Oxygen K-edge fine structures of water by x-ray Raman scattering spectroscopy under pressure conditions

Fine structure of the oxygen K edge was investigated for water at ambient pressure, 0.16, 0.21, 0.27, 0.47, and 0.60 GPa using x-ray Raman scattering spectroscopy (XRS). Similarity in near-edge structures at 0.16 and 0.60 GPa suggests little difference in the electronic state of oxygen in the low-pressure and high-pressure forms of water. Yet, we observed significant variation of preedge structure of the XRS spectra with compression. The intensity of the preedge peak at 535.7 eV has a minimal value at around 0.3 GPa, indicating that the number of hydrogen bonding increases first and then decreases as a function of pressure.     

Immobilization of paracetamol and benzocaine pro-drug derivatives as long-range self-organized monolayers on graphite

We show here by means of scanning tunneling microscopy (STM) at the liquid/solid interface that paracetamol and benzocaine molecules bearing a long aliphatic chain can be immobilized on highly oriented pyrolitic graphite (HOPG) as perfectly ordered two-dimensional domains extending over several hundreds of nanometers. In both cases, high-resolution STM images reveal that compounds 1 and 2 self-assemble into parallel lamellae having a head-to-head arrangement. The paracetamol heads of 1 are in a zigzag position with entangled n-dodecyloxy side chains while benzocaine heads of compound 2 are perfectly aligned as a double row and have their palmitic side chains on either sides of the head alignment. We attribute the very long-range ordering of these two pro-drug derivatives on HOPG to the combined effects of intermolecular H-bonding on one side and Van der Waals interactions between aliphatic side chains and graphite on the other side. The 2D immobilization of pro-drug derivatives via a non-destructive physisorption mechanism could prove to be useful for applications such as drug delivery if it can be realized on a biocompatible substrate.     

Theoretical investigation of carbon-sulfur triple bonds

By means of first principle calculations we have investigated a set of molecules that are presumed to contain carbon-sulfur triple bonds, namely HCSOH, H(3)SCH, cis-FCSF, F(3)CCSF(3), and F(5)SCSF(3). For HCSOH, FCSF, and H(3)SCH we used the CCSD(T) methodology and the correlation-consistent basis sets. On the other hand, F(3)CCSF(3) and F(5)SCSF(3) were studied at the B3LYP, M06-2X, MP2, and G3 levels of theory. We found that none of these molecules display a carbon-sulfur adiabatic bond dissociation energy (ABDE) as strong as diatomic CS (170.5 kcal mol(-1)), or a diabatic bond dissociation energy (DBDE) larger than the one found in SCO (212.0 kcal mol(-1)), although the DBDE of FCSF comes quite close at 208.3 kcal mol(-1). The CS ABDEs of F(3)CCSF(3), F(5)SCSF(3), and H(3)SCH are comparable to that of a single C-S bond. In contrast with the experimental results, F(3)CCSF(3) and F(5)SCSF(3) are predicted to be linear with C(3v) and C(s) symmetry, respectively, at the B3LYP/6-311+G(3df,2p) level. MP2/6-311+G(2df,2p) calculations support the C(3v) symmetry for F(3)CCSF(3), despite F(5)SCSF(3) not having a perfect linear structure; the CSC angle is 174.6°, which is nearly 20° larger than the experimental value. The analysis of the carbene structures of HCSOH and H(3)SCH revealed that they are not significant, because the triplet state is dissociative in these cases. However, for F(3)CCSF(3) and F(5)SCSF(3) , the carbene triplet states lie 0.81 and 0.77 eV above the singlet state, respectively. In the same vein, our investigation supports the presence of a strong double bond for HCSOH. The conflicting evidence available for F(3)CCSF(3) and F(5)SCSF(3) makes it very difficult to determine the nature of the CS bonds. However, the bond dissociation energies and the singlet-triplet splittings clearly suggest that these compounds should be considered as masked sulfinylcarbenes. The analysis of the bond dissociation energies challenges the existence of a triple bond in these five molecules, but from a strictly thermodynamic standpoint, cis-FCSF is found to be the candidate most likely to exhibit triple-bond character.     

Synthesis and characterization of novel fluorinated poly(oxomolybdates)

Two novel poly(oxofluoromolybdate) clusters, [Mo6O18F6]6- and [Mo7O22F3]5-, have emerged from systematic field studies on the hydrothermal fluorination of poly(oxometalates). They are accessible via the hydrothermal treatment of Mo(VI)-based precursors in the presence of MF additives (M = Li, Na, Cs, NMe4). The new fluorinated polyanions are stabilized by specific alkali-cation combinations, and their packing motifs can be tuned through the careful choice of cations and synthetic parameters. The [Mo6O18F6]6- anion can be described as a cored and fluorinated form of the Anderson-Evans cluster type. It is stabilized by the interaction of two alkali cations with the fluorinated faces of the ring-shaped anion. The partial replacement of these capping alkali cations by the bulky, organic NMe4(+) cation leads to the formation of [Mo7O22F3](5-)-based compounds. Thus, the extent of fluorination can be controlled through the polarizability of the cationic environment: in the [Mo7O22F3]5- anion, half of the fluoride atoms of the [Mo6O18F6]6- anion are replaced by a capping MoO4 tetrahedron, thereby rendering this anion a fluorinated, lacunar derivative of the alpha-[Mo8O26]4- octamolybdate anion. All compounds have been structurally characterized by single-crystal X-ray diffraction and solid-state NMR spectroscopy. The templating role of the alkali cations is discussed and explained with the help of electrostatic calculations.     

The study and characterization of nucleosides by mass spectrometry—I: The mass spectra of pyrimidine 2,2′-anhydronucleosides and their derivatives

The mass spectra of 2,2′-anhydrouridine, 2,2′-anhydrothymidine and 2,2′-anhydro-4-thiouridine are reported. The acetyl, trifluoroacetyl, trityl, pivaloyl and trimethylsilyl ether derivatives were also studied. Deuterium labeling in acetyl and trimethylsilyl groups aided characterization of many ions in the spectra, as well as helping to clarify hydrogen migration processes. The anhydronucleosides and their derivatives are readily distinguished from natural nucleosides by the presence of an ion containing the base moiety plus the anhydro-ring plus one hydrogen atom from the rest of the molecule. As for natural nucleosides the [base + H]⁺ and [base + 2H]⁺ ions are usually prominent, but in contrast to natural nucleosides, ions characteristic of the sugar moiety do not retain the 2′-oxygen atom (i.e. the oxygen atom of the anhydro-ring). The mass spectra of deuterium labeled derivatives suggest a test for the presence of a 3′-O-acetyl function (the O-acetyl group is lost from the molecular ion much more readily from the 3′- than from the 5′-carbon atom). The trimethylsilyl derivatives showed evidence in their mass spectra for migration of trimethylsilyl groups in addition to hydrogen atoms.     

Determination du plomb dans l'urine par spectrophotometrie d'absorption atomique

Lead is assayed in urine by atomic absorption spectrophotometry after separation by coprecipitation with a CaCO₃, HCl, (NH₄)₂HPO₄ solution at pH 9. Concentration of lead and removal of interfering substances are achieved simultaneously. Fresh or old urine samples acidified with HNO₃ and thymol can be used. Precipitation is quantitative. The precision of the method is ±0,004 ppm of lead.     

Electronic spectroscopy of the E<--X transition of NO-Kr and shielding/penetration effects in Rydberg states of NO-Rg complexes

We report the results of a (2+1) resonance-enhanced multiphoton ionization (REMPI) study of the E2Sigma+(4ssigma) Rydberg state of NO-Kr. We present an assignment of the two-photon spectrum based on a simulation, and discuss it in the context of previously-reported spectra of NO-Ne and NO-Ar. In addition, we report on spectra in the region of the vNO=1 level of the E, F and H' 4s and 3d Rydberg states of NO-Rg (Rg=Ne-Kr). Since the NO vibrational frequency is affected by electron donation from the rare-gas (Rg) atom to the NO+ core, as well as by the penetration of the Rydberg electron, the fundamental NO-stretch frequency reflects the interactions in the complex. The results indicate that the 4s Rydberg state has a strong interaction between the NO+ core and the Kr atom, as was the case for NO-Ar and NO-Ne. For the 3d Rydberg states, although penetration is not as significant as for the 4s Rydberg states, it does play an important role, with subtle angular effects being notable.     

Atomic scale insights on chlorinated gamma-alumina surfaces

The thermochemistry of chlorinated gamma-alumina surfaces is explored by means of density functional calculations as a function of relevant reaction conditions used in experiments and in high-octane fuel production in the refining industry such as hydrocarbon isomerization and reforming. The role of chlorine as a dope of the Br?nsted acidity of gamma-alumina surfaces is investigated at an atomic scale. Combining infrared spectroscopy and density functional theory calculations, the most favorable location of chlorine atoms on the (110), (100) and (111) surfaces of gamma-alumina is found to result either from direct adsorption or from the exchange of basic hydroxyl groups. Moreover, the modification of the hydrogen bond network upon chlorine adsorption is put forward as a key parameter for changing the Br?nsted acidity. In a second step, we use a thermodynamic approach based on DFT total energy calculations corrected by the chemical potentials of HCl and H2O to determine the adsorption isotherms of chlorine and the relative surface concentration of hydroxyl groups and chlorine species on the gamma-alumina surfaces. The determination of chlorine content as a function of temperature and partial pressures of H2O and HCl offers new quantitative data required for optimizing the state of the support surface in industrial conditions. The mechanisms of chlorination are also discussed as a function of reaction conditions.     

Shedding light on biomolecule conformational dynamics using fluorescence measurements of trapped ions

Biomolecule conformational change has been widely investigated in solution using several methods; however, much less experimental data about structural changes are available for completely isolated, gas-phase biomolecules. Studies of conformational change in unsolvated biomolecules are required to complement the interpretation of mass spectrometry measurements and in addition, can provide a means to directly test theoretical simulations of biomolecule structure and dynamics independent of a simulated solvent. In this Feature Article, we review our recent introduction of a fluorescence-based method for probing local conformational dynamics in unsolvated biomolecules through interactions of an attached dye with tryptophan (Trp) residues and fields originating on charge sites. Dye-derivatized biomolecule ions are formed by electrospray ionization and are trapped in a variable-temperature quadrupole ion trap in which they are irradiated with either continuous or short pulse lasers to excite fluorescence. Fluorescence is measured as a function of temperature for different charge states. Optical measurements of the dye fluorescence include average intensity changes, changes in the emission spectrum, and time-resolved measurements of the fluorescence decay. These measurements have been applied to the miniprotein, Trp-cage, polyproline peptides and to a beta-hairpin-forming peptide, and the results are presented as examples of the broad applicability and utility of these methods. Model fits to Trp-cage fluorescence data measured as a function of temperature provide quantitative information on the thermodynamics of conformational changes, which are reproduced well by molecular dynamics. Time-resolved measurements of the fluorescence decays of Trp-cage and small polyproline peptides definitively demonstrate the occurrence of fluorescence quenching by the amino acid Trp in unsolvated biomolecules.