Clifford W. Padgett

Synthesis and structures of alkaline earth metal salts of bis[(trifluoromethyl)sulfonyl]imide

Abstract A series of alkaline earth metal salts of the bis[(trifluoromethyl)sulfonyl]imide anion have been prepared and structurally characterized. The magnesium cation is fully hydrated with no direct interaction with the anion, although there is extensive hydrogen bonding involving coordinated and lattice water molecules and the anion. The calcium cation is heavily hydrated, but also directly interacts with two anions. As with the magnesium salt, hydrogen bonding plays a major role in determining the crystal packing. The strontium salt is anhydrous, and the eight-coordinate cation interacts directly with several anions to form a two-dimensional layered structure. The barium salt is a monohydrate, with a nine-coordinate cation. As with strontium, it also forms a layered structure. Despite the differences, all of the structures exhibit extensive fluorine segregation which results in the formation of hydrophilic and hydrophobic domains. In all but the magnesium salt, the anion is chelated to the metal and has a cisoid conformation with the trifluoroalkyl groups lying to the same side of the S N S plane. This is in keeping with our previous observations that the cisoid conformation is preferred when the anion is coordinated to a metal ion, while the transoid is preferred for noncoordinating cations.

A simple and effective 1,2,3-triazole based “turn-on” fluorescence sensor for the detection of anions

A novel and effective 1,2,3-triazole based fluorescence chemosensor has been synthesized for the specific detection of anions in homogeneous medium. Notably, the molecule, synthesized in one step using “Click chemistry”, is a simple 1,4-diaryl-1,2,3-triazole, containing a phenol moiety. The probe displayed the strongest response to fluoride ion through the “turn-on” fluorescence sensing mechanism when screened for selectivity and sensitivity against a series of anions (F−, Cl−, Br−, I−, H2PO4−, ClO4−, OAc−, BF4−). Fluorescence spectroscopy and Nuclear Magnetic Resonance Spectroscopy (NMR) studies substantiate 1 : 1 stoichiometry between the probe and fluoride anion. Kinetic studies and the single crystal X-ray spectroscopic evidence revealed the binding interaction occurs with the phenolic group and the anion.

Coupled molecular dynamics/continuum simulations of Joule heating and melting of isolated copper–aluminum asperity contacts

Atomic-level dynamics of Joule heating, melting and plastic dynamics at loaded nanometer-scale Cu and Al asperity contacts are modeled using an ad hoc coupling between a numerical solution to a heat transport equation, a virtual resistor network for describing electric current flow and a molecular dynamics simulation using the embedded atom method. Under constant voltage conditions the simulations demonstrate the formation of an Al melt that removes faceting from a Cu asperity via surface disordering at the melt–solid interface. Constant current simulations demonstrate initial disordering of both copper and aluminum at the interface. Flow from the aluminum melt increases the contact area, which lowers the resistance and drops the voltage to below that needed for melting. For the system with a loaded copper asperity, the interface recrystallizes and the dynamics transition from molten flow to plastic damage via dislocation emission. For an aluminum asperity, the asperity remains disordered after the voltage drop and no dislocation emission occurs into the copper or aluminum substrate.

Atomic and multi-scale modeling of non-equilibrium dynamics at metal–metal contacts

A coarse graining method that introduces Joule heating and improves heat transport in a classical molecular dynamics simulation is reviewed, and two example sets of simulations, opening of gold?gold nano-asperity contacts and nano-asperity sliding at loaded copper?aluminum interfaces are discussed. For the gold contact, dislocations nucleate from the edges of where the asperity contacts the substrates and move along the close-packed planes, resulting in stacking faults that form two subsurface Thompson tetrahedra. For a null voltage, a nanowire with a diameter much smaller than the initial contact area is created when the two tetrahedra are completed, and as the wire yields the partial dislocations retreat to the surface. Opening with Joule heating enhances dislocation mobility and intransient subsurface plasticity. Constant current simulations show melting and boiling of the nanowires depending on the voltage cap. Sliding of an aluminum asperity on copper with a null voltage shows dislocation formation in the copper and aluminum, while heating from an applied voltage eliminates damage in the copper. Sliding with a copper asperity enhances plastic damage in the copper substrate compared with the aluminum asperity, while Joule heating enhances aluminum pile-up in front of the copper asperity due to plowing.

Rubidium bis(trifluoromethanesulfonyl)imidate dioxane disolvate

The bis(trifluoromethanesulfonyl)imidate anion crystallizes with Rb as the title dioxane 1:2 solvate, Rb(+).CF(3)SO(2)NSO(2)CF(3)(-).2C(4)H(8)O(2), with the anion in a transoid conformation, as opposed to the cisoid form typically seen when there are significant cation-anion interactions. The Rb(I) cation is eight-coordinate, interacting with one anion in a chelating fashion and with two other anions through the remaining sulfonyl O atoms. The latter interactions link ion pairs through the formation of Rb(2)O(2) dimers about inversion centers at (0, 1/2, 0) and (1/2, 1/2, 0), forming extended columns which run parallel to the a axis of the unit cell. Rb-dioxane bridges crosslink these salt columns in the (010), (001) and (011) directions, resulting in a three-dimensional network solid. One dioxane solvent molecule is disordered over two half-occupancy sites.

Multiscale Modeling of Metal-Metal Contact Dynamics under High Electromagnetic Stress: Timescales and Mechanisms for Joule Melting of Al-Cu Asperities

An analysis and initial results from a multiscale continuum-atomistic simulation of the Joule heating and melting of Cu-Al asperity contacts is presented. An analytic expression is given for the time needed to reach the Al melting point for an asperity as a function of the voltage drop and the asperity contact area. The coupled continuum-atomistic simulations capture the initial stages of the formation of Al-Cu alloys that arises from the solvation of Cu atoms into the Al melt. Implications of these results for understanding contacts in electromagnetic launchers are discussed.

2,6-Diiodopyridine.

The title compound, C(5)H(3)I(2)N, crystallizes in the polar space group Fmm2, with crystallographic mm2 symmetry imposed on the molecule. Molecules are linked through C-H...N hydrogen bonding to form chains which are, in turn, joined through weak I...I halogen-bonding interactions to form layers. The pyridine ring lies parallel to the polar z axis and has the N atom pointing in the +z direction. The layers stack in a polar fashion normal to the a axis and the absolute structure has been determined.

Smartphones and Time Zones

Using the Sun to tell time is an ancient idea, but we can take advantage of modern technology to bring it into the 21st century for students in astronomy, physics, or physical science classes. We have employed smartphones, Google Earth, and 3D printing to find the moment of local noon at two widely separated locations. By reviewing GPS time-stamped photos from each place, we are able to illustrate that local noon is longitude-dependent and therefore explain the need for time zones.

A square-planar hydrated cationic tetrakis(methimazole)gold(III) complex

The cationic pseudo-square-planar complex tetrakis(1-methyl-2,3-dihydro-1H-imidazole-2-thione-κS)gold(III) trichloride sesquihydrate, [Au(C4H6N2S)4]Cl3·1.5H2O, was isolated as dark-red crystals from the reaction of chloroauric acid trihydrate (HAuCl4·3H2O) with four equivalents of methimazole in methanol. The Au(III) atoms reside at the corners of the unit cell on an inversion center and are bound by the S atoms of four methimazole ligands in a planar arrangement, with S-Au-S bond angles of approximately 90°.

Enantioselectivity and Enzyme-Substrate Docking Studies of a Ketoreductase from Sporobolomyces salmonicolor (SSCR) and Saccharomyces cerevisiae (YOL151w)

Models for two ketoreductases were created and used to predict the stereoselectivity of the enzymes. One was based on the crystal structure of Sporobolomyces salmonicolor. This model was used to predict the stereoselectivity for 46 ketone reductions using this enzyme; only 6 were incorrectly predicted. The stereochemistries of the products were compared to the experimental values found in the literature. The Prelog rules were also used to predict the stereoselectivity for this enzyme; however the Prelog rules seem to be highly substrate dependent. As a result, predicting stereoselectivity of KREDs is more complicated than is allowed for with just substrate size and geometry. This enzyme showed Prelog docking geometry for 13 substrates if the enzyme is assumed to prefer an anti-Prelog docking geometry. For SSCR the molecular modeling proved to be a better method for predicting stereoselectivity of the enzymes. The second model was a homology model for YOL151w based on the enzyme crystal structure of Sporobolomyces salmonicolor carbonyl reductase, SSCR. In this homology model, 14 compounds were docked and the predicted stereochemistry was compared to the literature values. Of these, 5 were incorrectly predicted.