Prediction of viscosities and vapor–liquid equilibria for five polyhydric alcohols by molecular simulation

Reverse nonequilibrium molecular dynamics in the canonical ensemble and coupled–decoupled configurational-bias Monte Carlo simulations in the Gibbs ensemble were used to predict the low-shear rate Newtonian viscosities and vapor–liquid coexistence curves for 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2-methyl-1,3-propanediol, and 1,2,4-butanetriol modeled with the transferable potentials for phase equilibria-united atom (TraPPE-UA) force field. Comparison with available experimental data demonstrates that the TraPPE-UA force field yields very good predictions of the viscosities and vapor–liquid coexistence curves. A detailed analysis of liquid structure and hydrogen bonding is provided.     

Modelling the purchase propensity: analysis of a revolving store card

We investigate the incremental roles of information that becomes available only after a revolving loan has been granted in explaining and predicting the time taken until the borrower makes a second purchase. Using data relating to a store card, granted around the time of first purchase and used in Belgium, we find that characteristics of a first purchase and remaining credit available for use enhance the explanatory and predictive power of application characteristics. The relationship differs between good and poor payers.     

Mechanistic studies of Hangman salophen-mediated activation of O-O bonds

Stopped-flow kinetic studies of a HSX-Mn-SalophOMe (1) catalyst provide spectroscopic evidence for the direct generation of a manganese(V) oxo salophen from a manganese(III) perbenzoate. The O-O bond heterolysis reaction that produces the oxo is not facilitated by intramolecular proton transfer from the acid hanging group of the HSX platform. Instead, the hanging group stabilizes the catalyst against oxidative degradation and, consistent with recent predictions of theory, is geometrically matched to promote the end-on coordination of a H2O2 substrate prior to its oxidation at the manganese(V) oxo center.     

Positive exponential sum method of inverting Laplace transform applied to photon correlation spectroscopy

A computer program for the positive exponential sum method of inverting Laplace transform of photon autocorrelation curves has been written and applied to both simulated and experimental data. The method recovers the decay time spectra from autocorrelation curves by replacing the decay time distribution by a set of delta peaks whose envelope roughly follows this distribution. Spacing of this set becomes slightly denser with decreasing noise and is about two peaks per decade under usual conditions (noise level about 10^–3). Occasional irregularities in peak spacing and an appearance of weak artifact peaks in regions where the decay time spectrum is of zero density may make the physical interpretation of results difficult.     

The peak constrained positive exponential sum method of inverting Laplace transform applied to correlation spectroscopy

As a contribution to the basic problem of correlation spectroscopy, a method has been developed for the Laplace transform inversion with a given number of maxima in the nonnegative inverted function() (i.e. in the distribution function of decay times recalculated as a density function on a logarithmic scale) by the least squares method. The resulting solution consists of the given number of-functions, each of which may be accompanied on one or both sides by one or several histogram bins decreasing away from the-function. When applied to simulated data for quasielastic light scattering (QELS), the method yields good agreement of the calculated distributions with the simulated ones, except that it yields sharp edges to the histogram bins and artefact-functions at the maxima of all the bands. An example shows the method to be a useful tool in interpreting QELS data.     

Molecular chemistry of consequence to renewable energy

Energy conversion cycles are aimed at driving unfavorable, small-molecule activation reactions with a photon harnessed directly by a transition-metal catalyst or indirectly by a transition-metal catalyst at the surface of a photovoltaic cell. The construction of such cycles confronts daunting challenges because they rely on chemical transformations not understood at the most basic levels. These transformations include multielectron transfer, proton-coupled electron transfer, and bond-breaking and -making reactions of energy-poor substrates. We have begun to explore these poorly understood areas of molecular science with transition-metal complexes that promote hydrogen production and oxygen bond-breaking and -making chemistry of consequence to water splitting.     

Testing of the constrained regularization method of inverting Laplace transform on simulated very wide quasielastic light scattering autocorrelation functions

Provencher's constrained regularization method of inverting the Laplace transform was tested on 7 decades wide simulated quasielastic light scattering (QELS) data. The standard method with integration and logarithmic grid was shown to undersmooth seriously theG(Γ) distribution in the region of largeΓ (small relaxation timeτ). The regularization can be considerably improved by switching the integration off. Then, smooth distributions of relaxation timeτ of the generalized exponential type are reproduced essentially correctly with a tendency to replace asymmetric peaks by more symmetric ones with shoulders (in the Gaussian distribution ofτ) or side peaks (in the Gaussian distribution of 1/τ) on the slow decrease sides. In distributions with singularities such as edges of histogram bins or delta functions, the coarse shape of the distribution is recovered essentially correctly, but smoothing of singularities causes a distortion of wide regions of the relaxation spectrum usually in the form of sinusoidal waves. The bias introduced by taking the square root of theg ₂ function was shown to worsen sometimes the CONTIN results considerably. Thus, the use of Provencher's CONTIN program with logarithmic grid and integration switched off is recommended for the analysis of very wide QELS autocorrelation curves.     

The preparation and electrochemistry of manganese(II) complexes of an unsaturated pentadentate macrocyclic ligand

The preparation of a series of six and seven coordinate manganese(II) complexes [Mn^(II)(L)X]⁺, and [Mn^(II)(L)X₂]^2? (X = halide, water, triphenylphosphine oxide, imidazole, 1-methyl imidazole and pyridine) incorporating the pentadentate planar macrocylic ligand L is described. Cyclic voltammetry of these complexes in acetonitrile each shows a reversible one-electron reduction wave near - 1.4 V vs a Ag/AgNO₃ reference electrode. Quantitative reduction of these complexes by controlled potential electrolysis at a platinum gauze at - 1.4 V yields the corresponding one-electron reduction products which have been shown by ESR spectroscopy to be manganese(II)-ligand radical species, the electron being thought to reside on the di-imino pyridine moiety of the macrocyclic ligand. No metal reduced species could be isolated even in the presence of π-acceptor ligands such as CO or phosphines.