Two levels of conformational pre-organization consolidate strong CH hydrogen bonds in chloride-triazolophane complexes

Structural rigidity is verified as a pre-organizational factor that acts together with the macrocyclic effect such that synthesis helps in paying the cost of bringing together electropositive CH donors ready for H-bonding with chloride.     

Molecular logic gates using surface-enhanced Raman-scattered light

A voltage-activated molecular-plasmonics device was created to demonstrate molecular logic based on resonant surface-enhanced Raman scattering (SERS). SERS output was achieved by a combination of chromophore-plasmon coupling and surface adsorption at the interface between a solution and a gold nanodisc array. The chromophore was created by the self-assembly of a supramolecular complex with a redox-active guest molecule. The guest was reversibly oxidized at the gold surface to the +1 and +2 oxidation states, revealing spectra that were reproduced by calculations. State-specific SERS features enabled the demonstration of a multigate logic device with electronic input and optical output.     

Thermodynamic parameters and counterion binding to the micelle in binary anionic surfactant systems

Competitive counterion binding of sodium and calcium to micelles, and mixed micellization have been investigated in the systems sodium dodecylsulfate (NaDS)/sodium decylsulfate (NaDeS) and NaDS/sodium 4-octylbenzenesulfonate (NaOBS) in order to accurately model the activity of the relevant species in solution. The critical micelle concentration (CMC) and equilibrium micelle compositions of mixtures of these anionic surfactants, which is necessary for determining fractional counterion binding measurements, is thermodynamically modeled by regular solution theory. The mixed micelle is ideal (the regular solution parameter β(M)=0) for the NaDS/NaOBS system, while the mixed micelle for NaDS/NaDeS has β(M)=-1.05 indicating a slight synergistic interaction. Counterion binding of sodium to the micelle is influenced by the calcium ion concentration, and vice versa. However, the total degree of counterion binding is essentially constant at approximately 0.65 charge negation at the micelle's surface. The counterion binding coefficients can be quantitatively modeled using a simple equilibrium model relating concentrations of bound and unbound counterions.     

From atomic to molecular anions: a neutral receptor captures cyanide using strong C-H hydrogen bonds

The multifaceted character of cyanide as an acceptor of hydrogen bonds from a receptor has been examined for the first time using electronic-structure theory and spectroscopic measurements (UV/Vis and NMR titrations). Motivated by the similar size and charge of the cyanide pseudohalide and the monoatomic chloride ion, strong interactions of cyanide with a rigid macrocyclic triazolophane receptor have been predicted by theory and confirmed by experimental findings. It was found that both anions bind with similar strength in the gas phase (computed) and in the solution phase (experimental) via C-H hydrogen bonds. Theoretical calculations predict that the heterodiatomic cyanide prefers to bind in the plane of the macrocycle along the north-south axis. Examination of the possible binding modes reveal low computed barriers for in-plane rotation. The predicted model is consistent with the experimental data. Overall, the binding of a molecular anion within the cavity of a triazolophane receptor has been characterized where the computed and experimental binding energies are consistent with the classification of cyanide as a pseudohalide in the context of supramolecular chemistry.     

An evaluation of the time dependence of the anisotropy of the water diffusion tensor in acute human ischemia

We have performed MRI examinations to determine the water diffusion tensor in the brain of six patients who were admitted to the hospital within 12 h after the onset of cerebral ischemic symptoms. The examinations have been carried out immediately after admission, and thereafter at varying intervals up to 90 days post admission. Maps of the trace of the diffusion tensor, the fractional anisotropy and the lattice index, as well as maps of cerebral blood perfusion parameters, were generated to quantitatively assess the character of the water diffusion tensor in the infarcted area. In patients with significant perfusion deficits and substantial lesion volume changes, four of six cases, our measurements show a monotonic and significant decrease in the diffusion anisotropy within the ischemic lesion as a function of time. We propose that retrospective analysis of this quantity, in combination with brain tissue segmentation and cerebral perfusion maps, may be used in future studies to assess the severity of the ischemic event.     

First symmetry tests in polarized Z0 decays to bbg

We have made the first direct symmetry tests in the decays of polarized Z0 bosons into fully identified bbg states, collected in the SLD experiment at SLAC. We searched for evidence of parity violation at the bbg vertex by studying the asymmetries in the b-quark polar- and azimuthal-angle distributions, and for evidence of T-odd, CP-even or CP-odd, final-state interactions by measuring angular correlations between the three-jet plane and the Z0 polarization. We found results consistent with standard model expectations and set 95% C. limits on anomalous contributions.     

The isomerisation of (Z)-3-[2H1]-phenylprop-2-enone as a measure of the rate of hydroperoxide addition in Weitz-Scheffer and Julia-Colonna epoxidations

(Z)-3-[2H1]-Phenylprop-2-enone is isomerised by hydroperoxide to an equimolar mixture of the (Z)- and (E)-isomers prior to epoxidation. Poly-(L)-leucine (10 mole %) accelerates the addition of hydroperoxide by an order of magnitude and sequesters hydroperoxide from THF.     

Cosmic rays and dosimetry at aviation altitudes

Recent concerns regarding the effects of the cosmic radiation field at aircraft altitudes on aircrew have resulted in a renewed interest in detailed measurements of the neutral and charged particle components in the atmosphere. CR-39 nuclear track detectors have been employed on a number of subsonic and supersonic aircraft to measure charge spectra and LET spectra at aircraft altitudes. These detectors are ideal for long term exposures required for these studies and their passive nature makes them suitable for an environment where interference with flight instrumentation could be a problem. We report here on measurements and analysis of short range tracks which were produced by high LET particles generated mainly by neutron interactions at aviation altitudes. In order to test the overall validity of the technique measurements were also carried out at the CERN-CEC field which simulates the radiation field at aviation altitudes and good agreement was found with dose values obtained using mainly heavy ion calibration.     

Electrophilic cleavage of the iron-carbon sigma bond: Mechanism and the stereochemistry at iron

Electrophilic cleavage of the iron-carbon sigma bond in the resolved chiral complexes CpFE(CO)(PPh₃)R (where R = Me, Et, CH₂CO₂ (menthyl), and CH₂O (menthyl)) by I₂, ICl, and HgI₂ reveals that the reaction proceeds with net retention of configuration at iron, but stereospecifity is not high and is insensitive to the nature of the alkyl group. In every case, recovered starting material is partially racemized, but the source of this is not yet certain. Formation of CpFe(CO)(PPh₃)I from cleavages using ICl indicates that initial electrophilic attack is at the metal. A mechanism is discussed which accounts for net retention at metal in terms of steric and ion pairing rather than electronic effects. Cleavage using HO₂CCF₃ forms CpFe(CO)(PPh₃)O₂CCF₃ of ca. 50% enantiomeric excess as shown by the use of an optically active Eu NMR shift reagent, but net retention or inversion at iron cannot be inferred.