Structure and protonation of the bis-ethylene adduct [Pt(micro-PBut2)(eta2-CH2=CH2)]2. Pt-H-P agostic interaction and proton scrambling

The bis-ethylene derivative [Pt(micro-PBu(t)(2))(eta(2)-CH(2)=CH(2))](2) was prepared and characterized by X-ray diffraction; its protonation affords [Pt(2)(micro-PBu(t)(2))(micro-PBu(t)(2)H)(eta(2)-CH(2)=CH(2))(2)](CF(3)SO(3)), with a rarely observed P-H-M agostic proton in rapid exchange with those of the adjacent ethylene molecule.     

Determination of furanic compounds in traditional balsamic vinegars by ion-exclusion liquid chromatography and diode-array detection

A method for the determination of furanic compounds in traditional balsamic vinegars is proposed. It is based on ion-exclusion chromatographic separation and diode-array detection of furans through an isocratic elution with 0.01 N phosphoric acid and 16% acetonitrile. Preliminary trials on standard compounds stability in heat-acidic conditions are also performed. In all the 19 samples analyzed, 2-furoic acid, 5 HMF, and furfural are found. No sample contains 4-hydroxy-2,5-dimethyl-3-(2H)-furanone (DHMF); 2-acetylfuran; or furfuryl alcohol. Three unknown compounds are also detected. The last eluting of these compounds is identified as 5-acethoxymethylfurfural, and, notwithstanding a partial hydrolysis in our chromatographic conditions, its quantitation can be carried out.     

Self-assembled monolayers of cobalt(II)- (4-tert-butylphenyl)-porphyrins: the influence of the electronic dipole on scanning tunneling microscopy images

Self-assembled monolayers (SAMs) of cobalt(II) 5,10,15,20-tetrakis(4-tert-butylphenyl)-porphyrin, a promising material for optical, photoelectrochemical, and chemical sensor applications, were prepared on Au(111) via axial ligation to 4-aminothiophenol, and studied by several surface science techniques. Scanning tunneling microscopy (STM) and spectroscopy (STS) measurements showed the apparent topology of the Au(111) herringbone structure reconstruction, but with bias-dependent contrast images and asymmetric I/V characteristics. Photoelectron spectroscopy confirmed the presence of metalloporphyrins on the surface, whereas near-edge X-ray absorption (NEXAFS) measurements revealed that the porphyrin ring was tilted by about 70 degrees with respect to the surface plane. The above effects are ascribed to the presence of oriented molecular dipole layers between the metal and the organic material as confirmed by a comparison with first-principles density-functional theory calculations. The measured bias-dependent STM profiles have been reproduced by a simple monodimensional tunneling model.     

Efficient crystallization-induced dynamic resolution of alpha-substituted carboxylic acids

Herein we present a novel route to enantiomerically enriched chiral alpha-substituted carboxylic acids by crystallization-induced dynamic resolution (CIDR) of their diastereomeric salts with chiral amines. Thus, the racemic alpha-bromo acid 3 is converted reliably with (1R,2S)-2-amino-1,2-diphenylethanol in the presence of a catalytic amount of tetrabutylammonium bromide into its R-enantiomer 4 in 90% yield with 88% ee. Similarly, the racemic alpha-thiobenzoyl acid 5 could be resolved to 90% ee in 74% yield. Further enrichment to enantiomeric homogeneity could be achieved in both cases by crystallization. In a telescoped, two-step process, S-alpha-thiobenzoyl acid 6 (>or=99.6% ee) was prepared from the racemic bromide 3 in 63% yield. State-of-the-art parallel experimentation enabled rapid screening for suitable dynamic resolution conditions. Kinetic studies defined the influence of temperature, tetrabutylammonium bromide concentration, molarity, and solvent polarity on the resolution rate, product yield, and enantiomeric excess.     

Progress toward a biomimetic synthesis of phomoidride B

[reaction: see text]. An intramolecular cyclization strategy for effecting a biomimetic synthesis of the core structure of the fungal secondary metabolites phomoidrides A and B is described. The cyclization substrate 20 is prepared in eight steps from dibromide 10. Treatment of 20 with triethylamine in acetonitrile results in a rapid cyclization to give 21 and 22 in good yield.     

Nature of the chemical bonding in D3h [MH3M]+ cations (M = Be,Mg)

Motivated by the particularly short metal-metal distance that has been predicted for the D_3h [BeH₃Be]⁺ cation, comparable to those anticipated for triple bonds, we investigate the nature of the bonding interactions in the D_3h [MH₃M]⁺ cations (M = Be, Mg). CCSD(T)/cc-pVQZ calculations are used to determine optimized geometries for all of the various species, including those “capped” by He or Ne atoms (as proxies for an inert gas matrix). The primary tools that are then used to investigate the nature of the chemical bonding are spin-coupled generalized valence bond calculations and the analysis of localized natural orbitals and of domain-averaged Fermi holes. The various results for all of the systems considered indicate the presence of highly polar three-center two-electron M─H─M bonding character instead of any significant direct metal-metal bonding.     

Guaiazulene-based phenolic radical scavengers: synthesis, properties, and EPR studies of their reaction with oxygen-centred radicals

A variety of phenolic derivatives 4, carrying the guaiazulene moiety, were prepared starting from guaiazulene. Compounds 4 react with oxygen-centred radicals exhibiting chromotropic behaviour. The radical scavenging power of these compounds was evaluated by different methods. Compounds 4 are less efficient than some of the most common radical scavengers but show quite selective behaviour towards different oxygen-centred radicals. A correlation is found between the antioxidant activity of the compounds 4 and the corresponding phenolic O-H bond dissociation energy. Some aspects of the reaction of the compounds 4 with oxygen-centred radicals were elucidated by EPR and DFT studies.     

Advanced Sunscreens: UV Absorbers Encapsulated in Sol-Gel Glass Microcapsules

Undesirable phototoxic and photoallergic reactions accompanying a justified increased use of sunscreen active ingredients within cosmetic products have encouraged the development of new products safer for human use. The sol-gel microencapsulation technology developed utilizes an interfacial polymerization process, allowing for the achievement of transparent silica glass microcapsules with sizes ranging between 0.3–3 microns and a characteristic core-shell structure. Within the sol-gel microcapsule structure a UV absorber core, constituting roughly 80% of the final product weight, is enclosed within a silica shell. These advanced sunscreen actives are then incorporated into a suitable cosmetic vehicle to achieve high Sun Protection Factors (SPF), while affording an improved safety profile, as the penetration of the UV absorbers is markedly reduced.     

Theoretical characterization of charge transport in chromia (alpha-Cr2O3)

Transport of conduction electrons and holes through the lattice of alpha-Cr2O3 (chromia) is modeled as a valence alternation of chromium cations using ab initio electronic structure calculations and electron-transfer theory. In the context of the small polaron model, a cluster approach was used to compute quantities controlling the mobility of localized electrons and holes, i.e., the reorganization energy and the electronic coupling matrix element that enter Marcus' theory. The calculation of the electronic coupling followed the generalized Mulliken-Hush approach using the complete active space self-consistent-field (CASSCF) method and the quasidiabatic method. Our findings indicate that hole mobility is more than three orders of magnitude larger than electron mobility in both (001) and [001] lattice directions. The difference arises mainly from the larger internal reorganization energy calculated for electron-transport relative to hole-transport processes while electronic couplings have similar magnitudes. The much larger hole mobility versus electron mobility in alpha-Cr2O3 is in contrast to similar hole and electron mobilities in hematite alpha-Fe2O3 previously calculated. Our calculations also indicate that the electronic coupling for all charge-transfer processes of interest is smaller than for the corresponding processes in hematite. This variation is attributed to the weaker interaction between the metal 3d states and the O(2p) states in chromia than in hematite, leading to a smaller overlap between the charge-transfer donor and acceptor wave functions and smaller superexchange coupling in chromia. Nevertheless, the weaker coupling in chromia is still sufficiently large to suggest that charge-transport processes in chromia are adiabatic in nature. The electronic coupling is found to depend on both the superexchange interaction through the bridging oxygen atoms and the d-shell electron-spin coupling within the Cr-Cr donor-acceptor pair, while the reorganization energy is essentially independent of the electron-spin coupling.