Investigation of Electron Behavior in Nano‐TiO2 Photocatalysis by Using In Situ Open‐Circuit Voltage and Photoconductivity Measurements

The in situ open‐circuit voltages (V_oc) and the in situ photoconductivities have been measured to study electron behavior in photocatalysis and its effect on the photocatalytic oxidation of methanol. It was observed that electron injection to the conduction band (CB) of TiO₂ under light illumination during photocatalysis includes two sources: from the valence band (VB) of TiO₂ and from the methanol molecule. The electron injection from methanol to TiO₂ is slower than that directly from the VB, which indicates that the adsorption mode of methanol on the TiO₂ surface can change between dark and illuminated states. The electron injection from methanol to the CB of TiO₂ leads to the upshift of the Fermi level of electrons in TiO₂, which is the thermodynamic driving force of photocatalytic oxidation. It was also found that the charge state of nano‐TiO₂ is continuously changing during photocatalysis as electrons are injected from methanol to TiO₂. Combined with the apparent Langmuir–Hinshelwood kinetic model, the relation between photocatalytic kinetics and electrons in the TiO₂ CB was developed and verified experimentally. The photocatalytic rate constant is the variation of the Fermi level with time, based on which a new method was developed to calculate the photocatalytic kinetic rate constant by monitoring the change of V_oc with time during photocatalysis.     

The Pentagonal Column and the ReO3-Type Structure

The pentagonal column (PC), consisting of MX₇ bipyramids sharing their equatorial edges with five MX₆ octahedra, is a structural feature frequently occurring in association with frameworks of the ReO₃-type, where it enables accomodation of an anion deficiency. The present article discusses the geometry of the PC and its compatibility with the ReO₃-type structure, which naturally emerges from the finding that a PC may result from twinning of an ReO₃-type structure on the unit cell level. A new symbolism is presented which facilitates visualization of a large variety of crystal and defect structures of MX_3–x stoichiometry. This symbolism may also serve as a convenient aid in deriving models of possible new structures that may be of value in the interpretation of HREM images.     

Low temperature luminescence spectra of the d10s2 complexes Cs2MX6 (M = Se,Te and X = Cl,Br). The Jahn—Teller effect in the Γ−4(3T1u) excited state

The emission spectra of the title compounds in microcrystalline form have been measured at 10 K. The extensive vibrational progression in the e_g mode is indicative of a tetragonal Jahn—Teller distortion in the Γ^?₄(³T_1u) excited state. The vibronic coupling of a threefold electronic state with a doubly degenerate e_g mode (T—e coupling), linear in the nuclear coordinates, has been reinvestigated considering spin—orbit coupling up to second order perturbation on energy levels which result from an a¹_1gt¹_1u electron configuration. For an estimation of Jahn—Teller coupling constants, the intensity distributions in the progressions were compared with the theoretical line shape functions which were obtained from a model which also permits the determination of potential energy minima and vibrational fundamentals of the excited state. The unusually large increase in the e_g vibrational frequency compared to the ground state is due to Jahn-Teller forces which distort the potential surface, yielding steeper excited state energy curves.     

Thermal studies of hypervalent iodine reagents

The use of unconventional synthesis methods in the formation of CaO·2Al₂O₃ (CA₂) is justified because it reduces the formation temperature of the compound. CA₂ is formed by classical method at temperatures above 1,400 °C. The polymeric precursor method allows a significant temperature decrease in CA₂ synthesis reaching temperatures of 1,000 °C. This paper deals with CA₂ synthesis by “citrate” method which is often presented as Pechini method, starting from a mixture of citric acid, ethylene glycol and calcium, and aluminum nitrates. A method based on the formation of a polymeric precursor was also used, starting from a mixture of acrylic acid and nitrates of calcium and aluminum. The results showed a net difference in favor of samples obtained from acrylic acid, which by annealing at 800 °C for 1 h, contain pure CA₂. The samples obtained from citric acid, after annealing at 800 °C are amorphous. After annealing at 900 °C in all samples CA₂ is single phase.     

Quinolinic acid as a reagent for the gravimetric and volumetric estimation of palladium

Summary Like -amino-picolinic acid, quinolinic acid behaves as a highly selective reagent for palladium with which it forms a complex of composition Pd(C₇H₄O₄N)₂. The complex being very stable is precipitated from solutions with an acidity of 0.25 N to a maximum p_H of 2.1 and thus is separated from all the common ions and other platinum metals except copper which, however, can be kept in solution in presence of EDTA. The palladium complex is either weighed after drying or dissolved in a standard potassium cyanide solution which is later back titrated with silver nitrate solution.     

Photoassisted Oxygen Reduction Reaction in H2–O2 Fuel Cells

The oxygen reduction reaction (ORR) is a key step in H₂–O₂ fuel cells, which, however, suffers from slow kinetics even for state‐of‐the‐art catalysts. In this work, by making use of photocatalysis, the ORR was significantly accelerated with a polymer semiconductor (polyterthiophene). The onset potential underwent a positive shift from 0.66 to 1.34 V, and the current was enhanced by a factor of 44 at 0.6 V. The improvement was further confirmed in a proof‐of‐concept light‐driven H₂–O₂ fuel cell, in which the open circuit voltage (V_oc) increased from 0.64 to 1.18 V, and the short circuit current (J_sc) was doubled. This novel tandem structure combining a polymer solar cell and a fuel cell enables the simultaneous utilization of photo‐ and electrochemical energy, showing promising potential for applications in energy conversion and storage.     

Effect of molecular entanglements on craze microstructure in glassy polymers

Thin films of ten glassy polymers are bonded to copper grids and strained in tension to produce crazes, which are then examined in the transmission electron microscope. The average craze fibril extension ratio λ for each polymer is determined from microdensitometer measurements of the mass thickness contrast of the crazes. The extension ratio λ is found to increase approximately linearly with the chain contour length l_e between entanglements, as determined from melt elasticity measurements of the entanglement molecular weight of these polymers. These results are analyzed by comparing them with λ_max, the maximum extension ratio of an entanglement network in which polymer chains neither break nor reptate (i.e., permanent entanglement crosslinks are assumed). The values of λ_max are given by l_e/d where d, the entanglement mesh spacing in the unoriented glass, is computed from d = k(M_e)^1/2 with k determined either from small-angle neutron scattering results on isolated chains in the glass or from coil size measurements in dilute solutions of a θ solvent. The craze extension ratios fall somewhat below λ_max at low λ but increase to well above λ_max for polymers with high l_e. This comparison suggests a significant contribution due to chain breakage (or reptation) in the higher-λ crazes of large-l_e polymers, which may arise from the higher true stresses in the craze fibrils (which for a given applied stress increase proportionally to λ). The results also imply that a useful way to increase the “brittle” fracture stress and decrease the ductile-to-brittle transition temperature of a glassy polymer is to decrease its entanglement contour length l_e.     

Studies on the reduction and reductive alkylation of amino acid-derived spirocyclic 2,6-dioxopiperazines

The regio- and diastereoselective reduction and reductive alkylation of 3-spiro-2,6-dioxopiperazines are described via a two-step process, which involves addition of NaBH₄ or Grignard reagents, followed by TFA-mediated dehydration with a second NaBH₄ addition. The results show that the reactivity of 2,6-dioxopiperazines is limited by their steric hindrance and by the volume of the nucleophile, which preferably add to the C₆ carbonylic carbon with complete diastereoselectivity. The diastereoselectivity of the first step is mainly governed by electronic factors, which direct the addition of the nucleophile from the most hindered face, while in the second step, the NaBH₄ attacks from the less crowded face. This second step proceeds with partial or complete racemization.     

Vibrational relaxation of CO2(00°1) and N2O(00°1) by NO

The far-from-resonance transfers and the de-excitation processes in CO₂-NO and N₂O-NO systems have been studied by measuring fluorescence decay rate constants of CO₂ or N₂O excited to the (00°1) level by laser radiation. The diagrams giving the variations of these rate constants versus the molar fraction of CO₂ or N₂O have been set out. From these diagrams, the relative importance of the V-V transfer and V-T de-excitation rate constants is discussed. The transfer rate constants have been calculated from a semiclassical theory in which the interaction potential is a sum of four atom-atom Morse potentials. The disagreement observed between calculated and experimental values probably results from the attractive multipolar forces which the theory does not take into account.     

Darstellung und Eigenschaften von Phosphanboranen polyfunktioneller Phosphanliganden; Kristallstrukturen des Bis(boranatodiphenylphosphonio)methans und des Tetrakis[(boranatodiphenylphosphonio)methyl]methans

Preparation and Properties of Phosphane-Boranes Derived from Polyfunctional Phosphanes; Crystal Structures of Bis(boranatodiphenylphosphonio)methane and Tetrakis[(boranato-diphenylphosphonio)methyl]methane Bis(boranatodiphenylphosphonio)methane has been prepared in pure form from Ph₂PCH₂PPh₂ (dppm) via two synthetic routes and its crystal structure determined. The molecule adopts a conformation very similar to that of the parent dppm ligand, which reflects the influences of the repulsive charges in the functional groups. Analogous phosphine-boranes were obtained from 1,1,1-tris(diphenylphosphinomethyl)ethane and tetrakis(diphenylphosphinomethyl)methane. The crystal structure has also been determined for the tetra-borylated derivative of the latter. A highly symmetrical arrangement has been found which corresponds to a relaxed conformation of the four dipolar groups. From C(CH₂PPh₂)₄ and H₂BBr · SMe₂ an ionic, spirobicyclic Product was obtained, which resembles the monocyclic analogues previously described in its Properties and spectroscopic data.     

Determination of aluminium in bulk coal samples by neutron activation analysis

The determination of Al₂O₃, in bulk coal samples to an accuracy of about 0.2% Al₂O₃ has been achieved by means of a thermal-neutron activation technique based on the reaction ²⁷Al(n,γ)²⁸Al. In the analysed samples, which had widely different compositions, the Al₂O₃ concentrations ranged from 1 to 11% and the ash contents from 7 to 40%. Al₂O₃ concentrations measured by x-ray fluorescence showed a linear relationship with both the 1.78-MeV γ-ray count following the decay of ²⁸Al and the thermal-neutron count near the samples during irradiation. The linear relationship, which was obtained by regression analysis of the experimental data, determined the Al₂O₃concentrations with a standard deviation of 0.24% Al₂O₃. The particle sizes in the samples ranged from —0.5 to —40 mm, the moisture contents ranged from 1 to 6%, and the sample weights ranged from 8 to 11 kg. However, inclusion of these parameters in the regression analysis did not significantly improve the results for Al₂O₃.     

A highly variable opto-acoustic phase angle within the So → S1 band of glyoxal

The S₀ → S₁ transition in the opto-acoustic spectrum of glyoxal vapor exhibits a variable phase angle which follows the pattern of absorption intensity. This unusual phenomenon results from a quenching of T₁ by S_o to form a long-lived intermediate, M, which then produces heat via an M + M reaction.     

From Surface‐Inspired Oxovanadium Silsesquioxane Models to Active Catalysts for the Oxidation of Alcohols with O2—The Cinnamic Acid/Metavanadate System

Silsesquioxane dioxovanadate(V) complexes were investigated with respect to their potential as a catalyst for the oxidative dehydrogenation of alcohols with O₂ as an oxidant. The turnover frequencies determined were comparatively low, but during the oxidation of cinnamic alcohol an increase in activity was observed in the course of the process, which was inspected more closely. It turned out that during the oxidation of cinnamic alcohol, not only was the aldehyde formed but also cinnamic acid, which in turn reacts with the silsesquioxane complex employed to give NBu₄[O₂V(O₂CC₂H₂Ph)₂], which can also be obtained from NBu₄VO₃ and cinnamic acid and represents a far more active catalyst, not only for cinnamic alcohol but also for other activated alcohols and hydrocarbons. The rate‐determining step of the conversion corresponds to an hydrogen‐atom abstraction from the C? H units, as shown by the determination of the kinetic isotope effect in case of 9‐hydroxyfluorene, and the reoxidation of the reduced catalyst proceeds via a peroxo intermediate, which is also capable of oxidizing one alcohol equivalent. Furthermore the influence of the organic residues at the carboxylate ligands on the catalyst performance was investigated, which showed that the activity increases with decreasing pK_s value. Moreover, it was found that during the oxidation the catalyst slowly decomposes, but can be regenerated by addition of excessive carboxylic acid.     

ESR study of the photo-induced decomposition of polypropylene in the presence of N,N,N′,N′-tetramethyl-p-phenylenediamine

The mechanism of uv (λ > 325 nm) photodegradation of polypropylene (PP) containing N,N,N′,N′-tetramethyl-p-phenylenediamine (T₄MPD) has been investigated by means of ESR spectroscopy. The observed spectra after uv irradiation of both isotactic-PP (IPP) and stereoblock-PP (SPP) samples in vacuum at 77 K consisted principally of a broad singlet which was assigned to a T₄MPD cation radical (T₄MPD^+·). On the other hand, the spectrum observed after irradiation of an atactic polypropylene (APP) sample at 77 K in vacuum was resolved into several components which decayed almost up to ca. 263 K to give rise to the broad singlet of T₄MPD^+·. One component was a sharp quartet which was assigned to a methyl radical, ·CH₃·. The other component, a singlet, was attributed to a trapped electron, e_t^?.By comparison of the ESR spectrum of deuterated T₄MPD with that of the normal compound it was found that 60 ～ 70% of the methyl radicals arose from the added T₄MPD due to β-scission, which also formed the N,N,N′-trimethyl-p-phenylenediamine radical, T₃MPD·. The T₃MPD· radical presumably captures an electron at lower temperatures to become a carbanion, T₃MPD^?, which releases the electron to reproduce the T₃MPD· radical at elevated temperatures. This production of the radical T₃MPD· due to the liberation of an electron provides an explanation for the observed increase in intensity of the decay curve in the temperature range from ? 168 K to 185 K. The remaining fraction, 30 ～ 40%, of the total methyl radicals was produced from the PP matrix by an energy transfer from the excited T₄MPD¹ to the PP matrix. The broad singlet which appeared in the temperature range near 195 K was attributed to an acyl radical ～CH₂CH(CH₃)CH₂?O from the observed g-value. By photoillumination of this sample this broad singlet was converted reversibly into the quartet which was assigned to the radical ～CH₂CH(CH₂·)CH₂CHO.     

Applications of charge stripping/charge exchange spectra of organic ions

The application of a new technique which involves the combination of charge stripping and charge exchange processes has been investigated. Ions are charge-stripped in the second field-free region of a triple-sector mass spectrometer (BEE geometry), and then subjected to a charge exchange reaction in the third field-free region. The resulting charge stripping/charge exchange (CS/CE) spectrum is free from interference, which is otherwise common in charge stripping spectra. Comparisons between charge stripping and CS/CE spectra are made in cases where both kinds of spectra are obtainable. In order to assess the applicability of this new technique to studies of isomeric ion structures, species for which charge stripping spectra have previously been unobtainable have been chosen. CS/CE spectra of [C₆H₆]⁺˙, [C₆H₅]⁺, [C₈H₁₀]⁺˙ and [C₇H₇O]⁺ ions from a variety of precursors are recorded: in most cases, sufficient differences are observed to permit distinction between isomeric structures (or mixtures of structures). Previous studies which have shown that stable doubly charged molecular ions of ethane cannot be formed by the charge stripping technique are confirmed from its CS/CE spectrum.     

Double phosphate crystallization in phosphate liquids

The discussion concerns the interactions of oxides of metals in groups III–VIII not with individual molten phosphates but with mixtures of them, which provides a range in M₂O:P₂O₅ in the phosphate liquid from 0.5 to 2.0; this constitutes a new approach to these reactions, and it enables one to determine unambiguously the ranges in which double phosphates are formed and to define the conditions under which the individual crystals are produced from those liquids.Kiev University. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 27, No. 3, pp. 307–311, May–June, 1991. Original article submitted February 19, 1991.     

Mass spectral and atomic absorption studies of selenium vaporization from a graphite surface

Selenium desorbs from a graphite surface primarily as SeO₂ which dissociates in the gas phase to form atomic Se with an appearance temperature of 1175 K in atmospheric GFAAS. Mass spectrometry studies in vacuum show that the SeO₂ desorption occurs at 425 K without the addition of any modifiers and that elemental Se does not desorb from the heated graphite flat.Vacuum MS studies show that the addition of Ni delays the appearance temperature of a significant amount of SeO₂ to 850 K. The SeO₂ lost at this temperature does not contribute to the Se absorbance signal which has a delayed appearance in atmospheric GFAA of 1575 K with the addition of Ni. Above 850 K, the remaining SeO₂ is attached to NiO which has formed on the surface. Then, NiO is reduced by the carbon forming CO and CO₂ which desorb at 1200 K while elemental Ni remains in the carbon. At the same time, SeO₂ is released from NiO and desorbs from the graphite in vacuum as SeO₂, SeO, Se and Se₂. Some reduction of SeO₂ on the surface has occurred at this high temperature. These Se species dissociate in the high temperature gas phase atmosphere of GFAA forming elemental Se which is then detected as a delayed Se AA signal. Much of the SeO₂ desorbed at low temperatures can be lost through diffusion from the furnace even when Ni is added, if the gas phase is not preheated.XPS and SEM studies confirm the existence of NiO on the surface at 1100 K with the remaining Se dispersed in the Ni so that it is not observable. Elemental Ni desorbs from the graphite at 1675 K. in vacuum MS, corresponding to the Ni signal observed in atmospheric GFAA. Following high temperature cleaning steps, elemental Ni is detected by XPS and MS to remain in the graphite. This amount of Ni is not significant enough to contribute to the Ni AA signal, nor does the elemental Ni affect the Se AA signal.An oxygenated surface, like that produced by the addition of Ni, also causes the appearance of the Se AA signal in atmosphere to be delayed to a higher temperature. The signal is enhanced, although not to the extent of that with the addition of Ni. SEM data show SeO_2(s,1) congregating at active sites on the oxygenated surface following a dry step. Under vacuum conditions, the desorption of SeO₂ from an oxygenated surface was not delayed. This indicates that atmospheric pressure is necessary to allow mobile SeO₂ species to move to oxygenated active sites which then delay the release. The mobile SeO₂ species are desorbed from the surface under vacuum conditions before reaching the active sites.     

Synthesis of triphenylphosphonopropionbetainetriorganotin(IV) salts [(C6H5)3P(CH2)2CO2SnR3]+X−, by nucleophilic displacement of anions from triorganotins

Trimethyl- and triphenyl-tin(IV) hydroxide act on triphenyl(2-carboxyethyl)-phosphonium hydrochloride, which is made from 3-chloropropionic acid and triphenylphosphine, to release water in the presence of dimethylformamide (DMF) as a catalyst. The water is azeotropically distilled to drive the reaction forward and produce triphenylphosphonopropionbetainetrimethyl- and triphenyl-tin(IV) chlorides in high yield. The latter product also results from the displacement of chloride from triphenyltin(IV) chloride by the phosphobetaine, (C₆H₅)₃P(CH₂)₂CO₂, which is made by treating the phosphonium hydrochloride with bicarbonate, and the compounds [(C₆H₅)₃P(CH₂)₂CO₂Sn(C₆H₅)₃]⁺X^? where X = Cl, Br, I, N₃, NCS, NO₃, B(C₆H₅)₄ and Co(CO)₄ are made in the same way. The acetate salt results from metathesis from the chloride and lead(II) acetate. A double salt, [(C₆H₅)₃P(CH₂)₂CO₂SnR₃]⁺ [R₃SnX₂]^?, is formed for X = Cl, Br and N₃ by adding additional (C₆H₅)₃SnX to the already-formed simple salts. Double salts are also obtained from the $\text{1}\text{1}$ reactions between the phosphobetaine and triphenyltin(IV) isocyanate and methyldiphenyltin(IV) chloride. The phosphonium chloride double salt could be converted to the thiophosphonium derivative by heating with elemental sulfur in ethanol. The products of these novel nucleophilic displacement reactions are high melting solids. Tin-119m Mössbauer data are consistent with five-coordinated, triorganotin(IV) formulations with the exception of the diphenyl(8-hydroxyquinolinato)tin(IV) chloride salt in which the tin atom is six-coordinated, and the diphenyltin system cis-oriented. The parameters otherwise do not change with the nature of the X group, which is the tetracarbonylcobaltate derivative is tetrahedral by infrared, establishing the ionicity of the products. The chloride exhibits a molar conductivity indicative of a $\text{1}\text{1}$ electrolyte in DMF. A bridging acetate structure in the solid is consistent with the lowered ν(CO₂) frequencies. The Mössbauer spectra of the double salts give simple doublets of lowered isomer shift (IS) and raised quadrupole splitting (QS) which may arise from a cross-linking ion pairing of the polymer chains in the solid, and the NMR spectra of the two methyltin derivatives shows only a single resonance line and tin satellites which is rationalized by a dynamic exchange process. The products are formulated as associated in carboxylate polymers with dangling triphenylphosphonium cations.     

A McLafferty rearrangement in an even-electron system: C3H6 elimination from the α-cleavage product of tri-n-butylamine

It is shown by deuterium labelling, linked-scan measurements and collision activation that the [M-C₃H₇˙]⁺ (α-cleavage) ion in the electron impact ionization spectrum of tributylamine loses C₃H₆ with transfer of one hydrogen specifically from the γ-position. The experimental data point towards a mechanism which involves the intermediate formation of a distonic diradical ion from an excited α-cleavage ion which then eliminates the neutral alkene.     

Poly[bis(μ2‐2‐aminopyrazine‐κ2N1:N4)(μ2‐nitrato‐κ2O:O)(nitrato‐κ2O,O′)disilver(I)]: an achiral two‐dimensional coordination polymer forming chiral crystals

The solution reaction of AgNO₃ and 2‐aminopyrazine (apyz) in a 1:1 ratio gives rise to the title compound, [Ag₂(NO₃)₂(C₄H₅N₃)₂]_n, (I), which possesses a chiral crystal structure. In (I), both of the crystallographically independent Ag^I cations are coordinated in tetrahedral geometries by two N atoms from two apyz ligands and two O atoms from nitrate anions; however, the Ag^I centers show two different coordination environments in which one is coordinated by two O atoms from two different symmetry‐related nitrate anions and the second is coordinated by two O atoms from a single nitrate anion. The crystal structure consists of one‐dimensional Ag^I–apyz chains, which are further extended by μ₂‐κ²O:O nitrate anions into a two‐dimensional (4,4) sheet. N—H...O and C_apyz—H...O hydrogen bonds connect neighboring sheets to form a three‐dimensional supramolecular framework.