Excited state dynamics of the11BO2 free radical

The excitation spectrum of the double-headed 00°0-00°0 band of BO₂ (A²Π_u-X²Π_g) was recorded by LIF. Special attention was paid to determine the dependence of the radiative lifetime of (00°0) A²Π state with J and the quenching by bath gases N₂, Ar, O₂. The determinations of fluorescence decay were made in real time. The mean radiative lifetime τ_r of the A²Π_3/2(00°0) state of ¹¹ BO₂ was determined to be 91 ± 4 ns (1σ).     

High Pressure — High Temperature — Synthesis and Properties of Chromium Kyanite, (Al,Cr)2SiO5

At pressures up to 30 kb and temperatures of 1000–1500°C up to 25% of the Al₃₊ in the Al₂SiO₂-polymorph kyanite could be replaced by Cr₃₊ (composition (Al_1.5Cr_0.5)SiO₅). The lattice constants a₀, b₀, c₀ increase by this substitution, whereas the triclinic angles α, β, γ remain virtually unaffected. Cr₃₊ substitutes for Al in the chains of (AlO₃₊)-octahedra parallel to c₀. The reflectance spectrum (240-1000 nm) exhibits the spin-forbidden ruby lines in addition to the spin-allowed transitions in Cr₃₊ in octahedral coordination. The half width (～3.8 kK), position, and shape of the 10 Dq band indicates a symmetry not higher than D_4h for the Cr₃₊-containing octahedra which is in accordance with structure refinement data for pure Kyanite (Burnham 1963).     

Rational Control of Conformational Distributions and Mixed‐Valence Characteristics in Diruthenium Complexes

The electronic characteristics of mixed‐valence complexes are often inferred from the shape of the inter‐valence charge transfer (IVCT) band, which usually falls in the near infrared (NIR) region, and relationships derived from Marcus‐Hush theory. These analyses typically assume one single, dominant molecular conformation. The NIR spectra of the prototypical delocalised (Class III Robin–Day mixed‐valence) complexes [{Ru(pp)Cp’}₂(μ‐C≡C?C≡C)]⁺ ([ 1 ]⁺: Cp’=Cp, pp=(PPh₃)₂; [ 2 ]⁺: Cp’=Cp, pp=dppe; [ 3 ]⁺: Cp’=Cp*, pp=dppe) feature a ‘two‐band’ pattern, which complicates band‐shape analysis using these traditional methods. In the past, the appearance of sub‐bands within or near the IVCT transition has been attributed to vibronic effects or localised d‐d transitions. Quantum‐chemical modelling of a series of rotational conformers of [ 1 ]⁺–[ 3 ]⁺ reveals the two components that contribute to the NIR absorption band envelope to be a π‐π* transition and an MLCT transition. The MLCT components only gain appreciable intensity when the orientation of the half‐sandwich ruthenium ligand spheres deviates from idealised cis (Ω P?Ru?Ru?P=0°) or trans (Ω P?Ru?Ru?P=180°) conformations. The increased steric demand of the supporting ligands, together with some underlying inter‐phosphine ligand T‐shaped CH???π stacking interactions across the series [ 1 ]⁺ to [ 2 ]⁺ to [ 3 ]⁺ results in local minima biased towards such non‐idealised conformations of the metal‐ligand fragments (Ω P?Ru?Ru?P=33–153°). Experimentally, this is indicated by appearance of multiple bands within the IR (C≡C) band envelopes and increasing intensity of the higher‐energy MLCT transition(s) relative to the π‐π* transition across the series, and the appearance of a pronounced ‘two‐band’ pattern in the experimental NIR absorption envelopes. These conformational effects and the methods of analysis presented here, which combine analysis of IR and NIR spectra with quantum‐chemical calculations on a range of energetically similar conformational minima, are expected to be quite general for mixed‐valence systems.     

Crystal Structure Determination of Thallium Carbodiimide,Tl2NCN

Orange‐red single crystals of thallium carbodiimide, Tl₂NCN, have been grown from an aqueous solution of cyanamide and thallium carbonate under strongly basic conditions. Tl₂NCN crystallizes in space group with a = 5.338(1), b = 6.626(2), c = 9.645(3) Å, α = 98.765(4)°, β = 98.685(4)°, γ = 113.178(4)°, and Z = 3; the structure can be considered a strongly distorted anti‐CdI₂ type. One finds two crystallographically different and irregular [NCN]Tl₆ octahedra in which the Tl–N distances of the three‐coordinate monovalent thallium ions lie between 2.52 and 2.72Å. The two symmetry‐inequivalent NCN^2? units adopt the carbodiimide shape, and the course of its molar volume as a function of the monovalent counter cation is analyzed.     

Quantitative millimetre wavelength spectrometry at pressures approaching atmospheric: Part I. Absolute determination of transition line strengths and sample concentrations with a confocal Fabry-Perot cavity spectrometer

The absolute determination of the millimetre wave power absorption coefficient of gas samples in a Fabry-Perot cavity whose resonant frequency is synchronised to a millimetre wavelength source is described. A theoretical treatment valid for pressures >200 Pa is developed. Absolute absorption coefficient measurements have been made on lines of SO₂ in the 50-60 GHz region that compared favourably with literature values. A calibration curve for SO₂ using the 59224.84 MHz line at 667 Pa over the concentration range 1-100% in N₂ has been produced. This technique could be especially useful for remote monitoring and process control applications because it is not necessary to scan the spectral line in order to determine the concentration of a species. The source runs at a single frequency coinciding with the peak of a millimetre wavelength (MMW) absorption line. This means that the technique could operate effectively at pressures up to and above atmospheric without the need for sophisticated MMW sources that can sweep across a wide frequency band.     

Copolymer Networks From Oligo(ε‐caprolactone) and n‐Butyl Acrylate Enable a Reversible Bidirectional Shape‐Memory Effect at Human Body Temperature

Exploiting the tremendous potential of the recently discovered reversible bidirectional shape‐memory effect (rbSME) for biomedical applications requires switching temperatures in the physiological range. The recent strategy is based on the reduction of the melting temperature range (ΔT _m) of the actuating oligo(ε‐caprolactone) (OCL) domains in copolymer networks from OCL and n‐butyl acrylate (BA), where the reversible effect can be adjusted to the human body temperature. In addition, it is investigated whether an rbSME in the temperature range close or even above T_m,offset (end of the melting transition) can be obtained. Two series of networks having mixtures of OCLs reveal broad ΔT_ms from 2 °C to 50 °C and from −10 °C to 37 °C, respectively. In cyclic, thermomechanical experiments the rbSME can be tailored to display pronounced actuation in a temperature interval between 20 °C and 37 °C. In this way, the application spectrum of the rbSME can be extended to biomedical applications.

     

Thermodynamic properties of nickel—selenium alloys

Vapor pressures of selenium in nickel—selenium alloys were determined by an isopiestic method between 450° and 1000°C and between 45 and 66.6 at% Se. Activities of selenium were evaluated according to three methods. For the NiAs-type Ni_1?x Se phase a statistical model was applied assuming random distribution of metal atoms and metal vacancies in the partially vacant (00 1/2) layers of the lattice. Good agreement between experimental and theoretical values was obtained. The interaction energy between nickel vacancies was found to be 8600 cal/g-atom.     

Spectroscopic and magnetic evidence for multiple-state emission from tris(2,2′-bipyridine) ruthenium (II) sulfate

Spectral changes of the charge-transfer photoluminescence of the title compound between 2 and 10°K have been interpreted in terms of a manifold of emitting states in thermal equilibrium. Analysis of the band shape changes by means of the Boltzmann law yields an energy gap of 10.2 cm^?1 in excellent agreement with the value obtained from published decay time measurements. A 75-kG magnetic field on the sample maintained at 1.65°K switches the emission back to that seen at 4.2°K. These results also corroborate the assignment of the lowest two states responsible for the luminescence to A₁ and E symmetry in the group D₃.     

Multiphoton laser-induced chemistry of trifluorobromomethane

The multiphoton induced chemistry of trifluorobromomethane (CF₃Br), using a pulsed TEA CO₂ laser operating in the R branch of the 00°1–02°0 transition, and the effects of pulse number, pressure, and exciting wavelength has been investigated. The photolysis products are C₂F₆, CF₄, CF₂Br₂, and Br₂. A reaction scheme is proposed to account for the observed products and their dependence on experimental conditions. The product yield are found to increase with CF₃Br pressure but to decrease with increasing added bath gas pressure (N₂ or NO). Varying the exciting wavelength caused a variation in product yields resulting in a spectrum similar to that for single photon absorption but red shifted by ≈10 cm^?1. It was also observed that at the wavelength employed ¹²CF₃Br is preferentially dissociated at pressures below 2 Torr. Finally a comparison is made between the results of this work and those done on other freons and SF₆.     

Miscibility Gap in the System Cesium Amide/Ammonia,CsNH2/NH3 — Thermodynamic Investigations

The system CsNH₂/NH₃ is investigated between —50 °C and 120 °C: Partial pressures of ammonia, DTA and density measurements are given as a function of concentration and temperature. Thermodynamic interpretations of the data show that the solutions deviate strongly from the behaviour of ideal mixtures. A monoammoniate of CsNH₂ is stable up to 17 °C. Dilute solutions (≤ 30 mole % CsNH₂) have a miscibility gap above 79 ± 5 °C with an almost colourless solution of lower density clearly separated from a yellow one. Phase separation is discussed on the basis of hydrogen bonds between NH ions as acceptors and NH₃ molecules both solvating cesium ions.     

Copolymerization of Vinyl Chloride and Sulfur Dioxide. II. Copolymerization Rates and Copolymer Compositions

The rate of copolymerization of vinyl chloride (VC) with sulfur dioxide and the composition of the poly (vinyl chloride sulfone) formed have been measured for comonomer liquid mixtures with X_VC = 0.1 to 1.0 and over the temperature range -95 to +46°C.

Polymerization was initiated by γ-irradiation (-95 to +46°C) and with the t-butyl hydroperoxide/SO₂/methanol redox system (-95 to -18°C). The copolymerization rates and copolymer compositions indicated two distinct temperature regions, with a change in mechanism around 0°C. For radiation initiation below 0°C, the rate versus comonomer composition relationship showed a maximum at an x_VC value which increased with increasing temperature. Above 0°C, the rate decreased with increasing temperature and was greatly retarded by SO₂. No high molecular weight copolymer or VC homopolymer was formed on irradiation of comonomer mixtures above ～55°C.     

Uber die Thermodynamik der Verdampfung von Scandium(III)-fluorid

On the thermodynamics of vapourization of scandium(III) fluoride The vapourization behaviour of solid ScF₃ was studied by the KNUDSEN effusion method. ScF₃ vapourizes congruently with practically stoichiometric composition in a high vacuum at temperatures from 1300 to 1600°K. Within the temperature ranges 1336–1400K and 1440–1528°K the vapourization was found to obey the equations respectively, assuming monomeric ScF3 to be the predominant gaseous species. By considering literature data for the vapourisation behaviour of AlF₃(s) and LaF3(s) the content of SC₂F₆ (g) in the saturated vapour over SeF₃ (s) was estimated to be approximately 0.3% at 1368 ° K. Second an Third Law calculations were performed using known and estimated thermodynamic data for ScF₃(g) and ScF₃(s), respectively, and the enthalpies and the enthalpies and entropies of vapourization at 298.15 ° K were obtained (data see “Inhaltsübersicht”). A. critical discussion of the experimental and theoretical results allowed to conclude that vapour pressures of ScF₃(s), which were determined by others within were found to be three to four orders of magnitude higher than the vapour pressures determined in this work where are are less probable.     

C−X Bond Activation by Palladium: Steric Shielding versus Steric Attraction

The C−X bond activation (X = H, C) of a series of substituted C(n°)−H and C(n°)−C(m°) bonds with C(n°) and C(m°) = H₃C− (methyl, 0°), CH₃H₂C− (primary, 1°), (CH₃)₂HC− (secondary, 2°), (CH₃)₃C− (tertiary, 3°) by palladium were investigated using relativistic dispersion-corrected density functional theory at ZORA-BLYP-D3(BJ)/TZ2P. The effect of the stepwise introduction of substituents was pinpointed at the C−X bond on the bond activation process. The C(n°)−X bonds become substantially weaker going from C(0°)−X, to C(1°)−X, to C(2°)−X, to C(3°)−X because of the increasing steric repulsion between the C(n°)- and X-group. Interestingly, this often does not lead to a lower barrier for the C(n°)−X bond activation. The C−H activation barrier, for example, decreases from C(0°)−X, to C(1°)−X, to C(2°)−X and then increases again for the very crowded C(3°)−X bond. For the more congested C−C bond, in contrast, the activation barrier always increases as the degree of substitution is increased. Our activation strain and matching energy decomposition analyses reveal that these differences in C−H and C−C bond activation can be traced back to the opposing interplay between steric repulsion across the C−X bond versus that between the catalyst and substrate.     

Über Ethylmethyldiphenylammoniumpolyiodide (EtMePh2N)Ix mit x = 3, 5: Darstellung und strukturelle Charakterisierung eines Triiodids (EtMePh2N)I3 und eines Pentaiodids (EtMePh2N)I5

Studies on Polyhalides. 41. On Ethylmethyldiphenylammoniumpolyiodides (EtMePh₂N)I_x with x = 3, 5: Preparation and Crystal Structures of a Triiodide (EtMePh₂N)I₃ and a Pentaiodide (EtMePh₂N)I₅ So far unknown compounds (EtMePh₂N)I_x with x = 3 and 5 have been prepared and structurally characterized. The triiodide (EtMePh₂N)I₃ crystallizes monoclinically in C2/c with a = 3406.1(3) pm, b = 893.1(1) pm, c = 1222.7(1) pm, β = 99.24(1)° and Z = 8. The crystal structure contains cationic and two kinds of anionic layers alternating along [1 0 0]. One anionic layer is composed of triiodide ions forming the typical and widespread observed herring‐bone pattern. The other one contains zigzag chains (I^– · I₂) along [0 0 1] as a so far not observed structural motif. The pentaiodide (EtMePh₂N)I₅ crystallizes triclinically in P 1 with a = 1020.7(1) pm, b = 1023.1(1) pm, c = 1269.5(1) pm, α = 81.88(1)°, β = 72.66(1)°, γ = 60.65(1)° and Z = 2. The crystal structure is divided into along [0 1 1] alternating puckered cationic and anionic layers. The anions have the common shape of a V, but are here of the rare isolated type. From a topological view two pentaiodide ions are connected to chairlike decagonal rings which are concatenated along [0 1 1].     

Wavelength dependence in photochemical vapor deposition of aluminum film using dimethylaluminum hydride

In photochemical vapor deposition of aluminum film on silicon using dimethylaluminum hydride, (CH₃)₂AlH, a surface reaction dominated below a (CH₃)₂AlH pressure of 0.3 m Torr at 200°C, which was induced only with the 160 nm band emitted from a deuterium lamp. A gas-phase reaction occurred above 0.3 mTorr at 200°C, which could be induced by both 160 nm and 240 nm emission bands from the lamp. To distinguish between surface ad gas-phase reactions, a thickness profile was used. At 240°C the surface reaction could be induced even by the 240 nm band, while the deposits formed under illumination of the two bands were thinner than those obtained with only the 240 nm band, indicating occurrence of vacuum ultraviolet (VUV)-enhanced desorption. The mechanism responsible for the observed wavelength dependence in unclear. The electrical resistivity of the films deposited at 200°C was 4.5 μΩ cm, which did not change with wavelength.     

A unique path to reach thermostable polypyrrole/Pd microfibers via chemical oxidative polymerization

Polypyrrole/palladium (PPy/Pd) nanocomposites, labeled by PPy/Pd-2/1-0, PPy/Pd-2/1-25, and PPy/Pd-3/1-0, are synthesized via a direct redox reaction between pyrrole monomer and PdCl₂ in the presence of sodium dodecyl sulfate (SDS) stabilizer in chloroform (CHCl₃)/acetonitrile (CH₃CN) binary organic solvents with 2:1 and 3:1 volume ratios at two temperatures involving 0 and 25 °C. A Pd-unloaded polypyrrole (PPy-2/1-0) is also synthesized similarly using iron(III) chloride (FeCl₃) oxidant for comparison purposes. The volume ratio of the solvents used as well as the temperature at which the oxidative polymerization takes place affects significantly the thermostability of the resulting nanocomposites. According to the thermogravimetric analyses, the stability order towards heat is found to be PPy/Pd-2/1-25?>?PPy/Pd-2/1-0?>?PPy/Pd-3/1-0?>?PPy-2/1-0. The nanocomposite PPy/Pd-2/1-25 shows clearly more thermostability compared to PPy/Pd-2/1-0 analog at temperatures above 400 °C. Furthermore, whereas three discrete maxima can be obviously found in the differential thermal analysis (DTA) thermogram of PPy-2/1-0 pure sample, no distinctive exothermic peak is observed in the curves of the three Pd-loaded nanocomposites.     

Untersuchungen zur Physisorption und Chemisorption von Wasser auf Zinkoxid-Oberflächen

The investigation of different zinc oxide samples by means of thermogravimetry and infrared spectroscopy has shown that the surface of the samples is covered by an approximately monoatomic layer of hydroxide groups. Furthermore, varying amounts of carbonate groups are found which are due to the presence of zinc hydroxide carbonate II [Zn₅(OH)₆(CO₃)₂]. Below relative water vapour pressures of p/p₀ = 0.2 (25°C), two hydrogen bridges connect one physisorbed water molecule with two chemisorbed surface hydroxide groups. In addition, about the same amount of water is physically adsorbed between vapour pressures of p/p₀ 0.2 and 0.8. At still higher relative humidity, a multimolecular layer is built up which reaches a thickness of about 200 water molecules at p/p₀ = 1.0. All samples show in the v-OH region of the IR. spectrum a broad absorption with four bands, A, B, C, and D. The position of the bands and the change of their intensities when rising the temperature of the samples up to 600°C indicate that both bands of longer wave lengths, C and D, arise from physically adsorbed water molecules, while the bands A and B are due to hydroxide groups located on the crystallographic faces (0001) and (0001 ), respectively.     

SPECTROSCOPIC STUDIES OF PURINES—I. FACTORS AFFECTING THE FIRST EXCITED LEVELS OF PURINE*

Abstract— Studies of purine absorption and emission in seven solvents differing greatly in dielectric constant and hydrogen bonding potential, reveal a variety of solvent effects. For example, the resolution of structure in the absorption spectrum, the position and/or intensity of the X₂ absorption band, the intensity of fluorescence, the magnitude of the long wave-lenth tail, and the position of the X₁ absorption band are differentially affected—in the order listed—by the solvents tested. Even though it is possible to correlate the extent of decrease in the n-π* tail with increasing solvent dielectric constant, probably alterations in all of these spectroscopic parameters depend most critically upon the ability of the various solvents to form hydrogen bonds with the hydrogen on N9 and/for with the non-bonding electrons on the purine nitrogens: it is tentatively concluded that the probability of hydrogen bonding is directly correlated with the electronegativity of the aza nitrogens (N7 > N3 > N1). In solvents like isopropanol not all of the non-bonding electrons must be solvated maximally in most purine molecules since there is appreciable fluorescence under conditions where a long wavelength tail is readily observed in the absorption spectrum (alternatively some noa-bonding electrons may not te relevant to fluorescence quenching.) Decreases in fluorescence yield are associated with red shifts in the fluorescence maximum, and in the solvents of highest polarity the fluorescence yield is again small indicating that glycerol and water can enhance radiationless tunneling—presumably by altering Franck-Condon configurations and/or improving electronic-vibrational coupling between solute and solvent. The quantum yield is uniform throughout the atsorption band for a given solvent, but studies in aqueous buffers varying from pH 1 to 11 show that the fluorescence yield is greater for charged than for neutral molecules. Further, the fluorescence excitation peak is red shifted in powders. Since phosphorescence is the predominant emission at 777deg;K and increases in fluorescence can be correlated with the presumed solvation of non-bonding electrons, the singlet excited state of lowest energy in ‘unperturbed’ purine must be n-π* in nature. The shape of the phosphorescence band and the decay lifetime of ? 1 sec at 77°K lead to the conclusion that the emitting triplet is a π-π* state. The eight vibrational structures in phosphorescence emission can be readily grouped into two progressions: there is an average separation of about 1300 cm^-1 between peaks within a given progression, and the two sets are mutually displaced by about 500 cm-^l. Individual vibrational peaks are favoured in different solvents and the whole band may be shifted up to 500 cm^-l. Even larger shifts are observed in charged purine molecules and in powders (up to 3000 cm^-l) and the presumed 0–0 band is not observed.     

Broadening and line mixing in the 20 (0)0<--01 (1)0, 11 (1)0<--00 (0)0 and 12 (2)0<--01 (1)0 Q branches of carbon dioxide: experimental results and energy-corrected sudden modeling

Using both a difference frequency spectrometer and a Fourier transform spectrometer, we have measured transitions in the 12 (2)0<--01 (1)0 band of carbon dioxide at room temperature and pressures up to 19 atm. The low-pressure spectra were analyzed using a variety of standard spectral profiles, all with an asymmetric component to account for weak line mixing. For this band, we have been able to retrieve experimental line strengths and the broadening and weak mixing parameters. In this paper we also compare the suitability of the energy-corrected sudden model to predict mixing in the two previously measured Q branches 20 (0)0<--01 (1)0, the 11 (1)0<--00 (0)0, and the present Q branch of pure CO(2), all at room temperature.