Optical potential calculations for molecular collisions

The Feshbach optical formalism is applied to elastic, nonreactive atom-diatom scattering on a single potential energy surface. The optical potential depends on GQ, the resolvent of E-QHQ, where Q projects onto open as well as closed channels. A method for generating GQ is developed which goes beyond the free-space approximation by partitioning the radial part of the intermolecular separation into a set of intervals, on each of which the projected interaction QVQ is represented by a constant diagonal form. The resulting GQ is used in calculations on a model collinear system. The calculations are carried out with various approximations on the full nonlocal optical potential equation for Pψ Emphasis is placed on two of these, one of which is characterized by a local homogeneous equation for Pψ, and the other by a local inhomogeneous equation for Pψ.     

Conductance of nano-tube junctions and its scaling law

The conductance is calculated by Landauer’s formula with a simple tight binding model for junctions connecting two different metallic carbon nanotubes. These junctions are formed by a pair of disclinations, a 5-membered ring and a 7-membered ring, without dangling bonds. Conductances of about eight hundred kinds of junctions are obtained. The conductance is determined only by the ratio i/R where i is the distance between the two disclinations and R is the circumference of the thinner tube. When i/R ? 1, the conductance is found to be almost proportional to (i/R)^-3. Even when the Fermi energy is shifted by doping, an extended scaling law holds, so far as the channel number is kept two.     

A versatile synthesis of 2-substituted 4-amino-1,2,4,5-tetrahydro-2-benzazepine-3-ones

A mild and general strategy for the synthesis of 2-substituted 4-amino-1,2,4,5-tetrahydro-2-benzazepine-3-ones is described. The seven-membered lactam is prepared by intramolecular amide bond formation from the intermediate amino acid, which is obtained either by reductive alkylation of a variety of amines with N-Boc,N-Me-ortho-formyl-Phe and Phth-ortho-formyl-Phe, or by reductive amination of a variety of aldehydes with N-Boc-ortho-aminomethyl-Phe.     

A copper-catalyzed domino radical cyclization route to benzospiro-indolizidinepyrrolidinones

In this Letter the synthesis of benzospiro-indolizidinepyrrolidinones is described by a domino atom transfer radical cyclization reaction using a copper catalyst. The structure of one of the products was established by single crystal X-ray diffraction. The investigated precursors, bearing a homo allyl substituent on the N-indole, result in a 5-exo-trig, followed by a 6-endo-trig cyclization. When the N-indole is substituted with an allyl group, only the spiro-cyclization occurs.     

Kinetics and mechanism of the liquid-phase oxidation of tert-butyl phenylacetate

The oxidation of tert-butyl phenylacetate in ortho-dichlorobenzene at 140°C occurs with short chains. The primary nonperoxide reaction products (tert-butyl α-hydroxyphenylacetate, tert-butyl α-oxophenylacetate, and benzaldehyde) are formed by the decomposition of a hydroperoxide (tert-butyl α-hydroperoxyphenylacetate) and (or) by the recombination of peroxy radicals with and without chain termination. Benzaldehyde and tert-butyl α-hydroxyphenylacetate undergo radical chain oxidation in a reaction medium to result in benzoic acid and tert-butyl α-oxophenylacetate. Homolytic hydroperoxide decomposition is responsible for process autoacceleration and results in benzaldehyde, which is also formed from hydroperoxide by a nonradical mechanism, probably, via a dioxetane intermediate. Both of the reactions are catalyzed by benzoic acid. Benzoic acid has no effect on hydroperoxide conversion into tert-butyl α-oxophenylacetate, which most likely occurs as a result of hydroperoxide decomposition induced by peroxy radicals. The rate constants of the main steps of the process and kinetic parameters have been calculated by solving an inverse kinetic problem.     

Identification of the geometrical isomers of α-linolenic acid using gas chromatography/mass spectrometry with a binary decision tree

Gas chromatography, using a highly polar column, low energy (30 eV) electron ionization mass spectrometry and multivariate curve resolution, are combined to obtain the mass spectra of all eight geometrical isomers of α-linolenic acid. A step by step Student's t-test is performed on the m/z 50-294 to identify the m/z by which the geometries of the double bonds could be discriminated. The most intense peak discriminates between cis (m/z 79) and trans (m/z 95) at the central (carbon 12) position. The configuration at carbon 15 is then distinguished by m/z 68 and 236, and finally the geometry at carbon 9 is determined by m/z 93, 173, 191 and 236. A three-question binary tree is developed based on the normalized intensities of these ions by which the identity of any given isomer of α-linolenic is accurately determined. Application of Bayes theorem to data from independent samples shows that the complete configuration is determined correctly with a minimum probability of 87%.     

An unusual behaviour of N-(tert-butoxycarbonyl)- and N-pivaloyl-(methylthio)anilines in metallation reactions

The metallation reaction of N-Boc-and N-Piv-(methylthio)anilines are here described. The results show that N-Boc derivatives are metallated only by superbases to give products substituted at the thiomethylic group. N-Piv derivatives show a different behaviour: ortho-derivative is metallated by both butyllithium and superbase at the thiomethylic carbon atom, while para-derivative is metallated in ortho to the N-Piv group by butyllithium and at the thiomethylic carbon atom by superbase. The meta-derivative is metallated only by superbase at the thiomethylic carbon atom.     

Two-photon ionization of an inner shell electron of the Cl atom

We have calculated, using second order perturbation theory, the two photon ionization cross section of a K-shell electron of chlorine forE=1.6 keV incident photons. Two classes of intermediate states must be considered, those in which a 1s electron moves to an emptyp-orbital, and those in which an electron from an occupiedp-orbital moves into the continuum. The first class of intermediate states is followed by the ejection of ap-electron into the continuum. The second class of intermediate states is followed by the transfer of a 1s electron into an emptyp-state. The largest contribution comes from 3p →d-continuum followed by 1s → 3p transition. Our result is σ⁽²⁾/I=2.06×10⁴¹ cm⁴/W where σ⁽²⁾ is the two photon ionization cross-section andI is the light intensity.     

A new method of deformation calorimetry applied to copper single crystals

A new method of deformation calorimetry is described by which the change of stored energy with deformation is measured. For the determination of the dynamic energy balance of plastic deformation, we deform metal single crystals by tension and measure the applied load K(t), the effective deformation rate E_exp and the temperature T(t) of our specimen with high accuracy during deformation. The load is determined by using a reference signal (load-fixpoint), the deformation rate by means of a tensionmeter which records the strain directly at the crystal. Measurement of specimen temperature is carried out with the aid of micro-thermistors of high temperature resolution, ΔT ? 2 · 10^?5 K, and time resolution Δt ? 0,1 s. An essential feature of our method is the procedure developed for evaluation of the T(t)-curves (which are severely influenced by unavoidable heat conduction) using the thermoelastic effect. It works by means of the response function of heat conduction which we record during elastic deformation of the specimen.Some results are given on the stored energy and its change of a copper single crystal deformed at room temperature.     

Interpretation of static and dynamic responses of a dissolved oxygen electrode in viscous broths

The response of an amperometric oxygen electrode is studied theoretically and experimentally for the case of significant liquid film resistance at the outer side of the membrane. The behaviour of the probe in a gas stream is predicted by using a transfer function which involves an electrode model including oxygen diffusion within the electrode. The effects of the liquid film which exists at the membrane surface when dissolved oxygen concentrations are measured, are taken into account by modifying the transfer function. The new expression obtained is used to model the step response of the probe in a 2-1 stirred tank fermentor filled with water or xanthane solutions at different concentrations. First, the results are used to correct automatically the steady-state voltage readings of the probe. Secondly, the probe transfer function is used to evaluate k₁a by dynamic measurements: the response to a step change in the gas concentration is transformed via the fast Fourier transform algorithm and k₁a is identified in the Fourier domain by a Gauss-Newton algorithm. Data acquisition, Fourier transform and k₁a identification are implemented on-line on a HP-87 computer. This method of obtaining k₁a values appears to be a generalized moment method. It is shown that it is necessary to consider the liquid film dynamics around the probe in the actual fermenting conditions to evaluate k₁a successfully.     

Liquid—liquid distribution in reversed micellar systems

The equilibrium distribution of a hydrophilic solute (M^z+ between an aqueous phase and a reversed micellar organic phase (consisting of a surfactant HA with aggregation number x, and dissolved in a hydrocarbon diluent) is analyzed quantitatively by treating the reversed micelles as a pseudophase. It is shown that when the M—A complex is strongly solubilized by the micellar pseudophase, the distribution coefficient (D) has a first-order dependence on the concentration of micellized surfactant (C_s). On the other hand, when the M—A complex is not solubilized by the reversed micelles, a plot of log D versus log C_s has a slope of (z/x); in this case the monomeric species HA is the active extractant and any effect that decreases surfactant aggregation (e.g. low aggregation number, small aggregation equilibrium constant) leads to an increase in the distribution coefficient.     

A diffusion model of Frenkel defects formation during high-energy particle passage in crystals

The cross-section σ(E) of Frenkel defect formation resulted from fast energy E particle elastic scattering by a crystal atom is calculated, taking into account the real dependence of the defect formation probability W(T) on the transferred energy value T essentially differing from a step-like probability. Thus obtained σ(E) is in agreement with experimental dependences for Si and Ge exposed to fast E ⩽ 1 MeV electron irradiation. The multiplication factor k(T) of the crystal atoms knocked-out in a cascade of atom-atom displacements is estimated with allowance for a more realistic probability function W(T). The resulted multiplication factor for a silicon crystal proved to be almost by an order of magnitude smaller than that determined by the known Kinchin-Pease relation. So, the suggested consideration predicts an essentially lower efficiency of radiation defect insertion approaching the estimated and experimental values.     

Kinetically distinct triplets in the photorearrangement of 2-phenylcyclohexanone to cis- and trans-6-phenyl-5-hexenals

A reinvestigation of the photochemistry of 2-phenylcyclohexanone reveals that the two aldehyde products, cis- and trans-6-phenyl-5-hexanal, come from triplets of different lifetimes. That the two distinct triplets are not simply the two conformers with phenyl axial and equatorial is demonstrated by the similar behavior of cis-4-t-butyl-2-phenylcyclohexanone. The trans isomer of this ketone is photostable. It is concluded that trans-enal arises by an almost concerted out-of-plane cleavage which forms a 1,6-biradical in the perfect geometry for disproportionation to trans-enal.The precursor to cis-enal may be a minor rotamer of 1 which is forced by nonbonded interactions into a cleavage mode which twists the biradical into a geometry suitable for at least partial formation of cis-enal. In both cases biradicals must be very short-lived and not rotationally equilibrated.     

Untersuchungen der Konformation von alternierenden Copolymeren ausL-Leucin undL-Ornithin in wäßrigen Lösungen

The conformation of the alternating copoly-(L.-leucyl-L-ornithine) was studied in solutions of various salts and in salt-free water as well by CD technique. This copolymer is shown to undergo a conformational transition from a disordered to aβ-structure by adding salts or with increasing the pH to a alkaline region. Such a tendency to form aβ-structure is enhanced by neutral salts like KF, NaF, and NaCl and remarkably enhanced by water structure breaking anions like LiClO₄ and NaClO₄. Theβ-structure induced by perchlorate ions is stable up to 90 °C. This finding can be interpreted in terms of the shielding effect resulted from a specific binding of perchlorate ions with positively charged side chains. Aβ-I-structure is also induced by water structure making anions like Li₂SO₄ and Na₂SO₄, but theβ-structure inducing molecular mechanism is probably different from the case of water structure breaking anions, due to its electrochemical bivalency. This becomes obvious from the fact that above 0.005 mole/l precipition occurs and from model considerations.     

Processus concurrents de photoisomerisation d'hexatrienes polymethyles: effet de longueur d'one

The ground-state conformer composition is shown to play a determining role in the orientation of the photochemical reactions of two hexatrienes. A pronounced wavelength effect is observed for Z-2,6-dimethyl-1,3,5-heptatriene; photocycloaddition leads to a bicyclo[3.1.0]hexene and [1,5] sigmatropic hydrogen migration occurs from an s-cis-s-trans conformation preferentially excited by longer wavelength light, whereas the Z-E interconversion is predominantly observed as a result of excitation of the s-trans-s-trans conformation by shorter wavelength light.     

Electroporation: A unified,quantitative theory of reversible electrical breakdown and mechanical rupture in artificial planar bilayer membranes

We present a quantitative theory of electroporation of artificial planar lipid bilayer membranes. Assuming that aqueous pores are involved in electroporation, we describe the pore population of the membrane by the density function n (r, t), where n (r, t) dr is the number of pores with radius between r and r + dr at time t. We further assume that there is a minimum pore size r_min, that pores of radius r_min are created and destroyed by thermal fluctuations, and that the pore creation rate is proportional to exp(aΔφ_m²/kT, where a is a constant, Δφ_m is the membrane voltage, k is Boltzmann's constant, and T is the absolute temperature. We use a simple formula for the conductance of a pore as a function of radius, the expression for the pore energy previously derived by Pastushenko and Chizmadzhev, and a simple model of the external circuit. We solve the equations numerically and compare the solutions to the results of charge pulse experiments.In a charge-pulse experiment a membrane suffers one of four possible fates: (1) a slight increase in electrical conductance, (2) mechanical rupture, (3) incomplete reversible electrical breakdown, resulting in incomplete discharge of the membrane, or (4) reversible electrical breakdown (REB), resulting in complete discharge of the membrane. In agreement with experiment, this theory describes these four fates and predicts that the fate in any particular experiment is determined by the properties of the membrane and the duration and amplitude of the charging pulse.     

Molecular energy parameters by solution thermodynamics

The relationships between statistical thermodynamics and equilibrium constants, either cumulative, stepwise or specific site constants are investigated. In order to show the link between equilibrium constants and statistical thermodynamic microscopic properties, a distinction has been introduced between non-reacting and reacting systems. The non-reacting systems are those for which continuous statistical distributions of enthalpies can be assumed. The distribution function can be obtained as an integral of the interparticle potential extended to the whole ensemble. Molecular partition functions ζ_A, are used to describe the properties of the ensembles. The reacting ensemble is represented by distinct distributions of enthalpies, each distribution being grouped around a mean value. Each level is representative of one species. Around each mean level the distribution is continuous as in non-reacting ensembles. The reacting ensemble of particles is described by a grand canonical molar partition function Z_M=(1+kγ(i)[A])^t where k is the specific site constant, γ(i) is the cooperativity function, [A] is the concentration of free ligand, and the power t indicates the maximum number of i sites in one class. The specific site constant k is proportional to the affinity of binding and is related to the depth of the minimum of the potential function. The factor γ_i is the cooperativity factor given by the value of the cooperativity function γ(i) at the ith level and indicates how the depth of the potential function is affected by previous binding of a ligand. The values of the stability constant and cooperativity factor can be optimized by a computer program. The derivatives of the partition function Z_M with respect to ln[A] correspond to the formation function 〈n〉 (first derivative) and to the buffer capacity B_C (second derivative). The derivatives of the partition function Z_M with respect to temperature are reaction enthalpy ΔH (first derivative with −(1/T)) and apparent heat capacity ΔC_p,app (second derivative with ln T). The denaturation heat obtained by integration of ΔC_p,app dT for many proteins explains why the denaturation enthalpy depends linearly upon T.     

Predicting the efficiency of antioxidants in polymers

The efficiency of antioxidants in polymers is determined by three processes: the chemical reactivity, the physical loss and a minimal effective concentration. In most cases, loss of the stabiliser by physical processes (evaporation, leaching) is more significant than the loss by chemical consumption. In this paper, a model is proposed that relates the protection time of an antioxidant to the migration in a polymer matrix. The parameters that determine the migration, i.e. mobility (D) and solubility (S) are predicted using theoretical modelling. A linear correlation was found between the protection time and the function −S ln(D). In some cases this correlation is better than that of the empirical model proposed by Moisan, ln(S²/D).     

The absolute configuration of peroxisomicines A1 and A2

Peroxisomicine A1 is a potentially antineoplastic compound isolated from the seeds of Karwinskia parvifolia. It is considered as a useful chemotype for the preparation of topoisomerase II targeted anticancer cells. Stereochemically, it is characterized by the presence of two stereocenters and a rotationally hindered and thus likewise stereogenic biaryl axis. In this contribution, the absolute configuration of peroxisomicine A1 and its epimer, peroxisomicine A2, was established by means of a five-step degradative procedure giving the respective R- and S-configured methyl 2-(2′-methyl-5′-oxotetrahydrofuryl)acetates. The configuration of the degradation product was obtained by means of optical rotation, ¹H NMR analysis using a chiral displacement reagent, and by experimental and quantum chemical circular dichroism (CD) investigations. Based on the results obtained here and considering our previous work on the relative configuration at centers versus axis of these compounds, peroxisomicine A1 resulted to be the P,3S,3′S-isomer and peroxisomicine A2 the P,3R,3′S-isomer.