Structure determination of gas-phase niobium and tantalum carbide nanocrystals via infrared spectroscopy

Niobium and tantalum carbide clusters have been isolated in the gas phase and irradiated with intense tunable infrared (IR) light. Stable neutral clusters are selectively ionized and subsequently detected in a mass spectrometer. By tuning the IR frequency, infrared multiphoton absorption spectra are obtained for a whole range of clusters. These mass-selective IR spectra lead to insights into the structures of small niobium and tantalum carbide clusters and nanocrystals.     

Caracte´risation ete´tude structurale du bichromate mixte AgKCr2O7

The crystal structure of AgKCr₂O₇ has been determined by single crystal X-ray diffraction. The structure is orthorhombic, space groupPnam, with cell constantsa = 15.175(7),b = 7.414(4), andc = 6.014(2)A? forZ = 4. The structure was refined to a finalR of 0.072 for 934 reflections. The configuration of the dichromate ion, pseudo-C_2v, is similar to that found in most other dichromate structures. The potassium ion is surrounded by eight oxygen atoms and the silver ion has a square planar coordination.     

Di-2-Pyridylketoxime,a new reagent for the spectrophotometric determination of iron(II)

Di-2-pyridylketoxime is proposed for the spectrophotometric determination of iron(II). The complex is stable and exhibits an absorption maximum at 534 nm in an alkaline citrate medium. Beer's law is obeyed and the molar absorptivity is 1.5·10³. Large amounts of copper and nickel can be tolerated. The method is simple, convenient and reproducible.     

Chimie organométallique du niobium VIII. Etude détaillée de la réaction d'acétyléniques perfluorés avec (η5C5H5)2NbH3: Mise en evídence de la coupure de liaison C-F,de la formation de liaison Nb-F et de la reduction du niobium. Structure cristalline d'un dérivé niobiacyclopentadiène

Hexafluorobutyne (CF₃CCCF₃ = Hfb) reacts with Cp₂NbH₃ (Cp = η⁵-C₅H₅) affording five new complexes I–V, whose structures have been studied by mass,

Ir, ESR and NMR spectroscopy. The X-ray study of II shows the presence of a planar NbC₄ ring Nb—C(1): 2.25(5); C(1)—C(2): 1.31(7); C(2)—C(3): 1.53(8); C(3)—(4): 1.34(7); C(4)—Nb: 2.23(5) Å. With trifluoropropyne (CF₃CCH = tfp), only the analog of III, i.e. Cp₂NbFf(tfpH), has been isolated.     

Gold cluster carbonyls: vibrational spectroscopy of the anions and the effects of cluster size, charge, and coverage on the CO stretching frequency

We report the vibrational spectra of the carbonyl complexes of anionic gold clusters in the range of the CO stretching frequency as measured in the gas phase using IR multiple photon dissociation spectroscopy. The investigated complexes contain between 3 and 14 Au atoms and up to 7 CO ligands. Special attention is given to the complexes that exhibit saturation CO coverage as well as to the monocarbonyl species. In conjunction with data from the corresponding cationic complexes we quantify how the CO stretching frequency varies with the charge state of the gold cluster. Our results provide a size- and charge-dependent basis to interpret values of the CO stretching frequency measured for CO on deposited gold clusters in terms of the charge states of the clusters.     

Oxidative stress effects on erythrocytes determined by FT-IR spectrometry

This study was designed to evaluate changes in erythrocyte contents during endurance moderate intensity exercise, a model of physiological oxidative stress. 16 endurance-trained subjects cycled 2 h at 55% of maximal aerobic capacity and blood was collected every 15 min. Transmission FT-IR spectrometry was used to analyze separately plasma and erythrocyte content changes during oxidative stress. Erythrocyte FT-IR spectra were corrected for hemoconcentration (Hc) before spectral areas integration of main IR absorbances belonging to phospholipids [nu(as)(CH(3)), [nu(as)(CH(2)), and nu(P=O)], proteins [nu(C=O) and delta(N-H)], and lactate [nu(C-O)] were used to determine erythrocyte content changes. Changes in nu(as)(CH(2)) and nu(P=O) absorbances while nu(as)(CH(3)) remained stable showed the magnitude of free radical attacks on phospholipids bilayer. Decrease in nu(C=O) and delta(N-H) absorbances while plasma and intracellular lactate, O(2) consumption, and Hc rose were linked to hemoglobin, and possibly spectrin, denaturation. Finally, the synergistic changes found between physiological, plasmatic and erythrocyte parameters showed that FT-IR spectrometry was a sufficiently accurate and sensitive method to determine acute changes in erythrocytes during moderate, physiological, oxidative stress.     

The site of Cr+ attachment to gas-phase aniline from infrared spectroscopy

Infrared spectroscopy of gas-phase Cr+ complexes of aniline was studied using the FELIX free electron laser interfaced to a Fourier transform ion cyclotron resonance spectrometer. For both the monomer complex Cr+(aniline) and the dimer complex Cr+(aniline)2 the spectra showed features indicating binding of the metal ion to the aromatic pi cloud, as opposed to the nitrogen atom. Agreement with DFT-calculated infrared absorption spectra for the ring-bound complexes was good using the MPW1PW91 functional, but the B3LYP functional predicted the wrong binding site. The spectroscopic results resolve the ambiguity in computational prediction of the preferred binding site and support the use of the MPW1PW91 functional for these systems.     

Isomerization of a peptidic fragment studied theoretically in vacuum and in explicit water solvent at finite temperature

The cis-trans isomerization of N-methylacetamide, a molecular model of the polypeptide chain, is examined via umbrella sampling Car-Parrinello MD and classical MD, in both gas and solution phases at 300 K. A new analysis of the C(O)-N bond interconversion and a full examination of the solvent shell structure are presented.     

Development of the force field parameters for phosphoimidazole and phosphohistidine

Phosphorylation of histidine-containing proteins is a key step in the mechanism of many phosphate transfer enzymes (kinases, phosphatases) and is the first stage in a wide variety of signal transduction cascades in bacteria, yeast, higher plants, and mammals. Studies of structural and dynamical aspects of such enzymes in the phosphorylated intermediate states are important for understanding the intimate molecular mechanisms of their functioning. Such information may be obtained via molecular dynamics and/or docking simulations, but in this case appropriate force field parameters for phosphohistidine should be explicitly defined. In the present article we describe development of the GROMOS96 force field parameters for phosphoimidazole molecule--a realistic model of the phosphohistidine side chain. The parameterization is based on the results of ab initio quantum chemical calculations with subsequent refinement and testing using molecular mechanics and molecular dynamics simulations. The set of force constants and equilibrium geometry is employed to derive force field for the phosphohistidine moiety. Resulting parameters and topology are incorporated into the molecular modeling package GROMACS and used in molecular dynamics simulations of a phosphohistidine-containing protein in explicit solvent.     

Optical parametric oscillation in quasi-phase-matched GaAs

We demonstrate an optical parametric oscillator (OPO) based on GaAs. The OPO utilizes an all-epitaxially-grown orientation-patterned GaAs crystal that is 0.5 mm thick, 5 mm wide, and 11 mm long, with a domain reversal period of 61.2 microm. Tuning either the near-IR pump wavelength between 1.8 and 2 microm or the temperature of the GaAs crystal allows the mid-IR output to be tuned between 2.28 and 9.14 microm, which is limited only by the spectral range of the OPO mirrors. The pump threshold of the singly resonant OPO is 16 microJ for the 6-ns pump pulses, and the photon conversion slope efficiency reaches 54%. We also show experimentally the possibility of pump-polarization-independent frequency conversion in GaAs.