Raman H-bond pair volume for water

The dispersion of the H-bond pair volume Delta V over the decoupled OD and coupled OH-stretching contours from HDO in H(2)O was determined from Raman intensities at pressures to 9700 bar at 301 K. The dispersion of Delta V was determined from -RT[partial differential ln(I(i)/I(REF))/ partial differential P](T) versus omega (in cm(-1)), where i refers to omega's over the stretching contours and I(REF) refers to the reference intensity at the isosbestic frequency. The maximum H-bond pair volume (defined for breakage) is 1.4+/-0.1 cm(3)/mol H-bond, which corresponds to the volume difference between a large dispersion maximum at 2,675 cm(-1) (near the OD stretch omega of HDO in dense supercritical water) and a large, broad minimum centered near 2,375 cm(-1) (just below the OD stretch omega of HDO in lda ice). The average DeltaV is 0.71+/-0.10 cm(3)/mol H-bond. Other minima near 2,625 cm(-1) (OD) and 3550 cm(-1) (OH) refers to bent H-bonds whose angles are approximately 150 deg. Isothermal pressurization of water lowers the molal volume by decreasing the concentration of long, weak H-bonds, and increasing the concentrations of bent H-bonds and short, strong, linear H-bonds. Such bending, shortening, and strengthening produces freezing to ice VI near 10 kbar at 301 K. The isobaric temperature derivative of the maximum H-bond volume is (partial differential Delta V/partial differential T)(P)< or =(2-5) x 10(-3) cm(3)/deg mol H-bond. The OH enthalpy dispersion curve for saturated NaBF(4) in water, yields a large maximum at 3,530-3,540 cm(-1) indicating that BF(4) (-) interacts preferentially with the dangling or "free" OH groups of water thus producing weak, strongly bent H-bonds having angles similar to those of the 3,550 cm(-1) high-pressure H-bond bending feature.     

Solution-phase, triangular ag nanotriangles fabricated by nanosphere lithography

A novel method to produce solution-phase triangular silver nanoparticles is presented. Ag nanoparticles are prepared by nanosphere lithography and are subsequently released into solution. The resulting nanoparticles are asymmetrically functionalized to produce either single isolated nanoparticles or dimer pairs. The structural and optical properties of Ag nanoparticles have been characterized. Mie theory and the Discrete Dipole Approximation method (DDA) have been used to model and interpret the optical properties of the released Ag nanoparticles.     

Theory of absorption and emission of radiation in molecular collisions: Semiclassical model

A semiclassical transition amplitude is derived for electronically inelastic molecular collisions in the presence of an intense electromagnetic field. The calculation of this amplitude involves the integration of classical trajectories for nuclei moving on effective potential energy surfaces. These surfaces, which exhibit an avoided crossing due to the coupling through the dipole interaction around the resonance nuclear configuration, are analytically continued to their complex intersection points, where trajectories switch surfaces. Other parts of these surfaces are similar to the field-free adiabatic surfaces, where one is shifted by ?ω. Metastable states, formed at some collision energies, are suggested in the field which are absent in the field-free system.     

Thermal addition of dibenzoylacetylene to cycloheptatriene and the photo-rearrangement of the thermal adduct

The reaction of cycloheptatriene (CHT) with dibenzoylacetylene (DBA), in refluxing toluene gave a mixture of products consisting of 6,7-dibenzoyltricyclo[3.2.2.0^2.4]nona-6,8-diene 6 and 3,4-dibenzoylbicyclo [3.2.2]nona-2,6,8-triene 7. Neat heating of a mixture of CHT and DBA around 95–100° gave besides, 6 and 7, small amounts of two dimers of DBA. Photolysis of 6 in benzene solution gave a lactonic, rearrangement product. 25. The structures of the various products have been established and reasonable mechanisms for their formation have been suggested.     

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&#x27; -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.     

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.     

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.