Effects of complex-valued electronic wave functions on nuclear motions

Moleculer species and colliding groups of atoms are considered for which the electronic wave functions are complex-valued, having arguments that depend parametrically on the nuclear coordinates. The effective Hamiltonian for nuclear motions in the adiabatic approximation that arises in the present case differs from the ordinary Born–Oppeneheimer Hamiltonian, the latter being obtained when restriction to real-valued electronic functions is made. The asymptotic boundary conditions imposed in collision theory lead to in- and out- states [8], and hence to complex-valued wave functions in the coordinate representation. The study of the influence of electron–molecule scattering on nuclear motions therefore necessitates the use of the new effective Hamiltonian, which leads to results differing from those predicted on the basis of the Born–Oppenheimer operator. It is shown that momentum-dependent potentials occurring in the new Hamiltonian might cause “distortions” to the vibrational patterns of some electron–molecule metastable states. Also, these terms can give rise to non-Born–Oppenheimer resonances when motions in an internuclear coordinate become unbounded. We derive expressions for the relevant level widths and line shapes, showing them to be subject to an isotope effect. Even when real-valued electronic functions may be used, the selections of complex-valued functions in their linear span is still optional. Although exact treatments lead to the same results in both real and complex cases we show how the choice of the argument of the electronic function to be non-zero and dependent on nuclear coordinates may be useful for the application of certain approximation schemes. It is demonstrated that for certain systems a suitable choice of the argument assures convergence when the related Lippmann–Schwinger Equation is iterated. It is also shown that in this way an nth order term in the series expansion of the T matrix [8] for moleculer systems can be made negligibly small.     

Spectrophotometric determination of thiourea in microgram quantities

Summary Copper(II) forms a stable colourless complex with thiourea. This property has been utilized to develop a Spectrophotometric method for the determination ofg quantities of thiourea. The complex absorbs in ultraviolet region and the determinations are carried out at 250 nm. The method permits the determination of thiourea from 10–60g with error not exceeding ± 1.0%.

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Zusammenfassung Kupfer(II) bildet mit Thioharnstoff eine farblosa Komplexverbindung, die sich auf Grund ihrer Absorption bei 250 nm zur spektrophotometrischen Bestimmung von Mikrogrammengen Thioharnstoff eignet. 10 bis 60g Thioharnstoff lassen sieh mit einem Fehler von höchstens ± 1,0% bestimmen.

     

Statistical analysis of measured actinide concentration bias in a Mixed Analyte Performance Evaluation Program

Data from seven Mixed Analyte Performance Evaluation Program studies have been used to identify specific problems with results and/or laboratories. These data have never been used to determine the bias or variability in data reporting for these laboratories in general. These historical analytical data were analyzed for accuracy and precision overall and separated by method of sample preparation, method of sample pretreatment, method of detection and sample size. The calculated mean percent relative bias was analyzed for the actinides (²⁴¹Am, ²³⁸Pu, ^239/240Pu, ^234/233U, and ²³⁸U) in the soil and water standards. Results from most analysis methods were at least slightly negatively biased. For the water standards, the overall 95% confidence interval for the mean percent relative bias was −1.6% to −0.5% indicating that the average actinide results slightly underestimated the true concentration. For the soil standards, the overall 95% confidence interval for the mean percent relative bias was −8.1% to −6.6% indicating that the average actinide results moderately underestimated the true concentration. This revised version was published online in July 2006 with corrections to the Cover Date.     

The infrared spectra of complexes with planar dithiooxamides X. The Cu(LH2)2X1 complexes

In this article, the preparation, X-ray powder results and vibrational properties of a new type of complex with planar dithiooxamides are discussed. In acetonitrile, Cu(I) can form complexes with the formula Cu(LH₂)₂X₁ (X = Cl, Br) where LH₂ is an N,N'-disubstituted dithiooxamide. Complexes with N,N'-dimethyldithiooxamide and N,N'-diethyldithiooxamide have been prepared. A thorough vibrational analysis with infrared and Raman techniques was performed, assisted by H/D and ⁶³Cu/⁶⁵Cu isotope substitution. The vibrational study indicates the presence of an inversion centre in the structure. This leads to the proposal of a very unusual six-coordinate structure for these compounds, with the two dithiooxamide ligands, having S-cis conformation, coordinated to the same copper and with the halogens bridged between two different copper atoms. Confirmation of this result was attempted by X-ray analysis, but although several methods were tried to obtain crystals, only powder X-ray data could be obtained, which could not give a definitive answer. Since such six-coordinate Cu(I) has never been described before, a less rigourous interpretation of the vibrational data was followed, leading to a square-pyramidal five-coordinate structure, with a rather weak bond between Cu(I) and a terminal halogen. The latter structure, with the data available up to now, seems a bit more likely, since five-coordination for Cu(I) has already been described.     

Determination of geosmin and 2-methylisoborneol in water using solid-phase microextraction and gas chromatography-chemical ionisation/electron impact ionisation-ion-trap mass spectrometry

A method for the determination of geosmin and 2-methylisoborneol (MIB) in water by solid-phase microextraction (SPME) is presented. Various SPME fibre chemistries have been compared for their efficiency in extracting MIB from water. Extraction conditions including the extraction time and temperature have been optimised. A 30 ml water sample is extracted for 20 min at 60 degrees C using a divinylbenzene fibre, and the extract analysed by gas chromatography with ion-trap mass spectrometry detection. d5-Geosmin and d3-MIB are added as internal standards to compensate for any variability in the SPME process which is not carried out to equilibrium. Chemical ionisation, using acetonitrile as the reagent gas, was found to give superior sensitivity to electron impact ionisation (EI) for the detection of MIB. EI was used as the ionisation mode for detection of geosmin. The method shows good linearity over the concentration range 5-40 ng l-1 and gives detection limits of 1 ng l-1 for both geosmin and MIB. Recovery (93-110%) and precision (3-12%) over this concentration range, for both raw and treated drinking waters, are comparable to currently employed methods such as closed-loop stripping analysis (CLSA). The method offers the advantage of being simple to use, with much shorter analysis times in comparison to CLSA.     

Theoretical investigations of Fe porphyrins. V. Low-lying electronic states of bisammineporphinatoiron(III)

Ab initio CI calculations are reported on the lowest doublet, quartet, and sextet states of [Fe^III(P)(NH₃)₂]⁺. The low-spin ground state is calculated as (d_xy² (d_π)³ with d_xy(d_π)⁴ higher by 0.15 eV. The near-ir bands at ～1 eV observed in low-spin ferriheme proteins are attributed to (π → d_π) transitions. The lowest high-spin state is ⁶A_1g, and the near-ir transitions of the high-spin ferriheme proteins observed at ～1.2 eV are attributed to higher ⁶[tripsextet] excited states [i.e., ring triplet, metal sextet]. The 30-ps “triplet” transient populated with low quantum yield observed in laser-flash studies on Fe^III(TPP)CI [TPP = tetrapbenylporphyrin] may be an ¹[tripsextet] state.     

Fermi resonances in the Raman spectra of alkali halide/BH4−

Raman spectra of a series of alkali-halide/BH^?₄ (and BD^?₄ crystals have been obtained. These spectra show some interesting examples of Fermi resonance type interactions between the stretching mode levels and overtone and combination band levels of the bending modes. Two resonances will be considered: (i) that between ν₁ and 2ν₄(A₁), and (ii) that between ν₃, 2ν₄ (F₂) and (ν₂+ν₄) (F₂).The F₂ resonance between ν₃, 2ν₄ and ν₂+ν₄ appears in the infrared spectrum and it has been studied on several occasions. However the equivalent Raman spectrum is of interest because the relative intensities of the bands are significantly different to those shown by the infrared spectrum.In the A₁ (and E) Raman spectrum of the stretching mode region there are two strong bands for each for the ¹⁰B and ¹¹B isotopes. The ν₁ would not be expected to show any ¹⁰B and ¹¹B splitting, but the observed bands are both closely resonating mixtures of ν₁ and 2ν₄(A₁). In fact the analysis shows that the stronger band has the higher proportion of 2ν₄ character, and the larger isotopic shift of the more intense band can then be seen to be reasonable.     

Protein biosensors based on biofunctionalized conical gold nanotubes

There is increasing interest in the concept of using nanopores as the sensing elements in biosensors. The nanopore most often used is the alpha-hemolysin protein channel, and the sensor consists of a single channel embedded within a lipid bilayer membrane. An ionic current is passed through the channel, and analyte species are detected as transient blocks in this current associated with translocation of the analyte through the channel-stochastic sensing. While this is an extremely promising sensing paradigm, it would be advantageous to eliminate the very fragile lipid bilayer membrane and perhaps to replace the biological nanopore with an abiotic equivalent. We describe here a new family of protein biosensors that are based on conically shaped gold nanotubes embedded within a mechanical and chemically robust polymeric membrane. While these sensors also function by passing an ion current through the nanotube, the sensing paradigm is different from the previous devices in that a transient change in the current is not observed. Instead, the protein analyte binds to a biochemical molecular-recognition agent at the mouth of the conical nanotube, resulting in complete blockage of the ion current. Three different molecular-recognition agents, and correspondingly three different protein analytes, were investigated: (i) biotin/streptavidin, (ii) protein-G/immunoglobulin, and (iii) an antibody to the protein ricin with ricin as the analyte.     

Characterization of a complex dispersion of multilamellar vesicles

Spherulites ^® are multilamellar vesicles made up of surfactant bilayers. These vesicles would potentially be very useful for the encapsulation and protection of molecules; however, traditional formulations of these vesicles are poor at retaining small hydrophilic molecules (below 1000 g/mol). In this study, we present new systems of Spherulites called complex dispersions. These are prepared by dispersing Spherulites in an oil medium, and then emulsifying this oily dispersion of Spherulites within an aqueous solvent. These new systems provide an additional oil barrier between encapsulated molecules and an external aqueous phase. We have used polarized light optical microscopy, X-ray diffraction and freeze–fracture electron microscopy to study a complex dispersion of Spherulites at all stages of its preparation. We first studied the sheared lamellar phase, followed by the dispersion of the multilamellar vesicles in the oily medium and finally the emulsification of the oily dispersion within the aqueous solvent. We compared our results on lamellar phases with previous results obtained with Spherulites directly dispersible in an aqueous medium. Since the formulation of our lamellar phase included a large percentage of oil as a component, we studied the localization of the oil in the lamellar structure. We also studied the influence of osmotic pressure on complex dispersions, because complex dispersions possess a double structure similar to that of water-in-oil-in-water emulsions and multiple emulsions are known to be sensitive to osmotic pressure. In conclusion, complex dispersions proved to be new potential carriers exhibiting some unique physical properties.     

Functional dynamics of ion channels: modulation of proton movement by conformational switches

Detailed comparative studies of proton relay in native and chemically modified gramicidin channels provide a unique opportunity to uncover the structural basis of biological proton transport. The function of ion channels hinges on their ability to provide surrogate solvation in narrow pore filters so as to overcome the dielectric barrier presented by biological membranes. In the potassium channel KcsA and in the cation channel gramicidin, permeant selectivity and mobility are determined by the proteinaceous matrix via hydrogen bonding, charge-dipole, and dipole-dipole interactions. In particular, main-chain carbonyl groups in these pore interiors play an essential role in the solvation of alkali ions and of protons. In this study, molecular dynamics simulations reveal how the translocation of H(+) is controlled by nanosecond conformational transitions exchanging distorted states of the peptidic backbone in the single-file region of a dioxolane-linked analogue of the gramicidin dimer. These results underline the functional role of channel dynamics and provide a mechanism for the modulation of proton currents by fluctuating dipoles.