Probing the transition from hydrophilic to hydrophobic solvation with atomic scale resolution

Picosecond and femtosecond X-ray absorption spectroscopy is used to probe the changes of the solvent shell structure upon electron abstraction of aqueous iodide using an ultrashort laser pulse. The transient L(1,3) edge EXAFS at 50 ps time delay points to the formation of an expanded water cavity around the iodine atom, in good agreement with classical and quantum mechanical/molecular mechanics (QM/MM) molecular dynamics (MD) simulations. These also show that while the hydrogen atoms pointed toward iodide, they predominantly point toward the bulk solvent in the case of iodine, suggesting a hydrophobic behavior. This is further confirmed by quantum chemical (QC) calculations of I(-)/I(0)(H(2)O)(n=1-4) clusters. The L(1) edge sub-picosecond spectra point to the existence of a transient species that is not present at 50 ps. The QC calculations and the QM/MM MD simulations identify this transient species as an I(0)(OH(2)) complex inside the cavity. The simulations show that upon electron abstraction most of the water molecules move away from iodine, while one comes closer to form the complex that lives for 3-4 ps. This time is governed by the reorganization of the main solvation shell, basically the time it takes for the water molecules to reform an H-bond network. Only then is the interaction with the solvation shell strong enough to pull the water molecule of the complex toward the bulk solvent. Overall, much of the behavior at early times is determined by the reorientational dynamics of water molecules and the formation of a complete network of hydrogen bonded molecules in the first solvation shell.     

Femtosecond nonresonant degenerate four-wave mixing at atmospheric pressure and in a free jet

2 , N₂, and CO₂. For CO₂ additional experiments have been performed at reduced pressure and in a molecular beam. By delaying the probe pulse a periodic recovery of the DFWM signal is observed. The period of these transients can be assigned unambiguously to rotational Raman transitions of the ground state within the laser bandwidth. The decay of the transients yields the collisional dephasing of the Raman-induced polarization. At zero delay also optical-field-induced birefringence of electronic nature contributes to the signal. The different time scales of the Raman and electronic effects allow us to estimate their relative strength. Received: 3 August 1998 / Revised version: 21 October 1998 / Published online: 24 February 1999     

Femtosecond dynamics of the collinear-to-spiral antiferromagnetic phase transition in CuO

We report on the ultrafast dynamics of magnetic order in a single crystal of CuO at a temperature of 207 K in response to strong optical excitation using femtosecond resonant x-ray diffraction. In the experiment, a femtosecond laser pulse induces a sudden, nonequilibrium increase in magnetic disorder. After a short delay ranging from 400 fs to 2 ps, we observe changes in the relative intensity of the magnetic ordering diffraction peaks that indicate a shift from a collinear commensurate phase to a spiral incommensurate phase. These results indicate that the ultimate speed for this antiferromagnetic reorientation transition in CuO is limited by the long-wavelength magnetic excitation connecting the two phases.     

Structural and magnetic dynamics of a laser induced phase transition in FeRh

We use time-resolved x-ray diffraction and magneto-optical Kerr effect to study the laser-induced antiferromagnetic to ferromagnetic phase transition in FeRh. The structural response is given by the nucleation of independent ferromagnetic domains (τ(1)~30 ps). This is significantly faster than the magnetic response (τ(2)~60 ps) given by the subsequent domain realignment. X-ray diffraction shows that the two phases coexist on short time scales and that the phase transition is limited by the speed of sound. A nucleation model describing both the structural and magnetic dynamics is presented.     

Nanoscale depth-resolved coherent femtosecond motion in laser-excited bismuth

We employ grazing-incidence femtosecond x-ray diffraction to characterize the coherent, femtosecond laser-induced lattice motion of a bismuth crystal as a function of depth from the surface with a temporal resolution of 193+/-8 fs. The data show direct consequences on the lattice motion from carrier diffusion and electron-hole interaction, allowing us to estimate an effective diffusion rate of D=2.3+/-0.3 cm(2)/s for the highly excited carriers and an electron-hole interaction time of 260+/-20 fs.     

Echanges isotopiques d'hydrogène T-H,T-D,D-H et H-D de la diazoacétone et du diazoacétaldéhyde: Vitesse de protonation et effets isotopiques. Communication préliminaire

The velocities of four acid-catalysed hydrogen isotope exchange reactions between diazoacetone ( 1 ) and water (T → H, T → D, D → H, and H → D) and of three exchange reactions between diazoacetaldehyde ( 2 ) and water (T → H, T → D, and H → D) have been measured. The exchange D → H of 1 was found to be submit to general acid catalysis (α_B = 0.6). The rates of protonation of primary and secondary diazocarbonyl compounds of similar structure were found to be quite similar, in spite of difference in mecanism (A-2 and A-S_E2, respectively).     

Hidden Heptacyclic Chiral N-Acyl Iminium Ions: A New Entry to Enantioenriched Polycyclic Azepanes and Azocanes by Superacid-Promoted Intramolecular Pictet-Spengler Reaction

Enantioenriched complex fused-tricyclic azepanes or bridged-polycyclic azocanes were constructed via a two-step sequence involving an enantioselective organocascade followed by superacid activation of the domino adduct. The activated oxa-bridged azepane acts as a key hidden heptacyclic chiral N-acyl iminium ion triggering a chemo- and diastereoselective intramolecular mono- or di-arylation.     

OH Concentration Measurements by Resonant Holographic Interferometry and Comparison with Direct Numerical Simulations

We apply a novel laser diagnostic technique — Resonant Holographic Interferometry (RHI) to measure the concentration of hydroxyl radical (∼2000 ppm) in a co-flow diffusion flame of diluted hydrogen and air stabilized on a Wolfhard-Parkerburner. This methodology is based upon the dispersion of light of frequency close to an electronic transition of a target molecule. The two-color setup utilized in RHI provides a two-dimensional distribution of the target species concentration and quantitative information can be obtained from the interferogram without requiring any calibration. To provide independent flame data for comparison, a two-dimensional numerical simulation was performed taking into account the effects of detailed chemical kinetics and transport phenomena. In spite of a number of simplifying assumptions made in the simulation, computational and experimental results are in good agreement with respect to the magnitude and width of the region where OH is found. We do observe a difference of approximately 1 mm in the flame position due to the simplifying assumptions made in the simulation. The comparison between the experimental and numerical results clearly demonstrated the potential of RHI in flame diagnostics. This revised version was published online in July 2006 with corrections to the Cover Date.     

Enantioenriched Methylene‐Bridged Benzazocanes Synthesis by Organocatalytic and Superacid Activations

Achieving in a straightforward way the synthesis of enantioenriched elaborated three‐dimensional molecules related to bioactive natural products remains a long‐standing quest in organic synthesis. Enantioselective organocatalysis potentially offers a unique opportunity to solve this problem, especially when combined with complementary modes of activation. Here, we report the sequential association of organocatalytic and superacid activations of simple linear achiral readily available precursors to promote the formation of unique highly elaborated chiral methylene‐bridged benzazocanes exhibiting three to five fully‐controlled stereocenters. This peculiar backbone, difficult to assemble by standard synthetic approaches, is closely related to bioactive natural and synthetic morphinans and benzomorphans. The formation of a highly reactive chiral 7‐membered ring N‐acyl iminium superelectrophilic ion, evidenced by low‐temperature in situ NMR experiments, triggers a challenging stereoselective Friedel–Crafts‐type cyclization.