5T2-1A1 Spin transition and residual paramagnetism in bis(2,4-bis(2-pyridyl)thiazole)iron(II) complexes: mössbauer effect and magnetism

The ⁵⁷Fe Mössbauer effect in [Fe(pythiaz)₂] (BF₄)₂ (I) and [Fe(pythiaz)₂] (C&O₄)₂ (II) has been studied between 298 and 4.2°K (pythiaz = 2,4-bis(2-pyridyl)thiazole). At 298°K compound I shows a doublet with ΔE_Q(⁵T₂) = 1.29 mm sec^?1 and δ^1S(⁵T₂) = +0.93 mm sec^?1 characteristic of a ⁵T₂ ground state. At 236°K, a second doublet, typical for a ¹A₁ ground state appears. The transition ⁵T₂ å ¹A₁ progresses as the temperature is lowered but levels off below ≈ 120°K. At 4.2°K, 59% of the intensity is due to the ¹A₁ state, and ΔE_Q(¹A₁) = 1.59 mm sec^?1 and δ^1S(¹A₁) = +0.26 mm sec^?1. In an applied magnetic field, V_zz(¹A₁) < 0 has been determined Similar results have been obtained with compound II.Debye-Waller factors f5_T2 and f1_A1. were determined from the Mössbauer spectra under the assumption of Curie-Weiss dependence of the magnetism for the ⁵T₂ and constant μ_eff for the ¹A₁ ground state. The resulting temperature dependence of f1_A1 is highly unusual thus suggesting complicated magnetic behaviour of both ground states in the transition region. Two mechanisms for the nature of the transition are discussed, a “spin-flip” mechanism being the physically more reasonable one. The assumption of a simple Boltzmann distribution (“spin equilibrium”) may be ruled out for the solid but could be encountered in solutions.     

The preparation of WMeCl3Y (Y = S OR Se) and some coordination compounds of WMeCl3Y (Y = O,S OR Se)

The reaction of WCl₄Y (Y = S or Se) with Me₂Mg leads to the formation of WMeCl₃Y. The compounds WMeCl₃Y (Y = O, S or Se) are very reactive and attempts to isolate them in pure form failed. However, a range of complexes which they form with some nitrogen and oxygen donors have been isolated and characterised.     

Crystal structures and phase transition of Rb2TeBr6 (300-12.5 K)

Using 298 and 160(3) K diffractometer intensity data the structure of Rb₂TeBr₆ has been determined by single crystal X-ray technique and refined to a final R_w of 0.041 and 0.037, respectively (K₂PtCl₆ type structure, space group Fm3m, Z = 4 with a₂₉₈ = 10.773(4) Å and a₁₆₀ = 10.713(6) Å). The powder diffraction pattern from 300 to 12.5 K was recorded by a low-temperature Guinier diffractometer and camera. Below 45(5) K a second-order phase transition leads to a tetragonally distorted structure corresponding to a softening of the T_1g(Γ) rotary phonon. The powder pattern measured with the diffractometer at 12.5 K was used for structural refinement (R = 0.092). This low-temperature phase shows a ferrorotative displacement of the TeBr₆-octahedra with a tilt angle of 4.7(1) deg. (space group $\text{I4}\text{m}\text{, Z = 2, a = 7.4726(3)}$ Å and c = 10.7008(5) Å). The structural results indicate that there is no stereochemical activity of the lone pair electrons at Te(IV) even at very low temperatures. Phase transitions of Rb₂TeBr₆ and Rb₂TeI₆ (A_2g(X)-condensation) are compared. The results of a lattice dynamic calculation shows that only the T_1g(Γ) condensation can be explained with a rigid ion model.     

The self-association of ethanol in non-polar solvents

The self-association of ethanol in solutions of n-hexane, cyclopentane, cyclohexane, carbon tetrachloride and benzene is studied on the basis of heat of dilution data published in the literature. The tetramer formation model fits the experimental data better than other self-association models. The corresponding self-association constants, K_1,4, and heats of reaction, ΔH_1,4, are calculated. The solvent effect is studied using the solvent polarity factor, E_T, and the existence of a linear dependance of log K_1,4 on E_T values is shown.     

Evidence for the formation of a triple radiative exciplex between excited aqua-uranyl(VI) and thallous ion

Results from steady-state and transient photokinetic and spectroscopic measurements, show the formation in aqueous solution of a triple radiative exciplex between excited UO²⁺₂ and TI⁺. Rate constants of exciplex formation and deactivation are evaluated The nature of bonding and the emittive properties of this species, of probable ¹(Tl·OUO-Tl)⁴⁺ structure, are discussed.     

Thermal decomposition and creation of reactive solid surfaces: IV. effect of NH4NO3 inclusion on the thermal genesis of chromia catalyst from a parent gel

The effect of NO₃^? contaminant on the thermal decomposition of chromia gel has been thoroughly investigated. NO₃^?free gel, prepared by treating a dilute solution of chromium nitrate with aqueous ammonia, and a simulated NO₃^?-contaminated one, obtained by mechanically mixing calculated amounts of the pure gel and NH₄NO₃ (4.7–33.3% by weight), have been subjected to thermogravimetric and differential thermal analyses. The thermal behaviour thus monitored has been physicochemically characterized by means of infrared and X-ray diffraction techniques. The results obtained have revealed a notable disparity between the reaction pathways conceded by the pure and the contaminated gel yielding chromias of different properties.     

Anomalies in Faradaic rectification polarography of aromatic compounds in non-aqueous solvents

The electrode reaction of p-dicyanobenzene^O/? in N,N-dimethylformamide is studied with the aid of the galvanostatic double pulse (GDP) method and faradaic rectification (FR) polarography. The results of the GDP measurements obey the theoretical requirements in all aspects. Conversely, the FR polarogram can be fitted with the theoretical polarograms only in the potential range ?-50 mV vs. the reversible half-wave potential of this electrode. Furthermore, the most suitable k⁰ values used in this curve-fit analysis depends on the frequency of ac input and differ greatly from the value determined by the GDP method. Attention is called to the fact that the p-dicyanobenzene^0/? system is just one of the 16 redox systems of aromatic compounds which show this type of behaviour in FR and GDP experiments. Their kinetic parameters are also tabulated, with some comments on the anomalies in FR polarography.     

The effect of solvents on the homopolymerization of 4-dimethylaminostyrene initiated with CCl3COOH

In the polymerization of 4-dimethylaminostyrene (DMAS) initiated with CCl₃COOH in solvents possessing different electron-donor and electron-acceptor properties and relative permittivity (c_r), the c_r value of the solvent correlates well with the polymerization rate. The greatest increase of rate of polymerization in the homogeneous system was observed in nitrobenzene. In this solvent, characteristics of the polymerization of DMAS initiated with CCl₃COOH were determined; differences between these characteristics and the rate equation for a similarly initiated polymerization of DMAS in benzene have been elucidated.     

The mechanism of photoionization of malachite green leucocyanide in polar solvents

The fluorescence lifetime and quantum yield of malachite green leucocyanide has been measured in cyclohexane, ethanol and acetonitrile and has been compared with the corresponding values for N,N-dimethylaniline in the same solvents. In cyclohexane, the data is essentially identical, but in polar solvents there are large differences. These are attributed to the fact that the leucocyanide photoionized to malachite green cation from its first excited singlet state.     

Rotation of molecules around specific axes: Axes reorientation under rotational excitation

Rotational energy levels of nearly spherical molecules in an isolated vibrational state are studied. It is shown that, in the limit of high rotational quantum number J, the rotational states may be interpreted as those of a stable rotation around axes properly oriented in the molecular frame. The orientation of the axes depends on J. Simple analytical solutions are given for the problem considered in the asymptotic and harmonic approximations. The results obtained possess a clear quantitative interpretation of the phenomena considered and, at the same time, agree quantitatively with the results of numerical diagonalization. The analogy between the effects of rearrangement of the rotational levels under the variation of J and the critical phenomena in macroscopic systems is discussed. The intensities of rovibrational transitions between totally symmetric vibrational states are calculated. A new selection rule is introduced which is due to a overlap of the rotational functions corresponding to the rotation around differently oriented axes.     

Photolysis in aprotic solvents of some alkylcobalt(III) complexes; an ESR and spin-trapping technique study

Photolysis of several alkylcobalt(III) complexes in the visible region λ > 420 nm has been studied by ESR and spin-trapping techniques with nitrosuderene, phenyl-t-butylnitrone (PBN) and 5,5′-dimethylpyrroline-N-oxide (DMPO) and their mixtures. DMPO and PBN give spin adducts of two different radicals; the first is a hydrogen atom coming from the equatorial ligand, and the second is an alkyl-free radical coming from the axial position of the cobalt(III) complexes. This indicates that excitation of the complex is followed by the expulsion of one hydrogen atom and homolytic cleavage of the metalalkyl bond.     

The electrochemical behavior of Os(II) complexes with α,α′-bipyridine and o-phenanthroline in non-aqueous solvents

The electrode reactions of Os(II) complexes with α,α′-bipyridine and o-phenanthroline have been studied in non aqueous solvents. Tris bipyridine and tris phenanthroline complexes of Os(II) give one-step oxidation waves and three-step reduction waves, while bis bipyridine complexes of Os(II) containing py₂, en, CN₂, Cl₂ and Br₂ as mixed ligands give one-step oxidation waves and two-step reduction waves. Most of these waves correspond to reversible single electron transfers. It is noticed that the half-wave potentials or the peak potentials of bis bipyridine complexes are shifted to more negative direction than those of the tris complex in both oxidation and reduction processes. This is attributable to less electron with drawing nature of substituted ligands than bipyridine. These potential data, the shifts of E_1/2 or E_p from those of tris complexes, and δE*, the potential difference between two consecutive single electron waves, are discussed in terms of molecular orbital scheme.     

Electrochemical reduction of α-unsaturated carbonyl compounds: Part VI. Effect of electrogenerated bases in very weakly protic solvents

Unsaturated carbonyl compounds were reduced electrochemically in carefully dried aprotic solvents. In the absence of protonating impurities, the i—V curves could be modified due to the de-activation of a part of the substrate by the action of the electrogenerated bases. In DMF solutions, the electrogenerated base which is able to deprotonate the ketone is mainly the dianion and discrepancies occur with, classical polarograms, of this class of reactants as observed at the level of the second step. For more conjugated ketones, a very cathodic step is assumed to correspond to the reduction of the conjugated anion of the ketone but the instability of this anion (inducing polymerisation reactions) prevents the acquisition of careful analytical evidence.     

Permeabilities and stabilities of large dihexadecylphosphate and dioctadecyldimethylammonium chloride vesicles

Sodium dihexadecylphosphate (DHP) vesicles, with a mean external diameter of 0.27 μm, were obtained by chloroform vaporization followed by gel filtration. The relative volume of the internal aqueous compartment, estimated measuring entrapment of [¹⁴C] sucrose, was 13 liters/mol. DHP and dioctadecyldimethyl ammonium chloride (DODAC) large vesicles were stable up to 20 days at room temperature. In contrast, small sonicated DHP and DODAC vesicles were stable for some hours after preparation. Both DHP and DODAC large vesicles were impermeable toward most solutes tested. Large DHP vesicles responded as ideal osmometers toward gradients of sucrose, NaCl, or NaOH. The behavior of large vesicles of synthetic amphiphiles and of phospholipids was found to be analogous in the gel state.     

On m-changes in atom-rigid-rotor collissions: influence of an initial rotation

Reorientation of the angular momentum of a diatomic molecule in collisions with atoms is investigated using classical mechanics. A factorisation formula for cross sections for rotational excitation is given. The factorisation allows the calculation of the state-specific cross section d σ (j′ m′ ← jm)/dΩ once the m-averaged cross sections d σ (j″← 0 )/dΩ for all possible j″ are known. The approach is applied to the Na₂-He system.     

Molecular determinants for drug—receptor interactions: Part 2. An ab initio molecular orbital and dipole moment study of the novel nootropic agent piracetam (2-oxopyrrolidin-1-ylacetamide)

From the ab initio molecular energies of the possible conformers and from a classical dipole moment analysis of 2-oxopyrrolidin-1-ylacetamide (υ = 4.02 D in dioxan at 30.0°C), the preferred conformation in solution of this novel nootropic agent has been determined. The exocyclic NCH₂ bond is rotated in one sense by 90° and the exocyclic CH₂C bond rotated in the same sense by 120° from the “planar” (OO)-cis conformation. The structures of the two enantiomers in solution differ from that of the crystalline molecule.