Single‐Molecule Sensing of Environmental pH—an STM Break Junction and NEGF‐DFT Approach

Sensors play a significant role in the detection of toxic species and explosives, and in the remote control of chemical processes. In this work, we report a single‐molecule‐based pH switch/sensor that exploits the sensitivity of dye molecules to environmental pH to build metal–molecule–metal (m‐M‐m) devices using the scanning tunneling microscopy (STM) break junction technique. Dyes undergo pH‐induced electronic modulation due to reversible structural transformation between a conjugated and a nonconjugated form, resulting in a change in the HOMO–LUMO gap. The dye‐mediated m‐M‐m devices react to environmental pH with a high on/off ratio (≈100:1) of device conductivity. Density functional theory (DFT) calculations, carried out under the non‐equilibrium Green’s function (NEGF) framework, model charge transport through these molecules in the two possible forms and confirm that the HOMO–LUMO gap of dyes is nearly twice as large in the nonconjugated form as in the conjugated form.     

Structural elucidation of V‐type nerve agents by liquid chromatography/electrospray ionization mass spectrometry

V‐nerve agents present information‐poor spectra, both in GC‐EI‐MS and LC‐ESI‐MS/MS, with dominant fragments/product ions corresponding to the amine‐containing residue. Hence, derivatives/isomers with the same amine residue exhibit similar mass spectral patterns, leading to ambiguity in the phosphonate structure. We present a simple approach for their structural elucidation based on two complementary experiments: ESI‐MS/MS of the original compound, which provides information about the amine moiety, and ESI‐MS/MS of the phosphonic acid hydrolysis products generated by N‐iodosuccinimide, which provides ions' characteristic of the phosphonate structure. This approach enables the structural elucidation of the original V‐agents with a higher degree of certainty.     

Two helium atoms inside fullerenes: probing the internal magnetic field in C60(6-) and C70(6-)

A 3He NMR resonance of C606- containing He is assigned to He2@C606-, thus showing that C60 can also accommodate two helium atoms. The ratio of the di-helium compound relative to the mono- is 1:200, 10 times lower than the equivalent counterpart of C70. The 3He NMR chemical shift of He2@C606- is 0.093 ppm downfield from the already known resonance of He@C606-. In the reduced endohedral mono- and di-helium C70, the 3He NMR chemical shift of He2@C706- is 0.154 ppm upfield from the peak of He@C706-.     

Vibrational spectroscopy and structure of polymer electrolytes,poly(ethylene oxide) complexes of alkali metal salts

Complexes of poly(ethylene oxide) (PEO), with various alkali metal salts are known to exhibit ionic conductivities which exceed 10^?5(Ωcm)^?1 at moderate temperatures. We have employed IR and Raman spectroseopy to study well characterized samples of the following polymer-salt complexes: PEO·NaBr, PEO·NaI, PEO·NaSCN, PEO·NaBF₄, PEO·NaCF₃SO₃, PEO·KSCN, PEO·RbSCN and PEO·CsSCN. Cation-dependent vibrational bands observed in the far IR and M-O_n symmetric stretching bands observed in the Raman support a cation-oxygen atom interaction, and indicate the polyether chain may wrap around the cations. In particular, NaX and KX complexes of PEO are believed to have a helical configuration for the polymer which differs from that of pure PEO. Some general rules are presented for polymer-salt complex formation.     

Dissociation dynamics of Ar3 in the time-dependent self-consistent field (TDSCF) approximation

The TDSCF approximation is applied to dissociation of Ar₃, which exhibits strong-coupling behavior (efficient internal energy transfer). The TDSCF results are in excellent agreement with those of exact classical dynamics. Combined with earlier findings for weak-coupling cases, this suggests validity of TDSCF for dissociation at all coupling strengths.     

Ab initio molecular orbital studies of polyethylene deformation

A molecular LCAO Hartree-Fock procedure was used to calculate total energies of axially stretched normal paraffins containing up to nine carbon atoms. The results are used to model the mechanical properties of polyethylene.     

On semigroups generated by restrictions of elliptic operators to invariant subspaces

Let A be the closed unbounded operator inL _p(G) that is associated with an elliptic boundary value problem for a bounded domainG. We prove the existence of a spectral projectionE determined by the set Γ = {λ;θ ₁≦argλ≦θ ₂} and show thatAE is the infinitesimal generator of an analytic semigroup provided that the following conditions hold: 1<p<∞; the boundary ϖΓ of Γ is contained in the resolvent setp(A) ofA;π/2≦θ<θ ₂≦3π/2 ; and there exists a constantc such that (I)││(λ-A)^-1││≦c/│λ│ for λ∈ϖΓ. The following consequence is obtained: Suppose that there exist constantsM andc such that λ∈p(A) and estimate (I) holds provided that |λ|≧M and Re λ=0. Then there exist bounded projectionE ⁻ andE ⁺ such thatA is completely reduced by the direct sum decompositionL _p(G)=E⁻L_p (G) ⊕E⁺L_p (G) and each of the operatorsAE ⁻ and—AE ⁺ is the infinitestimal generator of an analytic semigroup.     

Morphological control and photoluminescence of zinc oxide nanocrystals

Nanocrystals of the wide band gap semiconductor zinc oxide of controllable morphologies were synthesized by a simple thermal decomposition method. The predominating factor in determining the morphology (spheres, triangular prisms, and rods) was the solvent, selected on the basis of coordinating power. The nanoparticles were structurally analyzed, and the photoluminescence of each shape was compared. The intensity of the green band emission, common to many ZnO structures, was found to vary with morphology. The strongest green band intensity corresponded to the shape with the largest surface/volume ratio and could be attributed to surface oxygen vacancies. Control over the morphology of ZnO at the nanoscale is presented as a means to control the green band emission.     

Astrophysical neutrinos: flavor ratios depend on energy

Electromagnetic (and adiabatic) energy losses of pi's and mu's modify the flavor ratio (measured at Earth) of neutrinos produced by pi decay in astrophysical sources, Phi v: phi v mu: phi v tau, from 1:1:1 at low energy to 1:1.8:1.8 at high energy. The transition occurs over 1-2 decades of v energy, and is correlated with a modification of the neutrino spectrum. For gamma-ray bursts, e.g., the transition is expected at approximately 100 TeV and may be detected by km-scale v telescopes. Measurements of the transition energy and energy width will provide unique probes of the physics of the sources. Pi and mu energy losses also affect the ratio of ve flux to total v flux, which may be measured at the resonance (6.3 PeV): It is modified from 1/6(1/15) at low energy to 1/9 (practically 0) at high energy for neutrinos produced in p p (p gamma) interactions.