Two‐ and three‐dimensional hydrogen‐bonded structures in the 1:1 proton‐transfer compounds of 4,5‐dichlorophthalic acid with the isomeric monoaminobenzoic acids

The crystal structures of the 1:1 proton‐transfer compounds of 4,5‐dichlorophthalic acid with the three isomeric monoaminobenzoic acids, namely the hydrate 2‐carboxyanilinium 2‐carboxy‐4,5‐dichlorobenzoate dihydrate, C₇H₈NO₂⁺·C₈H₃Cl₂O₄⁻·2H₂O, (I), and the anhydrous salts 3‐carboxyanilinium 2‐carboxy‐4,5‐dichlorobenzoate, C₇H₈NO₂⁺·C₈H₃Cl₂O₄⁻, (II), and 4‐carboxyanilinium 2‐carboxy‐4,5‐dichlorobenzoate, C₇H₈NO₂⁺·C₈H₃Cl₂O₄⁻, (III), have been determined at 130 K. Compound (I) has a two‐dimensional hydrogen‐bonded sheet structure, while (II) and (III) are three‐dimensional. All three compounds feature sheet substructures formed through anilinium N⁺—H...O_carboxyl and anion carboxylic acid O—H...O_carboxyl interactions and, in the case of (I), additionally linked through the donor and acceptor associations of the solvent water molecules. However, (II) and (III) have additional lateral extensions of these substructures though cyclic R²₂(8) associations involving the carboxylic acid groups of the cations. Also, (II) and (III) have cation–anion π–π aromatic ring interactions. This work provides further examples illustrating the regular formation of network substructures in the 1:1 proton‐transfer salts of 4,5‐dichlorophthalic acid with the bifunctional aromatic amines.     

Substrate and field dependence of the SPINOE transfer to surface 13C from hyperpolarized 129Xe

The substrate and field dependencies of surface SPINOE enhancements using optical pumping and magic angle spinning NMR were monitored. Relaxation rates and enhancements were examined to gain an understanding of the parameters that determine the SPINOE enhancement. (13)C-labeled deuterated methanol was adsorbed on three different substrates (SnO(2), TiO(2), Ti/SiO(2)) with heats of adsorption for xenon ranging from 14.2 to 22.6 kJ/mol. The different heats of adsorption led to a range of xenon coverages and xenon relaxation rates. Using a simple model along with experimental values for the xenon surface polarization and cross- and self-relaxation rates, the (13)C signal enhancement could be predicted and compared with experimental enhancement values. Magnetic field dependence studies were also made by monitoring the (13)C enhancements via SPINOE from hyperpolarized xenon at fields of 0.075, 4.7, and 9.4 T. The pertinent parameters necessary to achieve maximum SPINOE enhancement are discussed.     

Picosecond transient photoreflectance measurements of ion-implanted GaAs

We have used picosecond transient reflectance techniques to measure the near-surface characteristics of ion-implanted GaAs. These non-destructive laser-based diagnostic techniques allow measurement of the modification of near-surface properties at relatively low implant fluences. Photothermal phenomena dominate these results and yield important information concerning the extent of implant-induced materials modification.     

Synthetic studies towards ptilomycalin A: total synthesis of crambescidin 359

A potentially biomimetic synthesis of the guanidine-containing marine natural product crambescidin 359 via a double Michael addition of guanidine to a suitably functionalised bis-enone is reported.     

COMPARATIVE PHOTOCHEMICAL ANALYSIS OF HIGHLY PURIFIED 124 KILODALTON OAT and RYE PHYTOCHROMES in vitro

Abstract A direct comparison of the photochemical interconversions between red (P_r-) and far-red (P_fr-) absorbing forms of highly-purified 124 kDa oat and rye phytochromes under identical experimental conditions was performed. In two different buffer systems at 5°C, the quantum yields for the P_r to P_tr and P_fr to P_r phototransformations under constant red and far-red illumination, φ _r and φ_fr respectively, were determined to be 0.152-0.154 and 0.060-0.065 for oat preparations and 0.172-0.174 and 0.074-0.078 for rye preparations. These values as well as the wavelength dependence of the photoequilibrium produced under continuous illumination throughout the visible and near-ultraviolet spectrum were based on the absorption spectra of the two phytochrome preparations and revised molar absorption coefficients. The molar absorption coefficients were estimated by quantitative amino acid analysis and shown to be identical for the two monocot phytochromes (i.e. 132 mM ⁻¹ cm⁻¹ at the red absorption maximum for the P_r form). Because these measurements were performed under identical experimental conditions, including buffer, temperature, light fluence rate, and instrumentation, the differences observed must reflect structural features inherent to the two different monocotyledonous phytochromes.     

Optimal population analysis

An easy method of approximating the electron density of a molecule by expressing it as a sum of squares of atomic orbitals is described. The coefficients are determined by minimizing the error in the electric field. The optimal population analysis is an integration of this optimized density over the orbitals belonging to one atom. The Mulliken population analysis can be interpreted as the integral of a Mulliken density that is a similar expression with fixed coefficients. The two densities are compared in détail using calculations on the water molecule and, more briefly, HF, LiH, BH, and C₂H₄. The error in the densities is identified and examined. The optimal population analysis is strongly recommended as a practical improvement to the Mulliken populations.     

Some interference effects occurring in the hydrogen/argon/entrained-air flame

An examination was made of the effects of various cations and anions on the determinations of calcium (4227 Å) by flame emission using turbulent hydrogen/ argon/entrained-air and hydrogen/oxygen flames. The effects of height of observation and mechanisms of interference are discussed for Cs⁺, Rb⁺, F^-, Br^-, I^-, NO₃^-, Cl0₄^-, S0₄^2-, PO₄^3-, P₂O₇^4-, acetate, oxalate, citrate, tartrate and EDTA. Ionization interferences were not detected in the hydrogen/argon/entrained-air flame, but all other types of interference were more serious with this flame than with the hydrogen/ oxygen flame. The study also suggests some methods by which the effects of the interferences may be overcome.