Fluids confined by nanopatterned substrates of low symmetry

By means of Monte Carlo simulations in the grand canonical ensemble (GCEMC) the phase behaviour of a ‘simple’ fluid (in which molecules possess only translational degrees of freedom) confined between planar nanopatterned substrates is investigated. The fluid—substrate interaction is purely repulsive except for circular regions of radius R attracting fluid molecules. Because the circular pattern is finite in size, system properties are not translationally invariant in any spatial dimension. Thus, the grand potential Ω is not a homogeneous function of degree 1 in any of its extensive variables, which precludes a ‘mechanical’ expression for Ω in terms of stresses and conjugate strains. Therefore, in order to determine the phase behaviour, Ω versus chemical potential μ is calculated by thermodynamic integration under isothermal conditions (T = const). Besides the more conventional gas and liquid phases, fluid bridges arise as a third type of thermodynamically stable phase depending on T and μ. In a bridge phase, molecules are preferentially adsorbed by attractive circular region, and span the gap between the opposite substrate surfaces.     

High resolution FTIR spectra and analysis of the ν11 fundamental and of the ν2 + ν11, ν5 + ν12 and ν7 + ν16 combination bands of 12C6D6

We report results from measurements of the high resolution FTIR spectrum for the fully deuterated benzene molecule C₆D₆ in the range 450–3500 cm^?1. Accurate spectroscopic constants have been obtained for the fundamental vibration ν₁₁ at 496.208 cm^?1 and improved ground state constants have been deduced from a fit of ground state combination differences. The J structure of the combination parallel bands ν₂ + ν₁₁ (at 2798.1 cm^?1), ν₅ + ν₁₂ (1802.5 cm_?1) and ν₇, + ν₁₆ (2619.3 cm^?1) of C₆D₆ has been analysed as well, from which improved values of the band origin and of the B and D _j constants of the excited states have been obtained. The strongest hot bands accompanying these parallel transitions have been assigned by means of the anharmonic force field calculated by Maslen et al. [1992, J. chem. Phys., 97, 4233]. In particular (ν₁₁ + ν₁₆) ? ν₁₆ is assigned to the band at 492.4 cm^?1 even though its shape is typical of a perpendicular transition (PAPE). New values for the ν₅, ν₁₂ and ν₁₆ band origins are determined from the band origins of combination bands and from calculated anharmonic constants. Numerous anharmonic constants are derived from the assignment of hot band and combination transitions.     

Curvy steps for density matrix based energy minimization: tensor formulation and toy applications

A unitary transformation-based framework is explained for varying the one-particle density matrix in a non-orthogonal expansion basis while preserving both the idempotency and electron number constraints. The equations are presented in a compact tensor formulation that permits the use of any representation for the matrices. A connection with the LNV method is established. Curvy steps, which are high order line searches along a chosen descent direction, are defined. Calculations with two toy model systems are presented to illustrate the nature of the displacements in this approach.     

The ammonia dimer equilibrium dissociation energy: convergence to the basis set limit at the correlated level

The low-energy region of the intermolecular potential energy hypersurface (PES) of the ammonia dimer was studied at the level of second-order Moller-Plesset perturbation theory (MP2) using a very large basis set. Individual minima were located on the PES employing the counterpoise (CP) correction to account for the basis set superposition error (BSSE). Apart from these canonical MP2 calculations local MP2 (LMP2) calculations were performed. For the latter the BSSE at the correlated level is inherently absent by virtue of the local truncation of the virtual space. Results from canonical and local MP2 calculations are compared and the reliability of the LMP2 method for intermolecular complexes and clusters is discussed. The canonical MP2 calculations predicted five minimum structures, the four most stable ones lying energetically very close. For these four structures single point MP2 energy calculations with a further extended basis set (1024 functions for the ammonia dimer) were performed. The equilibrium dissociation energies so obtained are close to the one-particle basis set limit, as illustrated by a remaining BSSE of less than 0.2 kJ mol^?1. The geometry optimizations at the LMP2 level, using the three most stable canonical MP2 structures as initial geometries, all collapsed to a single minimum corresponding to an asymmetric structural arrangement. A canonical MP2 single point calculation, at that geometry, revealed that the LMP2 minimum structure is virtually as stable as the lowest minima on the canonical MP2 PES. Based on these calculations the global minimum of the ammonia dimer was assigned to a part of the PES represented by an asymmetric structure with an equilibrium dissociation energy of 13.5±0.3 kJ mol^?1     

OPTIMAL GRAZING MANAGEMENT RULES IN SEMI‐ARID RANGELANDS WITH UNCERTAIN RAINFALL

Abstract. We study optimal adaptive grazing management under uncertain rainfall in a discrete‐time model. As in each year actual rainfall can be observed during the short rainy season, and grazing management can be adapted accordingly for the growing season; the closed‐loop solution of the stochastic optimal control problem does not only depend on the state variable, but also on the realization of the random rainfall. This distinguishes optimal grazing management from the optimal use of most other natural resources under uncertainty, where the closed‐loop solution of the stochastic optimal control problem depends only on the state variables. Solving this unusual stochastic optimization problem allows us to critically contribute to a long‐standing controversy over how to optimally manage semi‐arid rangelands by simple rules of thumb.     

PHOTOINDUCED ELECTRON TRANSFER ACROSS LIPID BILAYERS CONTAINING MAGNESIUM OCTAETHYLPORPHYRIN

Both photoinitiated (thermodynamically downhill) and photodriven (thermodynamically uphill) electron transfer reactions across lipid bilayers are sensitized by magnesium octaethyl porphyrin (MgOEP). It is shown that the reaction mechanism is via reduction of photoexcited MgOEP at the reducing (ascorbate) side of the bilayer and the charge carrier is likely the neutral protonated MgOEP anion. The MgOEP cation (or its neutral form) does not contribute to charge passage across the bilayer even though it is readily formed at the acceptor (ferricyanide or methyl viologen) side of the membrane. Photoelectric measurements on planar bilayers show that the time constant for reduction of excited MgOEP is about 10 microseconds with 10 mM ascorbate. The membrane transport of the mediator appears to be rate limiting when the reaction is photoinitiated and the interfacial reaction appears to be limiting when the reaction is photodriven. The quantum yield of the process is about 0.1 in the latter case and about 0.02 in the former. The former yield is increased to about 0.15 in the presence of a redox mediator, duroquinone. In these systems, the magnesium porphyrin is both sensitizer and trans membrane redox mediator.     

PHOTOCHEMICAL ADDITION OF AMINO ACIDS AND PEPTIDES TO DNA

Abstract— The quantum yields for photochemical addition of twenty of the amino acids commonly occurring in proteins to denatured calf thymus DNA have been determined in deoxygenated phosphate buffer at λ 254 nM and pH 7 using a fluorescamine assay technique. Fifteen were found to be reactive, with cysteine, lysine, phenylalanine, tryptophan and tyrosine being the most reactive. Alanine, aspartic acid, glutamic acid, serine and threonine were unreactive. Analogous quantum yields for a series of eighteen peptides of the form glycyl X (X being one of the commonly occurring amino acids) were also determined, along with the corresponding quantum yields for L-alanyl-L-alanine, L-alanyl-L-tryptophan, L-seryl-L-seryl-L-serine, L-threonyl-L-threonyl-L-threonine, and L-cystine- bis -glycine. All of the peptides were found to be reactive. The modified amino acids N^ε-methyllysine, N^ε, N^ε, N^ε-trimethyllysine and N^ε-acetyllysine, all occurring in minor amounts in the histone group of chromosomal proteins, were also found to be reactive as was N^α-acetyllysine. The quantum yields for photoaddition of a selected group of amino acids and peptides to denatured DNA and native DNA are compared. In some cases higher quantum yields for photoaddition to denatured DNA are observed while in other cases the reverse is true. The effect of oxygen on the quantum yields for photoaddition of selected peptides to DNA was examined. While for most systems studied the amount of reaction in aerated systems was less than in deoxygenated systems, in the case of glycyl-L-phenylalanine the reverse was true.