halide clusters Na _n F _n have most frequently cuboid structures and Na _n F _n?1 clusters are derived from Na _n F _n by removing a F^? centre. An excess electron localises on the site of the removed F^? ion. In this paper, other kind of clusters are studied at the SCF level: i) The non-cubic structures. ii) The cubic Na _n F _n?1 for which the excess electron has no defined site. Due to very large size of these species (at least 27 atoms!) some equivalent fictive linear and planar geometries are studied.     

Studies of cluster anions C n X− (X=N,P, As,Sb, Bi) produced by laser ablation

A series of cluster anions C _n X^? are produced from laser ablation of appropriate samples, where X is selected as a group-VB element. The recorded mass spectra of these cluster anions display a drastic even/odd alternation on ion intensities: For C _n N^?, only anions with oddn can be observed; For C _n P^? and C _n As^?, cluster anions with evenn are produced but with lower signal intensities; For C _n Sb^?, the signal intensity of clusters does not show even/odd alternation; Finally, for C _n Bi^?, the intensities of cluster anions with evenn are higher than those with oddn. This parity effect can be attributed to the linear structure of the cluster anions, and the parity reversal of C _n X^? from C _n N^? to C _n Bi^? can be explained from the electronegativity decreasing of the heteroatom X as it descends in the group. The Hückel model was applied to account the structural feature of these clusters.     

Thermal studies and spectral characterization of the chelate of bis-(η5-cyclopentadienyl titanium(IV) with salicylidene-4-methylaniline

A new chelate (η⁵-C₅H₅)₂Ti(SB)₂, whereSB=O, N donor Schiff base salicylidene-4-methylaniline, was synthesized. The course of thermal degradation of the chelate was studied by thermogravimetric (TG) and differential thermal analysis (DTA) under dynamic conditions of temperature. The order of the thermal decomposition reaction and energy of activation was calculated from TG curve while from DTA curve the change in enthalpy was calculated. Evaluation of the kinetic parameters was performed by Coats-Redfern as well as Piloyan-Novikova methods which gaven=1, ΔH=1.114 kJ·mol^?1, ΔE=27.01 kJ·mol^?1, ΔS=?340.12 kJ·mol^?1·K^?1 andn=1, ΔH=1.114 kJ·mol^?1, ΔE=20.01 kJ·mol^?1, ΔS=?342.60 kJ·mol^?1·K^?1, respectively. The chelate was also characterized on the basis of different spectral studies viz. conductance, molecular weight, IR, UV-visible and¹H NMR, which enabled to propose an octahedral structure to the chelate.     

A relativistic C.I. study on theJ=1/2 even spectrum of Sr II

We present a method for performing relativistic CI calculations in ground and excited atomic and ionic levels. An electron occupying a relativistic shellnκ in a given electronic configuration is described by a single numerical four-component Dirac-Fock orbital having the samen and κ quantum numbers to those of the shellnκ. Application of this method yields estimates for the I.P. (88 741 cm^?1) and the core correlation energy (?30916 cm^?1) for Sr II and for the total correlation energy in Sr III (?30916 cm^?1). Core-valence correlation energies for the |core 5s〉 (?4379 cm^?1), |core 6s〉 (?1191 cm^?1) and |core 13s〉 (?32 cm^?1) levels have been calculated for Sr II. Estimates for the total relativistic, Breit, vacuum polarization and self energy corrections for these levels are also reported. Configurations in which the core is not fully occupied have been found to yield significant contributions to the correlation energies of both ground and excited levels. Our results show that full scale relativistic CI calculations using numerical four-component Dirac-Fock orbitals are feasible and provide a useful ab-initio tool for the investigation of atomic properties within the framework of fully relativistic theories.     

Synthesis of boron-containing tyrosine derivatives based on the closo-decaborate and closo-dodecaborate anions

A series of new boron-containing tyrosine derivatives [B _n H _n?1O(CH₂)₄O-4-C₆H₄CH₂CH-(NH₃)COOH]^? and [B _n H _n?1O(CH₂CH₂)₂O-4-C₆H₄CH₂CH(NH₃)COOH]^? (n = 10, 12) were prepared by ring opening of the cyclic oxonium derivatives of the decahydro-closo-decaborate and dodecahydro-closo-dodecaborate anions, respectively, with ethyl N-trifluoroacetyl-L-tyrosinate.     

Proton conductivity of calix[n]arene-para-sulfonic acids (n = 4, 8)

High proton conductivity in calix[n]arene-para-sulfonic acid hydrates (n = 4, 8) reaching a value of 10^?1 Ohm^?1 cm^?1 at a relative humidity of 80% was revealed for the first time. This value is close to the record conductivity of solid proton conductors and acid water solutions. The dependence of proton transfer parameters and water quantity in the title compounds dependent on the relative humidity of air was investigated.     

Polymerization of methyl methacrylate by charge-transfer complex using n-butyl amine and carbon tetrachloride in dimethylsulphoxide

The charge-transfer complex formed by the interaction of an aliphatic amine, such as n-butylamine (nBA), and carbon tetrachloride (CCl₄) in dimethylsulphoxide (DMSO) initiates polymerization of methyl methacrylate (MMA) at 30°. The rate of polymerization is given by R_p = k[MMA]^0.83 [nBA]^0.5 [CCl₄]^0.5 when [CCl₄]/[nBA] is ? 1. When [CCl₄]/[nBA] > 1, R_p is independent of [CCl₄] and R_p = k[MMA]^1.46 [nBA]^0.5. The average rate constants are (1.42 ± 0.05) × 10^?6 1 mol^?1 sec^?1 in terms of MMA and (2.20 ± 0.06) × 10^?6 sec^?1 at 30° for higher and lower concentration of carbon tetrachloride respectively. A charge-transfer mechanism for polymerization is suggested.     

Non-isothermal decomposition kinetics of diosgenin

The thermal stability and kinetics of isothermal decomposition of diosgenin were studied by thermogravimetry (TG) and Differential Scanning Calorimeter (DSC). The activation energy of the thermal decomposition process was determined from the analysis of TG curves by the methods of Flynn-Wall-Ozawa, Doyle, ?atava-?esták and Kissinger, respectively. The mechanism of thermal decomposition was determined to be Avrami-Erofeev equation (n = 1/3, n is the reaction order) with integral form G(α) = [?ln(1 ? α)]^1/3 (α = 0.10–0.80). E _a and logA [s^?1] were determined to be 44.10 kJ mol^?1 and 3.12, respectively. Moreover, the thermodynamics properties of ΔH ^≠, ΔS ^≠, and ΔG ^≠ of this reaction were 38.18 kJ mol^?1, ?199.76 J mol^?1 K^?1, and 164.36 kJ mol^?1 in the stage of thermal decomposition.     

A Study of picosecond dehalogenation of chlorobenzene anions in liquids by positronium inhibition measurements

Radiation chemistry and results of Ps yields indicate that the following processes occur in the positron spur in solution of halogen-substituted hydrocarbons, RX_n: e⁺ + e^? → Ps, e^? + RX _n → (RX_n)^? → RX_n?1 + X^?, e⁺ + (RX_n)^? → Ps + RX_n, e⁺ + X^? → [X^?, e⁺]. Hence the trapped electron can form Ps only if (RX _n)^? is stable or has a lifetime that is longer than o comparable to the Ps formation time. Previous studies have shown that some of the strongly chlorinated benzenes (n = 4.5 give reasonable inhibition in benzene but not in linear hydrocarbons. The reason is very probably that the dechlorination time is much shorter in benzene than in saturated hydrocarbons because Cl^? is more strongly solvated in benzene than in non-aromatic hydrocarbons. To test those ideas further we have begun detailed studies of solutions of the possible “intermediate” inhibitors, viz. 1,2,3,5- and 1,2,4,5-C₆H₂Cl₄, in mixtures of C₆H₆/C₆H₁₄ different methyl-substituted benzene aniline, anisole, dioxane and ethylbenzene. The results are discussed and interpreted in terms of the spur model. The Ps inhibition efficiency of the two isomeric forms of tetrachlorobenzene studied, appears most probably to depend on intramolecular electron transfer with subsequent dehalogenation of the molecular anion on a picosecond timescale. The divergence in inhibitor efficiency obtained for the chlorobenzenes when dissolved in aromatic solvents compared to the same solutes when dissolved in a saturated alkane appears most probably to be caused by complex formation between the initially formed chlorobenzene anion and benzene molecules, which permits a rapid relaxation of the molecular anion with subsequent bond stretching and expulsion of the chloride anion.     

Paramagnetic centers related to Al,Sc, In,and Nb impurities in KTiOAsO<Subscript>4</Subscript>crystals

Electron paramagnetic resonance (EPR) is applied to study Al-, Sc-, In-, and Nb-doped KTiOAsO₄ (KTA) crystals. Paramagnetic hole centers O^? are observed after ionizing irradiation of KTA crystals. These centers are, as a rule, unstable at room temperature and are slowly annealed for about two weeks. Oxygen ions are bridging two cations in KTA. Near the impurity, two p-orbitals of oxygen atoms participate in covalent bonding with cations, whereas the third p-orbital remains free and under the radiation effect captures the hole thus forming the paramagnetic center of M ⁿ⁺-O^?-M^(n?1)+ (here M ⁿ⁺ is the lattice cation and M^(n?1)+ is the impurity cation of Al, In, Sc, or Nb). In the centers investigated the specific principal direction of the g-factor g ～ 2 is normal to the M ⁿ⁺-O^?-M^(n?1)+ plane, and the main value of g _max falls in this plane. The direction of the O^?-M^(n?1)+ bond is close to the selected direction of the hyperfine interaction with the impurity ion. The models of six hole centers and the found parameters of EPR spectra are discussed.     

Polyene spectroscopy: Vibronic evidence for a forbidden transition in DECA-2,4,6,8-tetraene

The dimethylpolyene deca-2,4,6,8-tetraene was studied by absorption, fluorescence excitation and fluorescence spectroscopy in glasses at 77 K and in n-alkane crystals at 4.2 K. A strong transition to a ¹B_u excited state is observed with an origin at 32400 cm^?1 in isopentane at 77 K and at 31280 cm^?1 in n-undecane at 4.2 K. A weak transition to a ¹A_g excited state is observed with an origin at 28738 cm^?1 in the n-undecane matrix. The radiative fluorescence lifetime is 500 ns. In undecane the transition from the ground state to the ¹A_g excited state exhibits a classic Herzberg—Teller vibronic pattern indicating a symmetry forbidden transition.     

An extraction-spectrophotometric determination of palladium with 4-(2-pyridylazo)resorcinol in the presence of diphenylguanidine

A new, sensitive spectrophotometric determination of palladium has been developed, based on the extraction of the red Pd(II) chelate with 4-(2-pyridylazo)resorcinol in the presence of N,N′-diphenylguanidine into n-butanol; the sensitivity of the method according to Sandell is S = 1.12 μg cm^?2, ?₅₃₀ = 9.4 × 10⁴ liters mol^?1 cm^?1, and palladium can be determined at concentrations from 0.21 to 1.91 μg ml^?1.