Facile synthesis of zero-, one-, and two-dimensional vanadyl pyrophosphates

Crystallization of Na(2)VOP(2)O(7) from its aqueous solution results in formation of a one-dimensional inorganic polymer {Na(2)VO(H(2)O)P(2)O(7)·7H(2)O}(n) (1). When this polymer is dehydrated at elevated temperatures this polymer undergoes a phase transition to form the two-dimensional framework β-Na(2)VOP(2)O(7), which although previously reported had been difficult to access. Exchanging lithium for sodium via ion-exchange chromatography results in formation of a discrete, cyclic, tetramer species, Li(8)[VOP(2)O(7)(H(2)O)·4H(2)O](4) (2). Isolation of crystalline β-Li(2)VOP(2)O(7) using a dehydration procedure analogous to the one employed for the sodium derivative was unsuccessful. In contrast, we show that β-K(2)VOP(2)O(7) can be obtained from the amorphous phase K(2)VOP(2)O(7)·nH(2)O (n = 0-7) upon thermal dehydration.     

Nitrogen fixation to cyanide at a molybdenum center

Facile methoxymethylation of N(2)-derived nitride NMo(N[(t)Bu]Ar)(3) provided the imido cation [MeOCH(2)NMo(N[(t)Bu]Ar)(3)](+) as its triflate salt in 88% yield. Treatment of the latter with LiN(SiMe(3))(2) provided blue methoxyketimide complex MeO(H)CNMo(N[(t)Bu]Ar)(3) in 95% yield. Conversion of the latter to the terminal cyanide complex NCMo(N[(t)Bu]Ar)(3), which was the subject of a single-crystal X-ray diffraction study, was accomplished in 51% yield upon treatment with a combination of SnCl(2) and Me(2)NSiMe(3).     

Two-phase flow instabilities in a silicon microchannels heat sink

Two-phase flow instabilities are highly undesirable in microchannels-based heat sinks as they can lead to temperature oscillations with high amplitudes, premature critical heat flux and mechanical vibrations. This work is an experimental study of boiling instabilities in a microchannel silicon heat sink with 40 parallel rectangular microchannels, having a length of 15 mm and a hydraulic diameter of 194 μm. A series of experiments have been carried out to investigate pressure and temperature oscillations during the flow boiling instabilities under uniform heating, using water as a cooling liquid. Thin nickel film thermometers, integrated on the back side of a heat sink with microchannels, were used in order to obtain a better insight related to temperature fluctuations caused by two-phase flow instabilities. Flow regime maps are presented for two inlet water temperatures, showing stable and unstable flow regimes. It was observed that boiling leads to asymmetrical flow distribution within microchannels that result in high temperature non-uniformity and the simultaneously existence of different flow regimes along the transverse direction. Two types of two-phase flow instabilities with appreciable pressure and temperature fluctuations were observed, that depended on the heat to mass flux ratio and inlet water temperature. These were high amplitude/low frequency and low amplitude/high frequency instabilities. High speed camera imaging, performed simultaneously with pressure and temperature measurements, showed that inlet/outlet pressure and the temperature fluctuations existed due to alternation between liquid/two-phase/vapour flows. It was also determined that the inlet water subcooling condition affects the magnitudes of the temperature oscillations in two-phase flow instabilities and flow distribution within the microchannels.     

Computational strategies for the optimization of equilibrium geometries and transition-state structures at the semiempirical level

Several improvements have been made to the gradient algorithms commonly used to optimize equilibrium and transition-state geometries at the semiempirical level. A gradient algorithm derived from a combination of a variable metric method (Davidon–Fletcher–Powell/Broyden–Fletcher–Goldfarb–Shanno) and Pulay's direct inversion in the iterative subspace method for geometry optimization (GDIIS) is compared with the variable metric method combined with an accurate linear search algorithm. The latter method is used routinely in the standard semiempirical program packages, MNDO, MOPAC, and AMPAC. The combined variable metric and GDIIS algorithm is also compared with GDIIS which uses a static metric. The performance of these algorithms is examined for a wide range of systems with respect to both choice of coordinate system (for cyclic molecules) and guess for the initial Hessian. The results show that the GDIIS method is up to ca. 40% more efficient than the variable metric combined with accurate line search algorithm: however, the exact savings vary depending on the coordinate system and initial Hessian. For noncyclic systems, variable-metric GDIIS is usually equal or superior to static-metric GDIIS, and consistently performs ca. 30% more efficiently than the variable metric combined with accurate line search algorithm. For the optimization of cyclic molecules, an improved estimate of the initial Hessian has increased the efficiency by at least a factor of two. Greater efficiencies (usually >40%) are also obtained when static-metric GDIIS is used to refine the geometry after the initial application of a transition-state search based on the variable metric combined with line search algorithm. On the basis of these results, we recommend several changes to the algorithms as currently implemented in the standard semiempirical program packages.     

Thermodynamics of Electrolyte Mixtures: HCl + NdCl<Subscript>3</Subscript> + H<Subscript>2</Subscript>O from 5 to 55 <Superscript>∘</Superscript>C

Electromotive-force (emf) measurements of cells containing solutions of hydrochloric acid and neodymium chloride were reported at constant total ionic strengths (I) of 0.01, 0.025, 0.05, 0.1, 0.25, 0.5, 1.0, and 1.5 mol-kg⁻¹ at 11 temperatures ranging from 5 to 55 ^∘C, and at I = 2.0 mol-kg⁻¹ at 25 ^∘C. Hydrogen and silver–silver chloride electrodes were used in these cells. Results from the emf measurements, the mean molal activity coefficients of HCl in HCl + NdCl₃ + H₂O mixtures, as well as the Harned interaction coefficients using Harned's rule are reported in the preceding article in this issue. The ion-interaction model of Pitzer is applied here for the evaluation of the Pitzer mixing coefficients, ^Sθ_H,Nd and ψ_H,Cl,Nd, as well as the linear representation of the temperature derivatives of ∂^Sθ_H,Nd /∂ T and ∂ψ_H,Nd,Cl/∂T. The activity coefficients at several ionic strength fractions y of NdCl₃ are given at 25 ^∘C. The results are interpreted in terms of ionic interactions.     

Solid-phase methods for the synthesis of cyanine dyes

We report here a series of studies that explore solid-phase methodologies for the synthesis of various cyanine dyes. The scope of the previously reported catch-and-release method using sulfonyl chloride resin(1) has now been extended to include pentamethine and water-soluble cyanine dyes. We also report a new and chemically distinct synthetic strategy, employing the stepwise attack of heterocyclic carbon nucleophiles on immobilized polyene-chain precursors, allowing the clean synthesis of hydrophobic and hydrophilic trimethine and pentamethine dyes from more easily obtained starting materials. Overall, both approaches appear to be robust and versatile strategies to delivering a wide range of cyanine-based dyes in high purity.