A high-pressure mass spectrometric and density functional theory investigation of the thermochemical properties and structure of protonated dimers and trimers of glycine

A new modification of pulsed-ionization high-pressure mass spectrometry (PHPMS) has been used to perform equilibrium thermochemical studies for relatively nonvolatile biomolecules such as amino acids. Binding enthalpy and entropy changes have been measured for proton-bound clusters of glycine, which are in good agreement with both theoretical (DFT) results of this work and a previous blackbody infrared dissociation experiment. Experimental data indicate that a number of conformers of the proton-bound dimer of glycine may coexist in the explored temperature range (360-460 K). Several new, conceptually different isomers (two of them zwitterionic) have been found by DFT calculations, one of which is 7 kJ mol(-1) lower in energy than the structure previously reported to be the energy minimum.     

Transport and kinetics at microheterogeneous electrodes: Part V. The methylviologen/platinum system

Results are reported for the reaction of methylviologen radical cation, MV⁺ with platinum colloidal particles, studied by stopped flow spectrophotometry. The rate of the reaction depends on the gaseous pretreatment of the particles. For particles reduced by hydrogen, the kinetics are usually first order with respect to MV⁺. The reaction is also first order in the concentration of platinum, and is inhibited in a first order manner by the product MV²⁺. This inhibition suggests that MV²⁺ is adsorbed on the particle surfaces, and this has been confirmed by ac, ring—disc electrode studies on macroscopic platinum electrodes. At high concentrations of MV⁺ some deviation from first order kinetics is observed. These results are all explained by a kinetic model in which either the desorption of MV²⁺ or the adsorption of MV⁺ is the rate limiting process. The rate of consumption of MV⁺ on an oxidised surface is an order of magnitude faster than that on the reduced surface. Ring—disc studies show that this is because the MV⁺ is not producing H₂ but is reducing the surface oxide. The results are shown to fit a simple model which takes into account this titration of the oxide layer. The model also explains why the rate on partially oxidised surfaces will appear to have an order greater than one in [Pt].     

Ion-Molecule reactions in methylamine and dimethylamine and trimethylamine systems

Fourier transform ion cyclotron resonance mass spectrometry has been used to measure the reaction rates for ions derived from methylamine with dimethylamine or trimethylamine. The use of the selective ion ejection technique greatly simplifies the elucidation of the ion-molecule reaction channels. The rate constants for proton transfer from protonated metwlamine, CH₃NH ₃ ⁺ (m/z 32), to dimethylamine and trimethylamine are 16.1 ± 1.6 × 10^?10 and 9.3 ± 0.9 × 10^?10 cm³ molec^?1s^?1, respectively. The rate constants for charge transfer from methylamine molecular ion, CH₃NH ₂ ⁺ (m/z 31), to dimethylamine and trimethylamine are 9.3 ± 1.8 x 10^?10 and 15.0 ± 5 × 10^?10 cm³molec^?1s^?1, respectively.     

Triple-period partial misfit dislocations at the InN/GaN (0001) interface: A new dislocation core structure for III-N materials

The lattice-misfit InN/GaN (0001) interface supports a triangular network of α-core 90° partial misfit dislocations. These misfit dislocations provide excellent strain relief. However, in their unreconstructed form the dislocation contains numerous high-energy N dangling bonds, which must be eliminated by reconstructing the dislocation core. Existing single-period (SP) and double-period (DP) dislocation reconstruction models eliminate these dangling bonds via a like-atom dimerization, such as N-N dimers. However, we show that these N–N dimers are unstable for the III-N materials, so an entirely new reconstruction mechanism is needed. A “triple-period” (TP) structural model is developed which eliminates N dangling bonds via the formation of N vacancies instead of N-N dimers. The model contains no N–N (or III–III) bonds, fully bonds all N atoms to four group-III neighboring atoms, and satisfies the “electron counting rule” by transferring charge from In dangling bonds to Ga dangling bonds.     

A search for charmed particles originating from the interactions of 300 GeV/c protons in emulsion nuclei

The associated production of charmed particles by the interactions of 300 GeV/c protons with complex nuclei has been sought using nuclear emulsions. The failure to observe any candidates for this process among some 60,000 interactions investigated implies, provided charmed particles lifetimes are in the range 10^?12 to 10^?14 s, a cross section for their associated production by the interactions of 300 GeV/c protons with nucleons of less than 1.5 microbarns at a 90% confidence level.     

Step structures on III-V phosphide (001) surfaces: how do steps and Sb affect CuPt ordering of GaInP2?

The observation of III-V phosphide (001)-(2 x 2) surfaces makes it possible to solve a long standing mystery of step structures. First-principles calculations show that a bulklike type-B step on a hydrogenated 2 x 2 surface is more stable than a rebonded one by 1.1 eV/unit step. In contrast, this energy difference for a H-free beta(2 x 4) surface is only 0.5 eV/unit step. The large difference explains why the CuPt ordering of GaInP is stronger in metal-organic chemical vapor deposition than in molecular beam epitaxy. However, a minute amount of Sb will preferentially attach to the 2 x 2 surface steps and induce additional step structures that cause ordering disruption.