Laser excited S2 → S1 and S1 → S0 emission spectra and the S2 → Sn absorption spectrum of azulene in solution

We present the S₁ → S₀ fluorescence spectrum, between 740 and 940 nm, of azulene solutions (10^?3 M in methanol) excited with a Q-switched ruby laser. The nitrogen-laser excited S₂ → S₁ fluorescence spectrum, between 700 and 930 nm, is also reported. The transient S₁ → S_n spectrum between 500 and 650 nm was studied, using synchronous nitrogen laser and dye laser excitation. The S₅ (¹B₁(3)) state of azulene was found to be located at 45500 cm^?1 and the cross section σ₂₅ of the transient absorption S₂ → S₅ is estimated to be 3 × 10^?18 cm²/molecule.     

Aflatoxin B1 and M1 in milk

The aflatoxin M₁ (AFLAM₁) is a mycotoxin that results from the hydroxylation of the aflatoxin B₁ (AFLAB₁). It contaminates the milk of animals fed with a diet containing its precursor. In this work, we determined the occurrence of AFLAB₁ and AFLAM₁ in milk, as well as the chromatographic conditions to quantify these mycotoxins. The extraction and quantification of AFLAB₁ and AFLAM₁ in naturally contaminated and artificially spiked milk samples which are produced and marketed in the state of RS were performed using the AOAC official method and UHPLC with fluorescence detection. We obtained a separation factor of 2.3 for AFLAB₁ and AFLAM₁ using a mobile phase consisting of 1% acetic acid:acetonitrile:methanol (55:10:35). The analytical curves had a wide linearity range and the limit of quantification (LOQ_m) concentrations of AFLAB₁ and AFLAM₁ were equal to 0.5 and 0.25 μg L⁻¹, respectively. Samples of pasteurized and ultra-high-temperature processed (UHT) milk showed natural contamination, and the levels for both aflatoxins ranged from 0.7 to 1.5 μg L⁻¹. Raw and concentrated milk samples only contained AFLAM₁, with a maximum average concentration of 1.7 μg L⁻¹. These concentrations, higher than permitted by legislation, confirm the existence of a health risk, as well as highlight the relevance of searching for alternatives to reduce this contamination.     

Fundamentals of lead acid cells part V. The oxidation of porous PbSO4/PbO2 on a pure lead base in sulphuric acid

Potentiostatic experiments are described in which porous electrodes of PbO₂ containing PbSO₄ are oxidised to PbO₂ in 5 M H₂SO₄. The process is analogous to the constant potential recharging of a lead acid cell. The form of the current transient depends upon the balance of PbSO₄ and PbO₂. For relatively small amounts of PbSO₄ a simple falling transient is obtained. For larger amounts the transient becomes more complicated and may be deconvoluted using a novel computer technique. Processes corresponding to various modes of electrocrystallisation are identified. The behaviour of newly prepared electrodes is compared and contrasted with the behaviour of electrodes stabilised by repeated cycling in 5 M H₂SO₄.     

Structure and electrical conductivity of the ferrocuprate solid solutions Sm1/2Gd1/2BaCuFeO5.05, Gd1/2Yb1/2BaCuFeO5.02 and Ho1/3Yb2/3BaCuFeO5.05

The unit cell parameters of the crystal structures of the ferrocuprate solid solutions Sm_1/2Gd_1/2BaCuFeO_5.05, Gd_1/2Yb_1/2BaCuFeO_5.02, and Ho_1/3Yb_2/3BaCuFeO_5.05 in air in the temperature range 293–1093 K have been determined by X-ray powder diffraction, and their electrical conductivity has been studied. The structural characteristics of layered ferrocuprates are determined by the size of rare-earth cations in their structures, whereas the conductivity of ferrocuprates depends on the electronic configuration of these cations. Original Russian Text ? A.I. Klyndyuk, 2007, published in Zhurnal Neorganicheskoi Khimii, 2007, Vol. 52, No. 3, pp. 370–374.     

Photolysis of ozone in the ultraviolet region. Reactions of O(1D), O2(1Δg) and O≠2

Reactions of O(¹D) and O₂(¹Δ_g) with ozone have been observed time resolved by the detection of the product O³P) and their rate constants have been determined. It is found that vibrationally excited molecular oxygen, O^≠₂, also produces O(³P) in reaction with ozone. These observations are supported by the results of quantum yield determinations of the ozone decomposition in UV-photolysis.     

Singlet methylene kinetics: Direct measurements of removal rates of ã1A1 and b̃1B1 CH2 and CD2

Rate constants for collisional removal of ã¹A₁ and b?¹B₁ CH₂ and CD₂ have been directly measured, using IR laser induced multiple photon dissociation to prepare the radicals, and time resolved laser induced fluorescence to observe them. For CH₂(ã¹A₁) removal by He, Ne, Ar, Kr, Xe, N₂, H₂, O₂, CO and CH₄, rate constants of 3.1, 4.2, 6.0, 7.0, 16, 8.8, 130, 30, 56 and 73 × 10^?12 cm³ molecule^?1 s^?1 were found respectively. These represent significant increases over the previously accepted values. Essentially no isotope effect is observed in the removal of CD₂(ã¹A₁) by the rare gases. The rate determining step in removal by the rare gases and N₂ is thought to be singlet—triplet intersystem crossing controlled by long range attractive forces, and the results are discussed in terms of both isolated and mixed state theoretical models of these processes. For the other molecular collision partners, bimolecular chemical removal channels are possible, and may account for the relatively fast rates observed. Radiative lifetimes of five Σ vibronic levels of CH₂(b?¹B₁) and three Σ vibronic levels of CD₂(b?¹B₁) have been measured and found to lie in the range 2.5–6.0 μs, and collisional quenching rates for CH₂(b?¹B₁) are found to be of the order of the gas kinetic collisional frequency.     

Threshold energies for production of CH2(3B1) and CH2(1A1) from ketene photolysis. The CH2(3B1) ↔ CH2(1A1) energy splitting

By measuring the relative CO quantum yields from ketene photolysis as a function of photolysis wavelength we have determined the threshold energy at 25° for CH₂CO(¹A₁) → CH₂(³B₁) + CO(¹Σ⁺) to be 75.7 ± 1.0 kcal/mole. This corresponds to a value of 90.7 ± 1.0 kcal/mole for ΔH_f298⁰[CH₂(³B₁)]. By measuring the relative ratio of CH₂(¹A₁)/CH₂(³B₁) from ketene photolysis as a function of photolysis wavelength we have determined the threshold energy at 25°C for CH₂CO(¹A₁) → CH₂(¹A₁) + CO(¹Σ⁺) to be 84.0 ± 0.6 kcal/mole. This corresponds to a value of 99.0 ± 0.6 kcal/mole for ΔH_f298⁰[CH₂(¹A₁)]. Thus a value for the CH₂(³B₁) ? CH₂(¹A₁) energy splitting of 8.3 ± 1 kcal/mole is determined, which agrees with three other recent independent experimental estimates and the most recent quantum theoretical calculations.     

The Solid Solution Series of the Sulfate Apatite System Na6.45Ca3.55(SO4)6(FxCl1−x)1.55

The crystal structures of 11 synthetic Na-Ca-sulfate-apatites, Na_6.45Ca_3.55(SO₄)₆(F_xCl_1−x)_1.55 with x=1 to 0, were refined using X-ray diffraction data yielding residuals between R1=0.0409 and 0.0629 in space group P6₃/m (Z=1). Lattice constants vary between 9.436(2) and 9.5423(1) Å (for a) and 6.919(2) to 6.8429(1) Å (for c). The sulfate tetrahedra and the two symmetrically independent cation polyhedra about M1 and M2 (occupied by Na and Ca, respectively) are generally very similar to the analogous polyhedra in phosphate apatites. A common structural feature of all members of the solid solution series is a deficiency in the total Cl- and F-content compared with the phosphate apatites. The mean value of (Cl+F) for the solid solution equals 1.55(6) atoms per unit cell compared with the ideal value of 2 atoms per unit cell observed in phosphate apatites. The solid solution series Na_6.45Ca_3.55(SO₄)₆Cl_1.55-Na_6.45 Ca_3.55(SO₄)₆F_1.55 shows a gap toward the side of fluoride rich compounds. Under ambient pressure the gap exists between 0<n_Cl/n_Cl+n_F<0.33, where n_Cl and n_F represent the numbers of Cl- and F-atoms per unit cell.     

Distinction of the delayed fluorescences S1 → S0 And S2 → S0 of pyrene by selective quenching Of S1 → S0

In the spectrum of the delayed fluorescence (DF) of pyrene, caused by triplet-triplet annihilation T₁ + T₁ → S_n + S_o (n = 1,2), a strong DF S₁ → S_o and a very weak DF S₂ → s₀ are observed. The DF S₁→ S_o is quenched selectively by compounds like N-diethylanine or triethylamine which do not quench T₁ of pyrene.     

Non-Condon effects in triplet-singlet spectra. The 3B1-1A1 system of NO2− in NaNO2 and NaHCO2

Spin-level resolved spectra of the ³B₁-¹A₁ (T₁-S_o) transition of NO₂^? in neat NaNO₂ and in a NaHCO₂ host at ? 4 K show different intensity envelopes for the totally symmetric bending progression, indicating a breakdown of the Condon approximation. The non-Condon effects are associated predominantly with the τ₂ spin level of the ³B₁ state and are particularly severe in NaHCO₂/NO₂^?. The results can be reasonably interpreted in terms of an empirical linear dependence of the transition moment on the bending coordinate, but the detailed origin of the effect is unclear.     

Interactions of Eu3+(aq) 7F, 5D1 and 5D0 levels with nitrate and inner-sphere (EuNO2+3)* exciplex formation

Steady-state and transient photokinetic and spectroscopic measurements on aqueous Eu(NO₃)₃ show different affinities of ⁷F, ⁵D₁ and ⁵D₀ Eu³⁺_aq towards nitrate ion. This may be rationalised by differences in the inner- and outer-shell hydration structures between ⁵D_O, ⁵D₁ Eu³⁺(_aq) and ⁷F Eu³⁺_(aq). Nitrate penetration into the inner-shell of Eu³⁺_(aq), and inner-coordination (EuNO²⁺₃)^* exciplex formation, occur solely in the long-lived ⁵D_O level of Eu³⁺_(aq).     

La(Li1/3Ti2/3)O3: a new 1:2 ordered perovskite

A new 1:2 ordered perovskite La(Li_1/3Ti_2/3)O₃ has been synthesized via solid-state techniques. At temperature >1185°C, Li and Ti are randomly distributed on the B-sites and the X-ray powder patterns can be indexed in a tilted (b⁻b⁻c⁺) Pbnm orthorhombic cell (a=a_c√2=5.545 Å, b=a_c√2=5.561 Å, c=2a_c=7.835 Å). However, for T?1175°C, a 1:2 layered ordering of Li and Ti along 〈111〉_c yields a structure with a P2₁/c monoclinic cell with a=a_c√6=9.604 Å, b=a_c√2=5.552 Å, c=a_c3√2=16.661 Å, β=125.12°. While this type of order is well known in the A²⁺(B²⁺_1/3B⁵⁺_2/3)O₃ family of niobates and tantalates, La(Li_1/3Ti_2/3)O₃ is the first example of a titanate perovskite with a 1:2 ordering of cations on the B-sites.     

Kristallstruktur, 1H-NMR- und massenspektrum von tricarbonylferracyclopentadien-tricarbonyleisen,C4H4Fe2(CO)6

The preparation, mass and ¹H NMR spectra and the crystal structure of C₄H₄Fe₂(CO)₆ are described.The compound can be prepared in a simple way by reaction of Fe₃(CO)₁₂ with thiophene (yield 17%). It forms monoclinic crystals (space group P2₁/c) with four formula units in the unit cell. The positions of all atoms (except H atoms) have been determined and refined until an R value of 0.058 was reached. Within the practically planar ferracyclopentadiene ring, multiple bond orders must be assumed for all bonds. One of the six CO groups in the molecule is bent and represents a strongly unsymmetrical bridging carbonyl group (“semibridging carbonyl group”).     

A multicommuted fluorescence-based sensing system for simultaneous determination of Vitamins B2 and B6

A multicommuted flow-through optosensor based on the direct fluorescence measurements of Vitamins B₂ and B₆ using a non-polar sorbent (C₁₈ silica gel) as solid sensing zone (to accomplish the separation and subsequent preconcentration/detection of the target analytes) have been developed. The proposed flow system was controlled by Java-written home-made software and designed using three-way solenoid valves for independent automated manipulation of sample and carrier solutions. The native fluorescence signal was simultaneously monitored at two pairs of excitation/emission wavelengths (450/519 and 294/395 for B₂ and B₆, respectively). The separation of the analytes was performed in the detection flow cell, using the differences in the sorption/elution process on the solid support between the two vitamins, due to their different polarity. Using an optimised sampling time, the analytical signal showed linearity in the range 0.01-0.4 and 0.15-3 μg ml⁻¹ with detection limits of 0.003 and 0.045 μg ml⁻¹ for B₂ and B₆, respectively, obtaining R.S.D. (%) values better than 2% for both analytes. The proposed methodology was applied to different pharmaceutical preparations, obtaining remarkably good results with recoveries ranging from 96 to 107.5%.     

Collision-induced dissociation of dye-laser excited Li2(A 1Σ+u)

Li₂ molecules in a vapour cell were excited into the A ¹Σ⁺_u state using a dye laser set at 6103 Å. In the presence of a foreign gas Li(2 ²P) atoms were detected by means of the Li(3 ²D-2 ²P) transition which was simultaneously excited by the 6103 Å radiation. From the pressure dependence of the normalized Li fluorescence intensity absolute rate constants for collision-induced dissociation Li₂(A) → Li(2 ²P) by the five rare gases were determined. The results are compared with recent work on the analogous process in the Li₂(B ¹Π_u) state and with other systems.     

Structural and vibrational studies of Li[Kx(NH4)1−x]SO4 and Li2KNH4(SO4)2 mixed crystals

Mixed crystals of Li[K_x(NH₄)_1−x]SO₄ have been obtained by evaporation from aqueous solution at 313 K using different molar ratios of mixtures of LiKSO₄ and LiNH₄SO₄. The crystals were characterized by Raman scattering and single-crystal and powder X-ray diffraction. Two types of compound were obtained: Li[K_x(NH₄)_1−x]SO₄ with x?0.94 and Li₂KNH₄(SO₄)₂. Different phases of Li[K_x(NH₄)_1−x]SO₄ were yielded according to the molar ratio used in the preparation. The first phase is isostructural to the room-temperature phase of LiKSO₄. The second phase is the enantiomorph of the first, which is not observed in pure LiKSO₄, and the last is a disordered phase, which was also observed in LiKSO₄, and can be assumed as a mixture of domains of two preceding phases. In the second type of compound with formula Li₂KNH₄(SO₄)₂, the room-temperature phase is hexagonal, symmetry space group P6₃ with cell-volume nine times that of LiKSO₄. In this phase, some cavities are occupied by K⁺ ions only, and others are occupied by either K⁺ or NH₄⁺ at random. Thermal analyses of both types of compounds were performed by DSC, ATD, TG and powder X-ray diffraction. The phase transition temperatures for Li[K_x(NH₄)_1−x]SO₄x?0.94 were affected by the random presence of the ammonium ion in this disordered system. The high-temperature phase of Li₂KNH₄(SO₄)₂ is also hexagonal, space group P6₃/mmc with the cell a-parameter double that of LiKSO₄. The phase transition is at 471.9 K.     

A Structural Investigation of La2(GeO4)O and Alkaline-Earth-Doped La9.33(GeO4)6O2

The structures of the oxyorthogermanate La₂(GeO₄)O and the apatite-structured La_9.33(GeO₄)₆O₂ have been refined from powder neutron diffraction data. La₂(GeO₄)O crystallizes in a monoclinic unit cell (P2₁/c) and is cation stoichiometric in contrast to previous reports. La_9.33(GeO₄)₆O₂ crystallizes in a hexagonal unit cell (P6₃/m) and the powder diffraction data show anisotropic peak broadening that is observed in electron diffraction patterns as incommensurate diffuse spots at hkq reciprocal planes (with q=1.6-1.7) and can be attributed to a correlated disorder in the “apatite channels”. This compound was doped up to a nominal composition close to M₂La₈(GeO₄)₆O₂ with M=Ca, Sr, Ba. The dopant ions preferentially occupy the 4f sites as the number of La vacancies decreases. The measured ionic conductivity of La_9.33(GeO₄)₆O₂ is about 3 orders of magnitude larger than for La₂(GeO₄)O at high temperatures and decreases with increasing dopant content from the highest value of about 0.16 S cm⁻¹ at 1160 K.     

The characterization of Ba3Re2O9 and Sr3Re2O9

The compounds Ba₃Re₂O₉ and Sr₃Re₂O₉ were prepared by the solid state reaction of the corresponding alkaline-earth oxide with ReO₃ at 750 to 900°C in sealed, evacuated, fused silica tubes. The two compounds are isostructural, having the nine-layer ABO₃ structure with vacant central octahedra. The unit cell parameters are given. The magnetic susceptibility for Ba₃Re₂O₉ indicates Curie-Weiss behavior with a Re⁶⁺ moment having localized electrons. The magnetic data for Sr₃Re₂O₉ suggest delocalized electron behavior from its temperature-independent susceptibility. Both compounds appear to have semiconducting properties, but the strontium analog is a better conductor. Both compounds are unstable when heated in air above 400°C. They are readily decomposed by chemical oxidizing agents.     

The 1,3B 1— 1A 1 systems of the nitrite anion in a NaHCO 2 host crystal

The lower energy electronic transitions of the NO ^?₂ anion in a NaHCO ₂ host crystal at 4 K are reported and analyzea. The ¹B ₁— ¹A ₁ system, whose origin is at 25 649 cm ^?1, is seen in absorption and fluorescence accompanied by a long progression in the symmetric bending mode (ν′ ₂ = 626, ν″ = 820 cm ^?1). The implied change in the ONO bond angle is 9 ± 3 ⁰. The ³B ₁ → ¹A ₁ phosphorescence is quite weak with an extrapolated origin at 18 059 cm ^?1. The ¹B ₁ ← ¹A ₁ absorption linewidths, the lower-resolution fluorescence photoexcitation spectrum, and the relative phosphorescence and fluorescence intensities suggest the involvement of an intermediate state in populating the triplet manifold after excitation of the ¹B ₁ state.     

Laser induced fluorescence of CH2 (ã1A1) produced in the photodissociation of ketene at 337 nm. The CH2 (ã1A1-X̃3B1)energy separation

The photodissociation of ketene, CH₂CO(X?¹A₁) → CH₂ (ã¹A₁) + CO(X ¹Σ⁺) has been observed at 337 nm, using a pulsed nitrogen laser. The CH₂ (ã ¹A₁) radical has been detected by laser induced fluorescence with a tunable dye laser. A laser excitation spectrum has been obtained from CH₂(ã ¹A₁) over the wavelength interval from 588.9 to 595.6 nm in the Σ ← Π vibronic subband of the CH₂ (ã ¹A₁); υ″ = 0, 0, 0?b? ¹B₁; υ′ = 0, 14, 0) transition. For the CH₂(ã ¹A₁ ; υ′= 0, 0, 0?X? ³B₁; υ′' = 0, 0, 0) energy separation an upper limit of (6.3 ^± 0.8) kcal/mole has been found. The radiative lifetime τ and the rate constant k for the removal of the 0₀₀ rotational level of the Σ(0, 14, 0) vibronic state have been measured directly. The values are τ = (4.2 ^± 0.2) μs and k = (7.4 ± 0.3) × 10^?10 cm³ molecule^?1 s^?1, respectively.