Heat capacity studies of the iron oxyhydroxides akaganéite (β-FeOOH) and lepidocrocite (γ-FeOOH)

The iron oxides and iron oxyhydroxides exist as several different polymorphs, and a thermodynamic understanding of these polymorphs can provide us with an understanding of their relative stability and chemical reactivity. This study provides heat capacity measurements for lepidocrocite (γ-FeOOH) over the temperature range (0.8 to 38) K and akaganéite (β-FeOOH) over the range (0.7 to 302) K. Fits of the heat capacity of the two samples below T = 15 K showed similar behavior to previously published fits of goethite (α-FeOOH), which required a linear term and an anisotropic gap parameter to model accurately the antiferromagnetic spin–wave contributions. The akaganéite measurements were compared to previously reported measurements all of which showed significant disagreement. It is believed that the measurements reported here are the most reliable. Also, the presence of adsorbed water contributes significantly to the heat capacity of akaganéite, and the standard molar entropy at T = 298.15 K of the hydrated form was calculated to be (81.8 ± 2) J · mol^?1 · K^?1.     

Polarizable protein packing

To incorporate protein polarization effects within a protein combinatorial optimization framework, we decompose the polarizable force field AMOEBA into low order terms. Including terms up to the third-order provides a fair approximation to the full energy while maintaining tractability. We represent the polarizable packing problem for protein G as a hypergraph and solve for optimal rotamers with the FASTER combinatorial optimization algorithm. These approximate energy models can be improved to high accuracy [root mean square deviation (rmsd) < 1 kJ mol(-1)] via ridge regression. The resulting trained approximations are used to efficiently identify new, low-energy solutions. The approach is general and should allow combinatorial optimization of other many-body problems.     

Environmental sensitivity of Ru(II) complexes: the role of the accessory ligands

A suite of Ru(II) complexes in which one ligand is pH responsive and the other two are varied in an effort to achieve improved photophysics has been synthesized and their potential as pH reporters assessed. The more general purpose of the study was to examine the role of the accessory ligands in heteroleptic reporter complexes and the degree to which such ligands can affect the performance of luminescent reporters. For this suite of complexes, judicious choice of the accessory ligand can alter both the pK(a)* and the dynamic range of response. It was found that the emission color and brightness were influenced by pH, but the lifetimes were only weakly affected. Surprisingly, some accessory ligands which should have improved luminescent properties essentially turned off the pH response. Several possible reasons for this observation are explored. It is suggested, and density functional theory (DFT) calculations support, that the relative π* levels of the pH sensitive and the accessory ligands are critical.     

Invariant complex structures on four-dimensional solvable real lie groups

In this paper, invariant complex structures on four-dimensional, solvable, simply-connected real Lie groups are classified where the dimension of the commutator is less than three. The resulting complex surfaces corresponding to these structures are also determined. The classification problem is reduced to determining certain complex “structure” subalgebras of the complexifications of the four-dimensional, solvable real Lie algebras. Most of the eleven types of non-abelian solvable real Lie algebras do have complex structure subalgebras; three do not. Only three types of algebras have solvable complex structure subalgebras, and only one possesses both abelian and solvable complex structure subalgebras. Each of the possible homogeneous surfaces is represented in the list of resulting manifolds.     

An Insoluble Surfactant Model for a Vertical Draining Free Film

A mathematical model is constructed to study the evolution of a vertically oriented thin liquid film draining under gravity when there is an insoluble surfactant with finite surface viscosity on its free surface. Lubrication theory for this free film results in three coupled nonlinear partial differential equations describing the free surface shape, the surface velocity, and the surfactant transport at leading order. We will show that in the limit of large surface viscosity, the evolution of the free surface is that obtained for the tangentially immobile case. For mobile films with small surface viscosity, transition from a mobile to an essentially immobile film is observed for large Marangoni effects. It is verified that increasing surface viscosity and the Marangoni effect retard drainage, thereby enhancing film stability. The theoretical results are compared with experiment; the purpose of both is to act as a model problem to evaluate the effectiveness of surfactants for potential use in foam-fabrication processes. Copyright 2000 Academic Press.     

Aliphatic thioacetate deprotection using catalytic tetrabutylammonium cyanide

A series of thiol-functionalized organic compounds were selected to analyze the scope and efficiency of a new thioacetate deprotection method using catalytic tetrabutylammonium cyanide (TBACN) to effect the transformation of a thioacetate group to a free thiol in the presence of a protic solvent. Particularly attractive are the mild reaction and workup conditions, reduced byproduct formation typically seen using literature methods and yields of greater than 80% for the free aliphatic thiols. This method is effective on aliphatic thiols with trityl, benzyl, p-halo-benzyl, phenethyl, phenoxyethyl, and cyclohexylethyl structural moieties, but it is not effective with thiophenols.