Theoretical analysis of the Jahn-Teller distortions in tetrathiolato iron(II) complexes

Crystallographic studies of [Fe(SR)(4)](2-) (R is an alkyl or aryl residue) have shown that the Fe(II)S(4) cores of these complexes have (pseudo) D2d symmetry. Here we analyze the possibility that these structures result from a Jahn-Teller (JT) distortion that arises from the e(3z(2) - r(2), x(2) - y(2)) orbital ground state of Fe(II) in T(d)symmetry. Special attention is paid to the influence of the second-nearest neighbors of Fe, which lowers the symmetry and reduces the full JT effect to a smaller, pseudo JT effect (PJT). To estimate the size of the PJT distortion, we have determined the vibronic parameters and orbital state energies for a number of [Fe(SR)(4)](2-) models using density functional theory (DFT). Subsequently, this information is used for evaluating the adiabatic potential surfaces in the space of the JT-active coordinates of the FeS(4) moiety. The surfaces reveal that the JT effect of Fe(II) is completely quenched by the tetrathiolate coordination.     

Spectrofluorimetric determination of piroxicam in the presence and absence of beta-cyclodextrin

The complexation between beta-cyclodextrin (beta-CD) and piroxicam (PX) was investigated by both fluorescence and absorption spectrometry. A 1:2 guest:host stoichiometry for the complex was established, and its association constant was calculated by applying a non-linear regression method to the changes brought about by the presence of beta-CD in both the fluorescence and absorbance spectra of PX. During the study of the influence of the pH on the fluorescence emission of the complex, an efficient enhancement of the signals at acidic pH was observed. This suggests that the protonated form of PX is included more effectively than the ionized form in the beta-CD cavity. Based on the results obtained, spectrofluorimetric methods for the determination of PX were developed. The best limits of detection and quantification were obtained using beta-CD at an acidic pH. The dynamic range in this latter case was 0.02-1 microgram ml-1. The method was applied satisfactorily to the determination of piroxicam in a pharmaceutical preparation.     

Molecular modelling of Jahn-Teller distortions in Cu(II)N6 complexes: elongations, compressions and the pathways in between

Ligand Field Molecular Mechanics (LFMM) parameters have been optimised for six-coordinate Cu(II) complexes containing amine, pyridine, imidazole and pyrazine donors. As found in previous LFMM applications, the new parameters automatically generate distorted structures with the magnitudes of the Jahn-Teller elongations in good agreement with experiment. Here, we explore the rest of the potential energy surface. The introduction of axial strain drives the LFMM structures via rhombic geometries to the compressed structure, the latter corresponding to the saddle point between successive elongation axes. Calculated barrier heights between compressed and elongated geometries also agree well with available experimental data. In every case bar one, the LFMM predicts that the crystallographically observed elongation axis corresponds to the overall lowest energy well. The structural predictions are confirmed by independent density functional theory (DFT) optimisations. LFMM calculations on bis(2,5-pyrazolylpyridine)copper complexes display a smooth variation in structure as a function of pyrazolyl substituent from elongated for R = H through to fully compressed for R = (t)Bu. This behaviour is driven by the steric interactions with the ground state varying smoothly as a linear combination of {d(x2-y2)}1 and {d(z2)}1.     

Properties of polyunsaturated phosphatidylcholine membranes in the presence and absence of cholesterol

Mixtures of cholesterol with phosphatidylcholine species containing the polyunsaturated acyl chains arachidonoyl or docosahexaenoyl were studied by (13)C magic angle spinning (MAS) NMR using both cross-polarization and direct polarization, by (31)P NMR and by differential scanning calorimetry. Several unique features of these systems were observed. The separation of cholesterol in crystalline form occurred at much lower molar fractions than with other forms of phosphatidylcholine. The crystals that were formed were sensitive to the history of the sample. At cholesterol molar fractions below 0.5, they dissolved into the membrane by sequential heating and cooling scans. With higher molar fractions of cholesterol, larger amounts of anhydrous crystals were formed after the first heating. This was accompanied by the formation of non-lamellar phases. The cholesterol crystals that were formed generally were not observed by direct polarization (13)C MAS NMR, even with delay times of 100 s. This suggests that the cholesterol crystals are in a more rigid state in mixtures with these lipids. This is in contrast with the terminal methyl group of the acyl chains that is too mobile to allow cross-polarization using 1 ms contact times.     

Direct electrochemistry of endonuclease III in the presence and absence of DNA

The electrochemistry of the base excision repair enzyme Endonuclease III (Endo III) in the presence and absence of DNA has been examined on highly oriented pyrolytic graphite (HOPG). At the surface modified with pyrenated DNA, a reversible signal is observed at 20 mV versus NHE for the [4Fe-4S]3+/2+ couple of Endo III, similar to Au. Without DNA modification, oxidative and reductive signals for the [4Fe-4S] cluster of Endo III are found on bare HOPG, allowing a direct comparison between DNA-bound and free redox potentials. These data indicate a shift of approximately -200 mV in the 3+/2+ couple upon binding of Endo III to DNA. This potential shift reflects a difference in affinity for DNA of more than 3 orders of magnitude between the oxidized 3+ and reduced 2+ protein and provides quantitative support for our model utilizing DNA-mediated charge transport to redistribute base excision repair enzymes in the vicinity of damaged DNA.     

Positive ions of SiH4: Jahn-Teller-like distortions for {{\rm SiH}_{4}^{+}} and eventually molecular collapse on further ionisation.

We have recently studied by quantum-chemical calculations some fifteen tetrahedral and octahedral molecules. These seemingly disparate numerical tabulations were, it was demonstrated, pulled together by comparison with model semiclassical scaling laws for (i) nuclear–nuclear repulsion energy at the equilibrium geometry and (ii) total energies likewise. Here, we again appeal to such model scaling predictions, but now for positive ions of SiH₄. We then report Hartree-Fock equilibrium geometries and MP2 corrections. For ${{\rm SiH}_{4}^{+}}$ , we assumed the symmetry to be C_2v , by analogy with ${{\rm CH}_{4}^{+}}$ for which experimental confirmation of this symmetry is available. Larger distortions, but still for C_2v symmetry, are found from our quantum-chemical studies in the case of ${{\rm SiH}_{4}^{2+}}$ . But for ${{\rm SiH}_{4}^{3+}}$ there is a marked tendency to return to a configuration quite close to tetrahedral symmetry. But non-convergence is found for ${{\rm SiH}_{4}^{4+}}$ . Finally relations to the admittedly simplistic semiclassical geometry scaling predictions of Lawes and March are conjectured.     

Virtual free ion magnetism and the absence of Jahn-Teller distortion in a linear two-coordinate complex of high-spin iron(II)

M?ssbauer spectroscopy and dc magnetization measurements have been used to characterize the low temperature magnetism of a rigorously linear, two-coordinate complex of high-spin Fe(II), Fe[(C(Si(CH3)3]2 (1). The local C-Fe-C chromophore of 1 exhibits novel slow, single-ion paramagnetic relaxation and fully resolved magnetic hyperfine splitting of its zero field M?ssbauer spectrum over the range approximately 100 to approximately 50K. The hyperfine field at 4.2 K is 152 T! This is the largest magnetic hyperfine field observed for iron to date regardless of spin, oxidation state, or coordination environment. This observation is attributable to the large unquenched orbital angular momentum corresponding to the degenerate ground (dxy, dx2-y2) orbital pair of 1 in local Dinfinityh symmetry. Maintenance of the ground-state degeneracy is required by the Jahn-Teller theorem leading to the unprecedented result that the magnitude of the magnetic moment of 1's 5Deltag ground state is essentially that of the parent free ion (5D4) ground term.     

External photoelectron emission spectra of ionic liquids in the presence and absence of iodide

External photoelectron emission spectra of typical ionic liquids were measured in the presence and absence of iodide. Threshold energies and quantum efficiencies of the photoelectron emission were estimated. From these values, the conduction band energy of electrons and the acceptor number of each liquid were estimated. Interactions between the ionic liquids and iodide are discussed on the basis of these results.     

Kinetic study of Berthelot reaction steps in the absence and presence of coupling reagents

Several reaction steps in the Berthelot reaction for the determination of ammonia have been separately studied. A reaction order of two has been confirmed for the reaction between HOCl and NH(3). The rate constant for this reaction has been determined to be 3.2 x 10(6)l.mole(-1).sec(-1). The first evidence for the formation of benzoquinonechlorimine is presented. Pentacyanoferrate coupling reagents which accelerate the production of indophenol have been found to operate on the reaction between NH(2)Cl and phenol. The rate constant for the final step of the reaction sequence has been determined to be 5.3 x 10(-3)l.mole(-1).sec(-1). A reaction between chlorimine and pentacyanoferrate compounds has been found to be responsible for the formation of a green product in the presence of excess of coupling reagent.     

Perfluoroalkylation in flow microreactors: generation of perfluoroalkyllithiums in the presence and absence of electrophiles

Perfluoroalkyllithiums were effectively generated from perfluoroalkyl halides in the presence and absence of electrophiles using flow microreactor systems. The in situ trapping with electrophile is conducted at much higher temperatures than those required for batch macro reactors. The subsequent trapping method is quite effective for highly reactive electrophiles that are not compatible with the lithiation process.     

The photodegradation of polyethylene and polypropylene in the presence and absence of added titanium dioxide

The durabilities of polyethylene and polypropylene, in the presence and absence of added titanium dioxide, towards photodegradation have been assessed by the rate of oxygen absorption upon irradiation, measurement of photo-induced carbonyl compound and hydroperoxide formation using i.r. spectroscopy, and by measuring the change in tensile properties caused by irradiation. Oxygen absorption measurements are relatively insensitive and this method appears to be unreliable. Measurement of carbonyl compound and hydroperoxide formation by i.r. spectroscopy is suitable for polyethylene but not for polypropylene. The most sensitive method of following degradation appears to be the measuring of change in tensile properties. In general, the lower the photo-reactivity of added titanium dioxide, the greater its protecting action provided that the pigment is used at sufficiently high loading. It appears that the manner in which a pigment is dispersed in a polymer affects the performance of the pigment as a stabilizer.     

Thermal behaviour of triglycidyl-isocyanurate (TGIC) in the presence and in the absence of polyester

Simultaneous thermal analysis and DSC measurements were used to characterize triglycidyl-isocyanurate as a technical product.

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Zusammenfassung Zur Beschreibung von technischem Triglycidyl isocyanurat wurden simultane Thermoanalyschen- und DSC-Untersuchungen durchgeführt.

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We thank G. Balázs for the DSC measurements, E. Molnár for the thermal analytical measurements and M. Mara for technical assistance.     

Evaluating presence/absence of target microbes in microbiological tests

A typical qualitative microbiological method performance (collaborative) study gathers a data set of responses about a test for the presence or absence of a target microbe. We developed 2 models that estimate false-positive and false-negative rates. One model assumes a constant probability that the tests will indicate the target microbe is present for any positive concentration in the test portion. The other model assumes that this probability follows a logistic curve. Test results from several method performance studies illustrate these estimates.     

Kinetics of the oxidative degradation of d-xylose in the presence and absence of cationic and anionic surfactants

The oxidative degradation of d-xylose by cerium(IV) has been found to be slow in acidic aqueous solution with the evidence of autocatalysis. The reaction is accelerated in the cetyltrimethylammonium bromide (CTAB) micellar medium but sodium dodecyl sulfate (an anionic surfactant) has no effect. The pseudo first-order rate constants have been determined at different [reductant], [oxidant], [H₂SO₄], temperature, and [CTAB]. The reaction rate increased with increasing [d-xylose] and decreased with increase in [H₂SO₄]. The CTAB-micelle-catalyzed kinetic results can be interpreted by the pseudophase model. The kinetic parameters such as association constant (K _s), micellar medium rate constant (k _m), and activation parameters (E _a, ΔH ^# and ΔS ^#) are evaluated and the reaction mechanism is proposed. The reaction rate is inhibited by electrolytes and the results provide an evidence for the exclusion of the reactive species from the reaction site.     

CH bond activation in the presence or absence of oxygen or nitrous oxide

The reaction of C₂H₆with lattice oxygen, O^2- (in the absence of gaseous oxygen), or “adsorbedℍ oxygen (in the presence of gaseous oxygen) over NiMoO₄ catalysts has been performed and compared to C₃H₈ activation. The results obtained indicate that adsorbed oxygen exhibits a higher reactivity to C₂H₆, while lattice oxygen is more reactive relative to C₃H₈. Kinetic studies of these two reactions in presence of molecular oxygen have indeed shown that the ethane oxidative dehydrogenation (ODH) is dependent on the oxygen partial pressure, whilst on the contrary propane ODH is not. In order to confirm the presence of “adsorbed” oxygen for ethane activation, ODH tests have been performed with N₂O. On increasing temperature, the O^- adsorbed species enhances the mild oxidation of ethane. The activation energy of ethane consumption E_C2H6, relative to propane (E_C3H8 = 133 kJ/mol) is 145 kJ/mol. A possible mechanism is proposed for the oxidative dehydrogenation of ethane. This revised version was published online in August 2006 with corrections to the Cover Date.     

Kinetics and mechanism of photochemical reactions of peroxomonosulfate in the presence and absence of 2-propanol

The photochemical decomposition of peroxomonosulfate (PMS) in the presence and absence of 2-propanol at 25°C was found to obey an overall first-order rate – d[PMS]/dt = k_?[PMS]. In the absence of 2-propanol, the quantum yield ≤ for the decomposition of PMS was found to depend upon the concentration of PMS at [PMS] > 2 × 10^?M, and is independent of concentration at [PMS] > 2 × 10^?2M. The quantum yield in the presence of 2-propanol was found to be 3.03 at [PMS] = 1 × 10^?2M and 4.45 at higher concentrations of PMS. In the pH range of 2–9.0 the quantum yield was found to be independent of pH, and the overall rate constant k_? was found to be 6.49 × 10^?3 s^?1 and 1.68 × 10^?3 s^?1, respectively, in the presence and absence of isopropanol. A suitable chain mechanism is proposed and explained.     

Density of electron states in crystalline systems calculated in the presence and absence of the magnetic field

It is demonstrated in terms of explicit calculations that the average density of states in a crystalline system remains unaffected by the presence of the external magnetic field. This is so on condition the magnetic field is weak enough to provide us with a large number of the Landau levels per energy unit. For a special case of a two‐dimensional crystalline system, the equivalence of the density of states obtained in the presence and the absence of the magnetic field can be shown in an analytic way. For three dimensions, the tightly bound s‐electron states in crystals having cubic symmetry are taken into consideration. Here the density of states calculated in the presence of the magnetic field is compared numerically with that of the field‐absent Bloch states acquired with the aid of Jelitto's method. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Kinetics of the acid hydrolysis of isoproturon in the absence and presence of sodium lauryl sulfate micelles

Kinetics of the hydrolysis of isoproturon by hydrochloric acid has been studied spectrophotometrically in the absence and the presence of anionic sodium lauryl sulfate (NaLS) micelles. The anionic micelle was found to increase the rate of reaction. The reaction followed first-order kinetics with respect to isoproturon and was linearly dependent upon [HCl]. In both aqueous and micellar pseudophases, the reaction was started with the protonation of the amino group of isoproturon followed by attack of water to yield phenylcarbamic acid and the corresponding amine, thus obeying the addition–elimination mechanism. The surfactant decreased the activation entropy. The binding constant in consistence with the rate constants was evaluated on the basis of pseudophase ion-exchange model. The added salts (NaCl and KCl) decreased the rate of reaction due to the exclusion of H⁺ from micellar surfaces.     

Effect of liquid paraffin on the stability of aqueous foam in the presence and absence of electrolytes

The foam stability of dodecyl diphenyl ether disulfonate solution with liquid paraffin droplets in the presence and absence of electrolytes was evaluated, and the stabilization mechanisms were deduced. The foam film is stabilized when the monovalent and divalent counterion concentration is lower than a critical value. However, the foam stability declined with the addition of trivalent counterions. There are two mechanisms we have speculated. Firstly, the counterions reduce the repulsive interaction between the polar groups of surfactants adsorbed at the air/water and oil/water interfaces in the pseudo-emulsion film. Secondly, comparing with the monovalent counterions, the multivalent counterions are not only able to interconnect head groups of adjacent ionic surfactant molecules which are adsorbed at the air/water or oil/water interface, but also to attract the couples of neighboring surfactant head groups adsorbed at the air/water and oil/water interfaces. The attractive interaction between both the interfaces promotes the emulsified droplets piercing the air/water interface.