A CNDO-MO calculation of VCl4

The electronic structure of the tetrahedral molecule VCL₄ is investigated within the CNDO-MO approximations. The metal and ligand valence orbitals, 3d, 4s, 4p; and 3s, 3p; respectively, have been systematically varied in an attempt to minimize the total energy; “optimum” V 4s(χ₄ = 1.10) and 4p(d ³ p ²) orbitals have been established, but V 3d(d ⁿ ) and Cl(-δ) valence orbitals are only seen to favor lower energy for expanded orbitals. Since determining the one-electron molecular orbital level which is occupied by the vanadium lone electron is a major aspect of this investigation, all calculations have been performed in triplicate: calculations assuming the unpaired electron occupies the 3a ₁, 2 _e and 4t ₂ molecular orbital (ground state electronic configurations² A ₁,² E, and² T ₂, respectively). The Hartree-Fock equations have been solved by Roothaan's SCF method for open shells, but off-diagonal multipliers between filled and partly filled molecular orbitals of the same symmetry have been neglected. As a qualitative estimate of the error introduced by this simplification, the pertinent overlap integrals between the eigenfunctions from calculations for the three possible configurations,² A ₁,² E, and² T ₂, are investigated as functions of the component 3d(d ⁿ ) and Cl(-δ) valence orbitals. The overlap integrals from the relevant² A ₁ and² T ₂ calculations are reasonably small, but the neglect of off-diagonal multipliers in calculations on the² E state is found to be a poor approximation. An ordering of the non-filled molecular orbitals in VCl₄ of 4t ₂ < 3a ₁ < 2e < 5t ₂ seems most consistent with the numerous calculations. This suggested ground state electronic configuration of² T ₂ introduces new aspects to the consideration of a (dynamic) Jahn-Teller effect in VCl₄. Experimental data pertinent to the electronic structure of VCl₄ has been briefly summarized, but unfortunately it is inadequate to confirm or deny the present calculations.     

Concentration dependent spectral changes of electrons trapped in MgCl2 and CaCl2 glasses. A pulse radiolysis study

The aqueous MgCl₂ and CaCl₂ glasses were subjected to pulse radiolysis at 87 K. The time evolution and concentration dependence of e^-_t optical absorption spectra was examined.For MgCl₂ glasses the increase of Mg²⁺ concentration from 2.6 to 5 mol dm^-3 is followed by the shift of λ_max from 665 to 615 nm. For CaCl₂ glasses the concentration change from 3.1 to 4.5 mol dm^-3 Ca²⁺ causes the shift of λ_max from 625 to 590 nm. The blue shift is also observed during partial decay of e^-_t on μs time scale.With increasing concentration of electrolyte the contribution of e^-_ir to the total e^-_t absorption decreases, whereas that of e^-_vis increases.The ratio of [e^-_ir]/[e^-_vis] measured on micro-second time scale is significantly higher for Mg²⁺ than Ca²⁺ glasses.     

New Dunham coefficients of the A 1 ∑ +-State of NaH and NaD

We determined new Dunham coefficients of the A ¹ ∑ ⁺-State of NaH and NaD from degenerate Four- Wave-Mixing (DFWM) spectra in the near UV and in the blue spectral region. In the case of NaH we combined these data with results of Rafi et al. and Orth et al.. The new set of coefficients describes the vibrational dependence of the rotational constants B and D and of the band origins from ? = 0 up to 25. The spectral positions of the lines in our DFWM-spectra can be reproduced by this coefficients with an accuracy better than 0.3 cm^-1 for J-values ≤ 15 and 2 cm^-1 for J-values ≤ 25. Especially for high J-values this is an improvement up to 30 times in comparison to Dunham coefficients recommended before. A RKR-potential of the A ¹ ∑ ⁺-state was calculated with the new coefficients. The Dunham coefficients of NaD were obtained by scaling the NaH coefficients with the reduced masses of the molecules. A comparison of our results to molecular constants determined from the measured NaD-spectra shows good agreement.     

A spectroscopic study of double electron transfer from Cu to Ar III in an ECR-microwave discharge

We present spectroscopic measurements of rate coefficients for the population of the ArI 6d[3/2] ^o and 8s[3/2] ^o states by two electron transfer from sputtered Cu to Ar²⁺ in an ECR microwave discharge. Rate coefficients are obtained from absolutely measured line intensities using a modified corona model. Though the binding energies of these states differ by 8 meV only the probability of populating the 6d level exceeds by a factor of ten that of populating the 8s level. This is attributed to an energy resonance between the two electrons when promoting to their final levels. The cross section for this two electron transfer comes out to be of the order of 10^?15 to 10^?14 cm².     

A novel stereochemical control : A persistent preference for exo electrophile acceptance in 2-bornanes

The conjugate bases of camphor oxime and 2-nitro-bornane, possessing a π array, accept elements of H⁺ and Br⁺ selectively from the “crowded” exo side. The present work adds another facet towards the understanding of stereochemical controls inherent in the bornane framework, the other two related being the endo addition of nucleophiles to camphor and the exo electrophile acceptance by camphor enolates. The present work describes, inter alia, a practical route to 2-endo nitrobornane and the novel 2,2-dinitrobornane.     

A discretisation of the Laplace transformation and an accurate method for the coulomb three-body problem

The atomic (H ^?,D ^?,T ^?,^∞ T ^?), electron-positron (e ^? e ⁺ e ^?),e ^?μ⁺ e ^? (leptonic) and mesomolecular (p p μ) systems were calculated using a method based on discretisation of the Laplace transformation. The results have rather high accuracy and can serve as a theoretical basis for experimental study of three-body systems. The results forH ^? permit the isotopic shift in the 1¹ S state of this system to be determined. The analysis of highly accurate results probably makes it possible to determine even negligible deviations ofpp potential from a point Coulomb. Moreover, all results presented can be used as tests in other methods for calculating three-body systems.     

A joint vibro-electronic mechanism in the dissociation of molecular hydrogen in nonequilibrium plasmas

A new mechanism Of H₂ dissociation in electrical discharges (10¹¹ ? n_e ? 10¹² cm^?3, 2.10^?16 ? E/N ? 3.10^?16 V cm², 300 ? T_g ? 1000 K, 3 ? p ? 30 torr) is presented and discussed. In this mechanism, called joint vibro-electronic mechanism (JVE), the electrons of the discharge create a strong vibrational disequilibrium with respect to the gas temperature (T_g) and promote electronic transitions from all vibrational levels of ¹Σ_g H₂ state to the repulsive ³Σ_u one. Moreover the V-V (vibration-vibration) and V-T (vibration-translation) energy exchanges are considered for building up the vibrational distribution of ¹Σ_g state. A complete set of e - D cross sections (e + H₂(¹Σ_g,ν) → e + H₂ (³Σ_u) → + 2H, ν = 0,14) is calculated by using an extension of the semiclassical Gryzinski theory in combination with the Franck-Condon principle. Dissociation rates calculated according to JVE are larger either than those obtained by the pure vibrational mechanism (PVM) discussed in our previous work or than those from the direct electronic impact mechanism (DEM) from the ground vibrational level. The behaviour of JVE rates as a function of gas temperature (T_g), of E/N, of electron density (n_e) and of pressure is then reported. The results show strong differences as compared, with the corresponding values obtained, with PVM. Finally the influence of the atoms as well as their recombination on the dissociation rates is discussed. The results have been obtained by solving a system of vibrational master equations.     

A simple sum rule in raman theory

It is shown that in the limit of large detuning energy and in the absence of coordinate dependence of the transition moment, the resonance Raman amplitude for a 0 → n transition on a harmonic potential surface is proportional to (δ²_? + δ²₊)^?1 × ?^∞_?∞ exp (?q²) · [?ⁿΔV(q)/?q¹¹] dq< where ΔV(q) is the difference between the arbitrary excited-state surface and the initial harmonic potential. The resonant and non-resonant detuning energies are given by δ_? = E ? hv and δ₊ = E + hv, where v is the incident laser frequency and E is the minimum separation between the potential surfaces.     

A novel acylated quercetin tetraglycoside from oolong tea (Camelia sinensis) extracts

A novel acylated quercetin tetraglycoside, namely quercetin 3-O-(2^G-p-coumaroyl-3^G-O-β-l-arabinosyl-3^R-O-β-d-glucosylrutinoside) was isolated from oolong tea (Camelia sinensis) extracts. Structural analysis of this compound was achieved by NMR, TOF-MS and high-resolution FAB-MS. Triglycosyl flavonols have previously been reported from tea leaves and tea seeds however this is the first report of an aromatic acylated and tetraglycosyl flavonol.     

First enantioselective synthesis of the novel antiinfective TAN-1057A via its aminomethyl-substituted dihydropyrimidinone heterocycle

Enantiomerically pure N²-Z-N²-MeAsnOH [(S)-14], prepared in 8 steps (23% overall yield) from asparaginic acid, was first subjected to a Hofmann degradation with PhI(OCOCF₃)₂ yielding (S)-N²-Z-N²-methyl-2,3-diaminopropanoic acid [N²-Z-N²-Me-L-A₂pr, (S)-15], and this in turn was protected to give N²-Z-N³-Boc-N²-Me-L-A₂pr [(S)-17]. Condensation of (S)-17 with HNC(SMe)NHCONH₂ followed by removal of the tert-butoxycarbonyl protecting group, cyclization and hydrogenolytic removal of the Z-group gave the heterocycle of TAN-1057A [(S)-1] with an e.e. of 87 in 36% yield [from (S)-14]. Coupling of (S)-1 with (S)-tris-Z-β-homoarginine (20a) in the presence of O-(7-azabenzotriazol-1-yl)-N,N,N′,N′-tetramethyluronium hexafluorophosphate (HATU) and iPr₂NEt in N,N-dimethylacetamide followed by hydrogenolysis afforded the most active A-diastereomer of the natural antibiotic TAN-1057 in 52% yield (from (S)-1 and 20a). Similarly, starting from (S)-1, a single diastereomer of the potent, less toxic TAN-1057A analogue 22b with a β-lysine side chain has been prepared. All described synthetic steps do not require column chromatography for purification of the products.     

A direct route for the preparation of Fmoc/OtBu protected iodotyrosine

Herein the synthesis of an Fmoc/O^tBu orthogonally protected iodotyrosine derivative is reported. This has been achieved via a simple two-step process in an overall 58% yield from commercially available Fmoc-Tyr(^tBu)-OH. The Fmoc/O^tBu orthogonally protected iodotyrosine was also shown to be amenable to Suzuki-Miyaura cross-coupling to deliver a novel bi-aryl tyrosine derivative.     

A proton and carbon-13 nuclear magnetic resonance study of neopentylmercury compounds

Substituent effects on ¹⁹⁹Hg¹H and ¹⁹⁹Hg¹³C spinspin coupling constants have been studied for neopentylmercury derivatives, (CH₃)₃CCH₂HgR(or X), where R is covalently bonded Me, Et, t-Bu, neopentyl, and vinyl, and X is easily ionizable CN, Br, Cl, OCOCH₃, and ONO₂. Linear relationships exist between the methylene J(¹³CH) and ²J(HgH), ⁴J(HgC) and ²J(HgC) and ³J(HgC); but deviations from linearity occur for the chloride, bromide, acetate, and nitrate in the relationships between ²J(HgH) and ⁴J(HgH), ²J(HGH) and ²J(HGC). These deviations are discussed in terms of hyperconjugative p_πd_π bonding between the methylene CH bonds and mercury.     

A general iterative method for obtaining kinetic parameters from TG data

An iterative method is described for determining the reaction order and activation energy from TG curves. The method makes use of equations to represent the temperature integrals which are derived using numerical relationships in terms of E, T, and empirical constants. Like the method of Reich and Stivala, the computation involves varying the value of n until the appropriate linear relationship gives an intercept of zero. The slope of the line is YE^x, where Y and X are constants in the equation -log I = YE^x(1/T) +log E^w+ U The method is tested using data obtained by means of a fourth order Runge-Kutta solution of the rate law for both Arrhenius and non-Arrhenius cases.     

A kinetic determination of ultramicroquantities of some biologically active substances

A new kinetic method for the determination of ultramicroquantities of adrenaline, noradrenaline, thyroxine, and 5-hydroxytryptophan is presented. The method is based on the effect of these organic substances on the oxidation of pyrocatechol violet with hydrogen peroxide, catalyzed by Cu(II) ions.In order to find out the experimental conditions under which this effect is optimum, the kinetics of the above-mentioned indicator reaction in the presence of the compounds to be determined was studied in detail.Adrenaline and noradrenaline were determined at concentrations ranging from 2.0 × 10^?6 to 8.0 × 10^?6M, thyroxine at concentrations varying from 1.0 × 10^?6 to 10.0 × 10^?6M, and 5-hydroxytryptophan over the concentration range 7.7 × 10^?7–32.0 × 10^?7M. In these determinations the standard deviation was lower than 10%.     

A test of the accuracy of atomic-beam laser spectroscopy

To determine the accuracy to which hyperfine splittings can be measured using a cw dye laser with rf sideband tuning, a series of atomic beam experiments with⁵⁹Co was performed. One hyperfine splitting of the first excited metastable atomic state was measured and compared with the results for the same splitting from magnetic rf resonance experiments. The uncertainty with the methods applied was found to be about 0.05 to 1% of the experimental linewidth in general; it is ±20 kHz or ±0.05% of the linewidth in the present Co experiment. The hyperfine splitting constants of the 3d ⁸ 4p ² F _7/2 state were found to beA=419.3(9) MHz,B=?77(17) MHz.     

A quantitative interpretation of the magnetic field effect on hyperfine-coupling-induced triplet fromation from radical ion pairs

Experimentally determined B_1/2 values characterizing the magnetic field dependence of molecular triplet production from radical ion pairs originating from photoinduced electron transfer are compared with semi-empirical values obtained according to B_1/2(hfi) = 2(B₁² + B₂²)/(B₁ + B₂) from the root-mean-square values for the hyperfine coupling of the two radicals, B₁ and B₂. The very good agreement is discussed.     

A structural study of ternary lanthanide orthoscandate perovskites

Ternary lanthanide scandates (Ln=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, and Ho) have been synthesized at ambient pressure. Their structure has been investigated at room temperature by Rietveld analysis of powder X-ray diffraction data. The Ln-scandates are orthorhombic perovskites, adopting space group Pbnm (? 62), a≈b≈√2a_p, c≈2a_p, Z=4. Heavy lanthanides (Er-Lu), and Y do not form perovskites at ambient conditions. Compositionally driven phase transitions were not observed. The unit-cell parameters decrease with increasing ScO₆ octahedron rotation and atomic number of the Ln cation. In common with lanthanide orthoferrites, the uniform structural evolution is interrupted at the middle-heavy part of the lanthanide sequence. This is probably due to an interplay between: (i) enlargement of the ScO₆ octahedra relative to BO₆ in other perovskites (e.g., FeO₆ in GdFeO₃); (ii) reduction in size of the first coordination sphere of Ln³⁺ coincident with the lanthanide contraction; (iii) coincident expansion of the second coordination sphere due to screening effects of O^I1 on O^I2, and entry of Sc to the lanthanide coordination sphere; (iv) complex mixing between oxygen and lanthanide lanthanide f- and scandium d-orbitals. In the series studied, Ln³⁺ are in eight-fold coordination (tetragonal antiprism), and are considerably displaced from the center of the LnO₈ polyhedron along [001]. Evolution of the crystallochemical characteristics through the Ln orthoscandate series is complex due to both the antipathetic distortions of A- and B-site coordination polyhedra and interaction of the orbitals of oxygen, Ln and Sc. Empirically obtained limits of Goldschmidt and observed ^viiit_o tolerance factors for ternary LnBO₃ compounds adopting the Pbnm structure are 0.795 and 0.841, respectively.     

The radiative lifetime of the A1II state of BH

Second-order polarization propagator calculations of the X¹Σ⁺ → A¹II transition moment as well as the radiative lifetime of the A¹II state of BH are reported. The calculated vibrational lifetimes are τ(v′ = 0) = 121 ns, τ(v′ = 1) = 129 ns, and τ(v′ = 2) = 137 ns. The τ(v′ = 0 lifetime agrees with the most recent experiment of τ(v′ = 0) = 125 ± 5 ns. We show that the electronic oscillator strength computed at the ground state equilibrium is rather different from the band absorption oscillator strength f₀₀, which demonstrates that theoretical electronic oscillator strengths should not be expected to agree with experimentally determined band oscillator strengths.     

Synthesis, structural aspects and cytotoxicity of the natural cyclopeptides yunnanins A, C and phakellistatins 1, 10

Yunnanins A and C, two cyclic heptapeptides occurring in the roots of Stellaria yunnanensis, and phakellistatins 1 and 10, a hepta- and an octacyclopeptide first isolated from marine sponges of the genus Phakellia, were efficiently synthesized using a combination of solid and solution-phase techniques. Structural analysis on the synthetic members of the yunnanin series showed that the synthetic sample of yunnanin A exhibited a configurational pattern at the Pro peptide linkages identical to the natural product (trans-Pro³, trans-Pro⁵), while yunnanin C was obtained as a complex mixture of geometric/conformational isomers; the major isomer (trans-Pro³) was indistinguishable from the natural cyclopeptide and co-occurred along with lower amounts of a mixture (1:1 ratio) of two different rotamers, both displaying cis geometry at the Pro³ linkage. In the phakellistatin series, the synthetic phakellistatin 1 (determined as cis-Pro¹, cis-Pro³, cis-Pro⁵) was identical to the natural one, while two different isomeric products of phakellistatin 10 could be obtained: a major one (trans-Pro¹, trans-Pro⁴, trans-Pro⁶) showing spectral properties superimposable with the natural metabolite, and a minor geometric isomer of the natural cyclopeptide. Interestingly, the synthetic cyclopeptides, although found to be chemically identical with their natural counterparts, did not display the same biological properties (in vitro cytotoxicity against a panel of cancer cell lines), leaving presently open the question whether or not the potent bioactivity reported in the literature should really be attributed to these natural cyclic peptides.     

A test of the R-centroid method

The R-centroid method is shown to be exact when the molecular dipole transition moment, D(R), varies linearly with internuclear distance R, but may fail to give accurate results in the region of a maximum in D(R). Using individual line strengths computed for the X¹Σ⁺—A¹Σ⁺ system in LiH in which D(R) goes through a maximum, it is shown that good results can still be obtained if only the strongest transitions are used in the R-centroid analysis.