The stability curve for short wavelength density fluctuations of deuterium in niobium

By means of coherent neutron scattering the temperature dependence of density fluctuations of deuterium in niobium has been measured for three concentrations (2.3, 9.8, and 18% D/Nb). Depending on the scattering vector, the extrapolation to the stability curve for these fluctuations yields temperatures, which are from 2 to 7 times lower than the stability curve for macroscopic fluctuations. Both suppression and anisotropy of the short wavelength stability curves are attributed to the existence of elastic interaction between deuterium atoms.     

Limitations of high-intensity soft X-ray laser fields for the characterisation of water chemistry: Coulomb explosion of the octamer water cluster

In this work, the Coulomb explosion of the octamer water cluster has been studied employing a theoretical approach. Instead of the usual methodology that makes use of classical molecular dynamics, time-dependent density functional theory has been applied to tackle the problem. This method explicitly accounts for the laser field and thus does not impose any constraint on the interaction between the laser pulse and the cluster. We focus on the effects of energetic changes in the system under high-intensity soft X-ray laser pulses. The motions of the ions and their velocities during this process show significant differences for the three applied laser intensities (10(14), 10(15) and 10(16) W cm(-2)). Very strong soft X-ray free electron laser (FEL) pulses must be short to allow for investigations of ultra-fast wet chemistry, according to the principle of collect and destroy.     

Differential reaction cross sections from rotationally resolved quantum scattering calculations: application to gas-phase S(N)2 reactions

Differential reaction cross sections have been computed based on previous rotationally resolved time-independent quantum-mechanical scattering calculations for the complex-forming S(N)2 reaction Cl(-) + CH(3)Br → ClCH(3) + Br(-). The results show almost isotropic cross sections for reactant molecules with high rotational quantum numbers. Backward scattering is disfavoured for reaction out of states with small rotational excitation, in particular the rovibrational ground state. This is a quantum-mechanical effect (interference of partial waves) that can partly be rationalized by simple classical arguments. In particular for higher vibrational excitations, an umbrella effect can be observed that favours the backward direction. It can be explained by the strong enhancement of the reactivity by opening a direct mechanism. The ion-dipole interaction exerts a torque onto the molecule which carries out a rotation by about 90° and then completes the reaction.     

Stereoselective Allylation of Ketones: Explanation for the Unusual Inversion of the Induced Stereochemistry in the Auxiliary‐Mediated Crotylation and Pentenylation of Butanone by DFT Calculations

The solvent makes the difference : While attack at the Re face of the intermediate oxocarbenium ion becomes less energy demanding with increasing size of R′, it is the solvent effect that makes this attack the main trajectory, leading to inversed stereochemistry (see picture).

     

Baryon pairs production ine + e − annihilation at\sqrt s = 2.4 GeV

Search for baryon pairs production ine ⁺ e ^? annihilation at \(\sqrt s = 2386 MeV\) is reported. The data relate to a luminosity of 161 nb^?1 collected by the DM2 experiment at DCI, the Orsay colliding ring. First measurements of directe ⁺ e ^? annihilation into \(\Lambda \bar \Lambda \) and ofe ⁺ e ^?→ \(p\bar p\) at this energy are presented. First observation of a goode ⁺ e ^?→ \(n\bar n\) candidate is reported and upper limits are given fore ⁺ e ^?→ \(n\bar n, \Lambda \bar \sum ^0 + c.c.\) and \(\Sigma ^0 \bar \Sigma ^0 \) .     

Structure and chemistry of apicidins, a class of novel cyclic tetrapeptides without a terminal alpha-keto epoxide as inhibitors of histone deacetylase with potent antiprotozoal activities

Apicidins are a class of cyclic tetrapeptides that do not contain the classical electrophilic alpha-keto epoxide yet are potent (nM) inhibitors of histone deacetylase and antiprotozoal agents. These compounds showed broad-spectrum activities against the apicomplexan family of protozoa including Plasmodium sp (malarial parasite), Toxoplasma gondii, Cryptosporidium sp., and Eimeria sp. These cyclic peptides contain a beta-turn amino acid (R)-Pip or (R)-Pro, (S)-N-methoxy Trp, (S)-Ile, or (S)-Val, and either (S)-2-amino-8-oxodecanoic acid or a modified (S)-2-amino-8-oxodecanoic acid. The isolation and structure elucidation of new apicidins from two Fusarium species, temperature-dependent NMR studies of apicidin, NMR and molecular modeling based conformation of the 12-membered macrocyclic ring, and selected chemical modifications of apicidin have been detailed in this paper. The cyclic nature of the peptide, the C-8 keto group, and the tryptophan are all critical for the biological activity.     

A seemingly well understood light-induced peroxide decarboxylation reaction reinvestigated with femtosecond time resolution

The photoinduced (266 nm) ultrafast decarboxylation of the peroxyester tert-butyl 9-methylfluorene-9-percarboxylate (TBFC) in solution has been studied with femtosecond time resolution. While the photodissociation of TBFC occurs too fast to be resolved, the intermediate 9-methylfluorenylcarbonyloxy radical (MeFl-CO(2)) decarboxylates on a picosecond time scale. The latter process is monitored by pump-probe absorption spectroscopy at wavelengths between 400 and 883 nm. The measured transient absorbance signals reveal a dominant fast decay with a lifetime of a few picoseconds and, to a minor extent, a slow component with a lifetime of about 55 ps. Statistical modeling of MeFl-CO(2) decarboxylation employing molecular parameters calculated by density functional theory suggests that the fast component is associated with the decarboxylation of vibrationally hot radicals, whereas the 55 ps decay reflects the dissociation of thermally equilibrated MeFl-CO(2) at ambient temperature. The vast majority of MeFl-CO(2) radicals thus decarboxylate on a time scale about an order of magnitude faster than expected from the time constant of 55 ps reported by Falvey and Schuster for this reference reaction. This literature value turns out to refer to decarboxylation rate of MeFl-CO(2) at ambient temperature.     

Theoretical investigations of proton-bound cluster ions

Several proton-bound cluster ions have been studied by means of coupled cluster calculations with large basis sets. Among these are complexes of a krypton or xenon atom with the cations HCO+, HN2+ and HNCH+. Various spectroscopic properties have been calculated in all cases. Effects of vibrational anharmonicity are particularly pronounced for the intramolecular stretching vibrations of Kr...HN2+ and Xe...HN2+. The proton stretching vibration of (N2)H+(N2) is predicted around 800 cm-1, with a large transition dipole moment of 1.15 D. Both (N2)H+(N2) and (HCN)H+(NCH) have linear centrosymmetric equilibrium structures. Those of (OC)H+(CO) and (HCC-)H+(CCH-) are asymmetric with barrier heights to the centrosymmetric saddle points of 382 and 2323 cm-1, respectively. The dissociation energy of the anionic complex Cl-...HCCH is calculated to be Do = 3665 cm-1, 650 cm-1 larger than the corresponding value for Br-...HCCH. The complex between a fluoride ion and acetylene is more strongly bound and shows strongly anharmonic behaviour, similar to the bihalides FHF- or ClHCl-. Strong Fermi resonance interaction is predicted between nu 3 (approximately proton stretch) and 2 nu 4 (first overtone of intermolecular stretch).