Angular momentum decoupling approximations in the quantum dynamics of reactive systems

Two methods for implementing angular momentum decoupling approximations in quantum mechanical reactive scattering examined. Applications of both reactive and nonreactive H + H₂ collisions indicate that for the most intense individual reactive transitions and for all degeneracy-averaged ones, these decoupling methods (especially the “proper” decoupling method) yield results in good agreement with those of fully-coupled calculations. However, for the less intense reactive transitions and all individual nonreactive transitions, very large errors can result from use of these approximate methods.     

Δ4,4' -4-chalcogenapyranyl-4h-chalcogenapyrans : Synthesis,electrochemical oxidation,and esr investigations of radical-cation states

The O-, S-, Se- and Te-containing Δ^4,4' -2,2?,6,6'-tetramethyl-, -tetra-t-butyl- and -tetraphenyl-4-(chalcogenapyranyl)-4H-chalcogenapyrans were prepared from the corresponding chalcogenapyran-4-ones. The thia-, selena- and tellurapyran-4-ones were prepared by sodium borohydride reduction of the appropriate chalcogen and addition of the disodium chalcogenide to a 1,4-diyn-3-one. The comparative electrochemistry of all three series was examined by cyclic voltammetry. Within each scries the first oxidation (E₁) became more difficult with increasing chalcogen size. The radical-cation states of the tetra-t-butyl analogues were examined by ESR. The O and S analogues gave well-defined five-line spectra, but the Se and Te analogues gave broad single lines. The g value increased with increasing size of the heteroatom. A linear correlation exists between g value and spin-orbit coupling constants λ for the chalcogens, suggesting that the electron densities on the heteroatoms are similar in each heteroatom analogue.     

Crystal Structure and Packing of Isocyclosporin A

The crystal structure of isocyclosporin A ( 1 ), a rearrangement product of the immunosuppressant drug cyclosporin A, has been determined at 193 (2) K. Crystals are orthorhombic with cell dimensions a = 26.684 (7), b = 26.936 (3) Å, c = 28.549 (7) Å, space group C222₁. The structure was solved by direct methods and refined by full-matrix least-squares methods to a conventional R value of 0.110. In contrast to the structure of cyclosprin A in solution and in the crystal, isocyclosporin A ( 1 ) has no regular secondary structural elements. The backbone adopts an open, irregular conformation with cis amide bonds between residue 2 and 3, and 3 and 4, respectively. All the other amide bonds and the ester linkage are trans. Contrary to crystal structures of cyclosporin derivatives, this crystal structure is stabilized by two transannular and four intermolecular H-bonds.     

An ab initio study of the geometry,harmonic and anharmonic force fields,and fundamental vibrational frequencies of cis- and trans- thiolformic acid

The geometry, harmonic and anharmonic force fields, and fundamental vibrational frequencies of cis- and trans-thiolformic acid are studied ab initio in the 4-31G basis set. An extensive comparison is made between changes in diagonal and off-diagonal quadratic and cubic force constants and diagonal stretching quartic constants in going from the chain to the ring structure in thiolformic acid and formic acid. The changes in the force constants are indicative of a much weaker interaction in the trans conformer between S? H and O?C, compared with O–H and O?C, in keeping with the weaker hydrogenbonding property of the S? H group in general.     

Gradient expansion of the atomic kinetic energy functional

It is demonstrated that the ground-state atomic kinetic energy functional T[?], where ? is the electron density, can be computed to surprising accuracy from the truncated gradient expansion: T[?] = + T₂[?] + T₄[?], with T_o[?] = $\text{3}\text{10}$(3π²)$\text{2}\text{3}$ ∫ ?$\text{5}\text{3}$ d_τ, T₂ [?] = $\text{1}\text{72}$ ∫ (??)²?^?1 d_τ, and T₄ [?] given by the formula of Hodges. Calculations of T₀, T₂ and T₄ are reported for He with ? both the Hartree—Fock and a very accurate density, and for Ne, Ar and Kr with ? the Hartree—Fock density. For Kr, T₀ + T₂ + T₄ is within 0.3% of the exact Hartree—Fock T, with T₂/T₀ = 0.05, T₄/T₂ = 0.17.     

The crystal structure of tetrakis(methyldiphenylphosphine)iridium(I) tetrafluoroborate with cyclohexane of solvation: Ir[P(C6H5)2CH3]4BF4 - C6H12

The crystal structure of tetrakis(methyldiphenylphosphine)iridium(I) tetrafluoroborate with cyclohexane of solvation, [Ir(PPh₂Me)₄]BF₄·C₆H₁₂, has been determined from a three-dimensional X-ray analysis. The compound has been analysed in space group C₂/c of the monoclinic system. There are twelve molecules (i.e. 1.5 molecules per asymmetric unit) in a cell of dimensions a = 36.804(8), b = 22.93(2), c = 21.676(4) Å, β = 121.41(1)°. Block-diagonal least-squares refinement has given a final R-factor of 0.060 for 7905 reflections having I > 3σ(I).The structure consists of two crystallographically distinct, but structurally similar molecules, one on a general position and one on a crystallographic two-fold axis. The phosphine ligands around the iridium atoms are in a very distorted square-planar arrangement. The reactions of the cation axe discussed in terms of this structure.     

Anionic-initiated polymerization of β-nitrostyrenes

β-Nitrostyrene and many of its derivatives have been shown to yield high polymers via anionic initiation with alkoxide ions in protic solvents, e.g., alcohols. A study was conducted to determine the effect of certain substituents on the polymerization characteristics of representative monomers of this series and the properties of their polymers. A kinetic study was conducted and the relative rates of propagation were unexpectedly found to correlate well with brown's σ⁺ values. The rates of initiation of two representative monomers, β-nitrostyrene and p-methoxy-β-nitrostyrene, with sodium ethoxide were found to be 3.51 and 2.86 liter/mole sec, respectively. The rate of chain transfer in ethanol was studied qualitatively by using gel-permeation chromatography (GPC) to obtain molecular weight distribution (MWD) curves. The low values of the M _w/M _n ratios indicated little chain transfer in the protic solvent.     

Self-assembled luminescent CdSe-ZnS quantum dot bioconjugates prepared using engineered poly-histidine terminated proteins

We report a simple and versatile approach for the conjugation of luminescent CdSe-ZnS core-shell quantum dots (QDs) to proteins through coordination of engineered C-terminal oligohistidine sequences. Several histidine tail containing proteins were self-assembled onto the QD surface using this method. A recombinant antibody specific for the high explosive 2,4,6-trinitrotoluene (TNT) was conjugated to QDs through a carboxy terminal histidine tail and the bioconjugate used to detect TNT by competitive immunoassay. TNT was detected over the range of 10 μg/ml down to 41 ng/ml using the scFv conjugated to QDs. These results open up the possibility to conjugate luminescent QDs to a whole range of proteins to form QD bioconjugates that can be effectively used in bio-oriented applications, such as sensing, imaging, immunoassay and other diagnostics.     

The growth of crystals in solution

The crystallization of sparingly soluble salts from their aqueous supersaturated solutions is discussed from the standpoint of two important applications; scale formation and biological mineralization. Theories of crystal growth are outlined and the importance of kinetic factors in determining the nature of the growing phases is discussed. The kinetic factors can be studied by using a highly reproducible seeded growth technique and under certain conditions secondary nucleation can also be induced on the surface of the inoculating seed crystals. The kinetics of crystallization of the alkaline earth surfaces and the calcium phosphates is discussed. In the latter systems, temperature, supersaturation, surface concentration, pH, ionic strength and the presence of foreign ions are important in determining the nature of the phase which grows on the added seed crystals. The mechanism of the retardation of crystal growth by added crystallization inhibitors is illustrated by the influence of organic phosphonate molecules upon the precipitation of calcium carbonate.