The importance of spin relaxation in the delayed fluorescence of molecular crystals

Deactivation rate constants of spin-orbital excited Br atoms in the reactions Br(²P_{$\text{1}\text{2}$}) + O₂ → Br(²P_{$\text{3}\text{2}$}) + O₂ (k₁), and Br(²P_{$\text{1}\text{2}$}) + NO → Br(²P_{$\text{3}\text{2}$}) + NO (k₄) have been measured with a photodissociative IBr laser on the electronic transition ²P_{$\text{1}\text{2}$}?²P_{$\text{3}\text{2}$} in the Br atom (λ = 2.7 μm). The values obtained are (6.4 ± 1.8) × 10^?14 cm³ s^?1 and (1.9 ± 0.6) × 10^?12 cm³ s^?1, respectively. Comparison with published data leads to the conclusion that, contrary to a widely accepted point of view, the high rate constants for the quenching of excited halogen atoms are due to resonant energy transfer processes and not to the paramagnetic nature of the quencher.     

Interactions magnétiques dans des groupements binucléaires [Fe2O7]8− au sein de Na8Fe2O7

The magnetic interaction in the structural units [Fe₂O₇]^8?, built of two corner-sharing FeO₄ tetrahedra, in Na₈Fe₂O₇ ($\text{Na}{}_2\text{O}\text{Fe}{}_2\text{O}{}_3\text{=}\text{4}\text{1}$) has been studied by magnetic susceptibility measurements (4.2–500 K). An exchange integral $\text{J}\text{K}{}_{\mathrm{<mtext>B</mtext>}}$ of ?37 K is obtained by comparison of the experimental values and the calculated ones using a Heisenberg-Dirac-Van Vleck-type Hamiltonian ? = $\text{?2J}\text{S}\text{?}{}_1\text{S}\text{?}{}_2$. The hypothesis of magnetically isolated [Fe₂O₇]^8? groups is corroborated by Mössbauer spectroscopy between 1.5 and 77 K. The susceptibility measurements of the solid solutions Na₈Fe_2?xM_xO₇ (M = Al, Ga; 0 ≤ x ≤ 0.2 for Al; 0 ≤ x ≤ 2 for Ga) leads to the same conclusion of the existence of isolated Fe³⁺Fe³⁺ pairs in Na₈Fe₂O₇. The type of substitution of Fe by Al or Ga is determined; homonuclear Fe³⁺Fe³⁺ and M³⁺M³⁺ pairs and heteronuclear Fe³⁺M³⁺ pairs are formed.     

Thermal decomposition of magnesian kutnahorite

This work investigates the thermal decomposition of magnesian kutnahorite, which belongs to the dolomite group.The DTA curve measured in static air using a small amount of sample (5.0 mg) is quite different from those published previously. This difference might be due to the effect of a self-generated CO₂ atmosphere.In a CO₂ flow of 100 ml min^?1, magnesian kutnahorite decomposes in four steps. Mg-kutnahorite → CaCO₃ + Mg₂MnO₄ + Mn₃O₄ + MgO → CaCO₃ + CaMnO₃ + MgO → CaCO₃ + CaMnO₃ + Ca₂MnO₄ + MgO → CaMnO₃ + Ca₂MnO₄ + MgO + CaO.However, in a mixed gas flow of CO₂ at 95 ml min^?1 and CO at 5 ml min^?1, it decomposes, like dolomite, in two steps. Mg-kutnahorite → CaCO₃ + (Mg,Mn)$\text{O}\text{-}$ → (Ca, Mn)O + (Mg,Mn)$\text{O}\text{-}$.It has been found that the equilibrium redistribution of Mn between (Ca, Mn)$\text{O}\text{-}$ and (Mg, Mn)$\text{O}\text{-}$ is achieved at the second decomposition step. This is supported by theoretical considerations.Consequently, when the O₂ partial pressure in the atmosphere is low enough to keep Mn in a bivalent state, the Mn bearing dolomite group mineral decomposes in a similar manner to dolomite itself.     

Synthèse et structure cristalline d'un nouvel oxyde mixte FeV2O6H0.5 relation avec la structure type diaspore

Single crystals of a new compound, FeV₂O₆H_0.5, have been obtained by hydrothermal synthesis at 650°C and 2 kbar. An electron microprobe analysis indicated that the chemical formula is FeV₂O₆. This compound has an orthorhombic symmetry, space group P2₁2₁2₁ with Z = 4. The unit cell dimensions are

$\text{a = 4.891}\text{A}\text{?}\text{, b = 9.553}\text{A}\text{?}\text{, c = 8.786}\text{A}\text{?}$

These parameters are related to a′, b′, c′ of the diaspore-type VO₂ by the relations: $\text{a ? a′, b ? b′, c ? 3c′}$. The structure, based on single crystal data, has been determined from Patterson and Fourier syntheses. A structural refinement gave a final R-factor of 2.4%. The new structure can be deduced from that of the diaspore-type VO₂, by displacement of one third of the cations from an octahedral site to an adjacent unoccupied tetrahedral site, which is located in a channel parallel to the c axis. The calculation of the cation and oxygen valences indicated that some O^2? were in fact OH^?. This conjecture was supported by thermogravimetric analysis. The chemical formula was indeed Fe³⁺₂V³⁺V⁴⁺V⁵⁺₂O₁₁(OH). Some intensities of the powder diffraction lines changed strongly when the preparation temperature was reduced from 650°C to 300°C. This was explained by an increase of the hydrogen ratio in the formula FeV₂O₆H_x, which implies a structural change. The different phases FeV₂O₆H_x can be considered as solid solutions between two extreme phases with different structure: one with the present one and the other with a diaspore-type structure.     

ESR studies on the reactions of sulphite radical anion,SO3.̄−, with nitroalkane compounds in aqueous solutions

The reactions of the sulphite radical anion, SO₃^{$\text{.}\text{?}$?} (generated from the Ti³⁺-H₂O₂-Na₂SO₃ system at pH 9), in aqueous solutions with some nitroalkane compounds were investigated by using a rapid-mixing flow technique coupled with electron spin resonance (ESR) which can detect the radicals having a lifetime of 5–100 ms. The SO₃^{$\text{.}\text{?}$?} radical added to the double bond of CN in the nitroalkane aci-anions which are the main form of nitroalkanes in aqueous alkaline solutions. From the observed hyperfine splitting constants of the SO₃^{$\text{.}\text{?}$?} adducts of nitroalkane aci-anions, the preferred conformation of the adducts was deduced.     

C2O(X3Σ−): absolute reaction rates measured by laser induced fluoresence

Absolute rate constants are reported for reactions of C₂O($\text{X}\text{?}$³Σ^?) under pseudo-first-order decay conditions. C₂O is generated by laser photodissociation of C₃O₂ at 266 nm, and detected by dye-laser induced fluorescence on the $\text{A}\text{?}$³Π_i-$\text{X}\text{?}$³Σ^? transition. Rate constants of (433 ± 12), (3.30 ± 0.12) and (1.12 ± 0.05) × 10^?13 cm³ molecule^?1 s^?1 are reported for reactions with NO, O₂ and isobutene. The NO value is approximate due to an apparent dark reaction between NO and C₃O₂. Upper limits of 1 × 10^?14 cm³ molecule^?1 s^?1 are reported for reactions with H₂, CO₂, C₃H₂ and C₂H₄. The C₂O + C₃O₂ reaction does not follow pseudo-first-order decay kinetics. Two explanations are proposed to explain this observation. Results are compared with previous relative rate measurements and are discussed in terms of their relevance to combustion chemistry.     

Phase equilibria in the system V2O3Ti2O3TiO2 at 1473°K

The phase equilibria in the V₂O₃Ti₂O₃TiO₂ system have been determined at 1473°K by the quench method, using both sealed tubes and controlled gaseous buffers. For the latter, $\text{CO}{}_2\text{H}{}_2$ mixtures were used to vary the oxygen fugacity between 10^?10.50 and 10^?16.73 atm. Under these conditions the equilibrium phases are: a sesquioxide solid solution between V₂O₃ and Ti₂O₃ with complete solid solubility and an upper stoichiometry limit of (V, Ti)₂O_3.02; an M₃O₅ series which has the V₃O₅ type structure between V₂TiO₅ and V_0.69Ti_2.31O₅ and the monoclinic pseudobrookite structure between V_0.42Ti_2.58O₅ and Ti₃O₅; series of Magneli phases, V₂Ti_n?2O_2n?1Ti_nO_2n?1, n = 4–8; and reduced rutile phases (V, Ti)O_2?x, where the lower limit for x is a function of the $\text{V}\text{(V + Ti)}$ ratio. The extent of the different solid solution areas and the location of the oxygen isobars have been determined.     

The rate constants of the gas-phase reactions K + Cl ⇆ K+ + Cl− and KCl + M ⇆ K+ + Cl− + M from measurements in atmospheric pressure flames

Mass spectrometric studies of the ions present in H₂/O₂/N₂ flames with potassium and chlorine added have demonstrated that ionization can occur in the forward steps of K + Cl ? K⁺ + Cl^? (II), KCl + M ? K⁺ + Cl^? + M (IV), where M is any third body. Variations of [K⁺] with time in these systems have been measured and establish that the rate coefficients (in ml molecule^?1 s^?1) of the ion-producing steps are k₂ = 5 × 10^?10T^{?$\text{1}\text{2}$} exp(?10 500/T) and k₄ = 2.2 × 10⁷T^?3.5 × exp(?60 800/T). Coefficients for ion-ion recombination have been obtained from k₂ and k₄ using the equilibrium constants of (II) and (IV) and are k_?2 = 1.7 × 10^?9T^{?$\text{1}\text{2}$} and k_?4 = 1.1 × 10^?17T^?3, with each one in the ml molecule^?1 s^?1 system of units. Replacement of the N₂ in one of these flames with sufficient Ar to maintain the temperature constant leaves the measured k₂ and k_?2 unchanged, but lowers the observed k₄ and k_?4. This confirms that ion-recombination in the backward step in (II) is a two-body process, whereas in (IV) it is termolecular.     

Phase diagram and some physical properties of V2O3+x1 (0 ? x ? 0.080)

The magnetic and electric properties of V₂O_3+x were investigated by measurements of magnetic susceptibility, electrical resistivity, magnetotorque, Mössbauer of doped ⁵⁷Fe, and NMR of ⁵¹V, and the results were compared with those of the (V_1?xTi_x)₂O₃ system or highly pressured V₂O₃. The results obtained are as follows: (1) The metallic state shows an antiferromagnetic ordering at T_N (x). The value of T_N for metallic V₂O₃, obtained by interpolation to x = 0, shows the coincidence between V₂O_3+x and the (V_1?xTi_x)₂O₃ system. (2) Magnetic susceptibility of V₂O_3+x is expressed as χ_M(V₂O_3+x) = (1?x)χ_M(V³⁺) + xχ_M(V⁴⁺). χ_M(V⁴⁺) obeys the Curie-Weiss law $\text{(χ}{}_{\mathrm{<mtext>M</mtext>}}\text{(}\text{V}{}^{\mathrm{4+}}\text{) =}\text{0.77}\text{T}\text{+ 17)}$. (3) In the insulating phase, the electrical resistivity ? is expressed as a common equation: $\text{? = 10}{}^{\mathrm{?1.8}}\text{exp}\text{(}\text{E}\text{kT}\text{)}$. This implies that the substitution of Ti or nonstoichiometry (V⁺⁴ + metal vacancies) has little influence on the carrier mobility (or bandwidth). (4) There is a critical length in the c-axis (? 14.01 Å) where the metal-insulator transition takes place. This suggests that the length of the c-axis plays an important role in the metal-insulator transition of V₂O₃-related compounds.     

On the structure of β-Ga2O3

The deviation from the symmetry $\text{2}\text{m}$ to 1 of β-Ga₂O₃ as reported by Wolten and Chase must be very small. It is apparently not measurable by X-ray intensity differences, by EPR of both Fe³⁺-and Cr³⁺-doped β-Ga₂O₃, by Mössbauer effect spectroscopy of Fe³⁺-substituted β-Ga₂O₃, or by optical and microwave resonance spectroscopy. Second harmonic generation, as measured by the Kurtz-Perry technique, and pyroelectricity are absent. It is pointed out also that single crystals of high-pressure phase Ga_2?xIn_xO₃ have been reported to be isostructural with the monoclinic β-Ga₂O₃.     

Gas phase oxidative-addition reactions of alkyl radicals at complexed cobalt(II) centres

Negative chemical ionisation mass spectrometry is used as a probe to identify reactions between hydrocarbon radicals and cornplexed cobalt(II) centres in the gas phase. Methane NCI mass spectra of a series of cobalt(II) complexes containing O₄, O₂N₂ and N₄ donor atom sets are characterised by adduct ions of the form [M + C_nH_2n+1]^? at m/z values above the molecular ion, [M]^?. Formation of such ionic species has been rationalised in terms of a one-electron oxidative-addition mechanism involving attack by hydrocarbon plasma-derived alkyl radicals at the metal centre prior to electron capture: Co^IIL_n + R^? → RCo^IIIL_n$\text{e}{}^{\mathrm{?}}$ [Co^IL_n]^?. The competing resonance electron attachment reaction: Co^IIL_n$\text{e}{}^{\mathrm{?}}$ also occurs within the ion source.     

Oxysulfure de scandium Sc2O2S

Sc₂O₂S is hexagonal, $\text{P6}{}_3\text{mmc}\text{, a = 3.5196(4)}$ Å, c = 12.519(2) Å, Z = 2, Dc = 3.807 g cm^?3, Dm = 4.014 g cm^?3, μ(MoKα) = 55.51 cm^?1. The final R value is 0.038 for 205 symmetry-independent reflections. This scandium oxysulfide has c = 12.52 Å, twice the value found in rare earth oxysulfides. An La₂O₂S cell combined with its reflection in a (001) mirror gives the Sc₂O₂S cell.     

The influence of the orientation of the NO molecule upon the chemiluminescent reaction NO+O3→NO2*+O2

A beam of state-selected NO molecules (J = Ω = $\text{3}\text{2}$) has been produced by an electrostatic hexapole and has been collided with O₃ molecules in a scattering chamber. The E-field dependence of the chemiluminescent cross section, σ_hr, has been investigated and resulted in the determination of the M-dependence of σ_hr: σ_hr (M)/σ₀ = 1.192±0.009, 0.0848±0.015, 1.177±0.015, 0.783±0.009 for M = $\text{3}\text{2}$, $\text{1}\text{2}$, $\text{?1}\text{2}$ and $\text{?3}\text{2}$, respectively. Application of the Legendre expansion technique and the density matrix formalism provided a deconvoluted σ_hr(γ), for a single angle of attack γ of the NO axis, expressed in simple model functions with adjustable parameters. From this analysis it is concluded that chemiluminescence only occurs when cos γ ≈ 1, the “end-on-head” orientation of NO yielding ≈ 30% of all collected light, and when cos γ ≈ ?0.275, the “broad-side-tail” orientation of NO yielding the remaining 70%. The steric factors belonging to these reactive orientations have been estimated and are S₁ = 0.25±0.07 and S₂ = 0.40±0.09, respectively. The observed dependence of σ_hr has been confronted with the rules of Woodward and Hoffman. Although there are indeed two symmetries (bpl and cpl) correlating the electron orbitals of the reactants and the products, these rules do not lead to an explanation of the steric effects of the NO+O₃ reaction.     

Energy levels of iodine monochloride near the first dissociation limit

Absorption transitions to vibrational levels close to the A state dissociation limit of ICI have been examined using a two-photon sequential absorption technique. The discrete rotational structures of I³⁷ Cl bands to within 0.7 cm^?1 of the limit have been selectively excited and analysed. A value of 17557.514 ± 0.030 cm^?1 has been obtained for the I(²P^o_{$\text{3}\text{2}$}) + Cl(²P^o_{$\text{3}\text{2}$}) dissociation energy D_e, relative to the minimum of the ICI ground state potential well. The two-photon technique can be used to excite and display separately the high resolution absorption spectra of different isotopic species of a molecule which are contained in a mixture.     

Über Ordnungs-Unordnungsphänomene bei Sauerstoff perowskiten vom Typ A2+3B2+M5+2O9

Perovskites of the type A²⁺₃B²⁺M⁵⁺₂O₉, where A²⁺ = Ba, Sr; B²⁺ = Mn, Co, Ni, Zn; M⁵⁺ = Nb, Ta, show order-disorder phenomena. At lower temperatures a thermodynamically unstable disordered cubic perovskite is formed ($\text{1}\text{3}$ formula unit—$\text{AB}{}_{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{M}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}\text{O}{}_3$—in the cell), which transforms irreversibly into a 1: 2 ordered high-temperature form with 3L structure (sequence (c)₃). For A²⁺ = Ba this lattice is hexagonal (space group $\text{P}\text{3}\text{m1}$; one formula unit in the cell); with A²⁺ = Sr a triclinic distortion is observed. For Ba₃CoNb₂O₉ a second transformation into a cubic disordered perovskite takes place at 1500°C. This transition is reversible and of the order-disorder type. The vibrational and diffuse reflectance spectra are discussed.     

Dynamic metal-ligand coupling in the infrared MCD spectra of the cobalt(II) tetrahalides

The axial single-crystal magnetic circular dichroism spectra of Cs₃ZnCl₅/Co²⁺ and Cs₃ZnBr₅/Co²⁺ have been measured over the 4000–7000 cm^?1 region of the ⁴A₂ → ⁴T₁ (F) transition at ambient and liquid-helium temperature. The B/D and C/D terms obtained give transition moment ratios, 〈t₂∥m∥t₂〉/〈e∥m∥t₂〉, in accord with the value required (?3^{$\text{1}\text{2}$}/2^{$\text{1}\text{2}$}) by a dynamic ligand-polarisation model for d-electron transition probabilities in tetrahedral metal complexes.     

A study of the near infrared emission bands of the hydroperoxyl radical at medium resolution

Emissions of the hydroperoxyl radical HO₂ in the spectral range from 1.0 to 1.6 μm were studied at low and medium resolution. The resolved spectrum shows the expected parallel band structure for the vibrational ovetone transition ²A″ (200-000); in the case of the vibronic transitions ²A′, 000 → ²A″, 000 and ²A′, 001 → ²A″, 000, however, comparison of experimental and computer simulated spectra shows that there also occur intense subbands with ΔK = 0, in addition to the ordinary ΔK = ± 1 transitions. The cause for the break-down of the type-C selection rule is not well known. In the reaction system of ethylene with discharged oxygen vibronic bands could be observed originating from ²A′ levels up to at least ν′₃ = 6. The most probable excitation mechanism for these high vibronic levels is the chemiluminescent reaction HCO + O₂ (¹Δ_g) → HO₂(²A′, 00ν′₂) + CO. From the computer fits to the spectra of HO₂ and DO₂ at medium resolution the origins of the 000-000 bands and the fundamental frequencies $\text{ν}$_3′ of the excited ²A′ state could be determined; the values are $\text{ν}$_o(HO₂)=7028 ± 3 cm^?1, $\text{ν}$_o(DO₂)=7034 ± 8 cm^?, $\text{ν}$_3′(HO₂)=927 ± 10 cm^?, and $\text{ν}$_3′(DO₂)=940 ± 28 cm^?1.     

Surface-hopping model for near-resonant electronic energy transfer

A surface-hopping model is applied to near-resonant electronic energy transfer in the NFBi and O₂I systems. Multiple surface crossings occur in NFBi at ca. 8 A, corresponding well with measured transfer cross section of 200 A². A Landau-Zener model yields the temperature dependence of the thermally averaged cross section for the laser pumping reaction, O^*₂(a¹Δ) + I(²P_{$\text{3}\text{2}$}) → O₂(X³Σ^?_g) + 1^*(²P_{$\text{1}\text{2}$}).     

The mechanism of ascorbic acid oxidation by Cu(II)-poly-4-vinylpyridine complexes

The mechanism of ascorbic acid (DH₂) oxidation with molecular oxygen catalysed by the polynuclear complex of Cu²⁺ with poly-4-vinylpyridine (PVP), partially quaternized by dimethylsulphate, has been studied. The half-conversion time of the reaction of DH₂ with Cu(II) PVP under anaerobic conditions is independent of [Cu²⁺]. At pH 3.5, t_0.5 (sec) = 0.8 + 5 × 10^?4 [DH₂]. The formation of an intermediate cupric-ascorbate complex is suggested (K_c ≈ 10⁴ M^?1). Free radicals of ascorbic acid are detected by the ESR-method combined with a flow technique. The small steady-state concentration of radicals indicates that their decay occurs inside the macromolecular complex. The rate constant of the PVP Cu(II) DH^? ternary complex dissociation is ≈0.4 sec^?1 (pH 3.5). The reaction of Cu(I) PVP with O₂ is not accompanied by formation of O₂^? outside the macromolecule bulk. The rate constant of this reaction is 1.3 ± 0.15) × 10² M^?1 sec^?1 (pH 3.5). The cyclic mechanism of the catalytic reaction is suggested to include interchange of the redox state of copper-ions. About $\text{2}\text{3}$ of the total copper ion exists in the form Cu(I) PVP during the reaction at pH 3.5. The rate of DH₂ oxidation under these conditions is limited by the rate of Cu(I) PVP reaction with O₂. At pH 4.5 the overall reaction rate is limited by the rate of interaction of Cu(II) PVP with DH^?.     

The quenching of excited iodine atoms by oxygen molecules as studied by opto-acoustic spectroscopy

The opto-acoustic spectrum of I₂ in the presence of various quenching gases — NO, O₂, CH₃I, SO₂, C₃H_S, N₂, and He — has been studied. Of these, the I₂/O₂ spectrum is quite different due to the near-resonant energy transfer I(²P$\text{1}\text{2}$) + O₂(³Σ) → I(²P$\text{3}\text{2}$) + O₂(^IΔ), wherein the resistance of the O₂((^IΔ) species to collisional relaxation severely distorts the acoustic signal. The photochemical production of excited ²P$\text{1}\text{2}$ iodine atoms commences at wavelengths considerably longer than the dissociation limit of the I₂B? state.