Mechanical Properties of Organic-Inorganic Hybrid Materials Determined by Indentation Techniques

The mechanical properties of hybrid materials consisting of polystyrene (PS), which was cross-linked with different proportions of the multifunctional cluster Zr₆O₄(OH)₄(methacrylate)₁₂ (Zr6) were investigated. With the help of (micro)indentation and scratch testing, the influence of the Zr6 clusters on mechanical properties, such as hardness, stiffness, creep, craze initiation, and scratch resistance was shown. There was only a slight increase in hardness and in indentation modulus with higher cluster loadings. While this observation was in agreement with the compression behaviour of the materials, the tensile properties showed a much stronger dependence on the Zr6 content. With increasing cluster loading, an increase of craze initiation stress, as determined by ball indentation experiments, was found. Performing scratch testing with constant load, a reduction of pile-up and a stronger recovery were observed for the hybrid materials compared to the neat PS. Scratch tests with a constant increase of load showed an increase of the critical load for crack opening during scratching.     

Mimicking primary processes in photosynthesis—covalently linked porphyrin quinones

Porphyrin quinones (P-Qs), covalently linked via different aliphatic bridges, have been synthesized and studies in their (porphyrin) cationic and (semiquinone) anionic radical states by EPR, ENDOR and TRIPLE resonance techniques. Electron transfer (ET) from the porphyrin donor to the quinone acceptor could be observed by time-resolved picosecond fluorescence spectroscopy (singlet ET) and by time-resolved EPR spectroscopy (triplet ET) in isotropic fluid solution and in anisotropic media (liquid crystals and reversed micelles). Steady-state in situ photoexcitation of P-Qs in CTAB cationic reversed micelles yielded the corresponding semiquinone radical anions. In TRITON X-100 reversed micelles both the radical cation of the porphyrin and the radical anion of the semiquinone could be detected, which occured in complete emission. In covalently linked porphyrin flavins ET from the photoexcited porphyrin fragment to the flavin and, in addition, energy transfer from the photoexcited flavin to the porphyrin could be observed.     

A Theoretical Study of Amine Bonding in Titanium Alkoxide Adducts

Summary. DFT calculations were carried out on Ti₂(OCH₃)₈ (NH₂CH₃)₂ and Ti₂(OCH₃)₈(NH₃)₂, which are model compounds for the previously isolated amine adducts Ti₂(OR)₈(NH₂ R′)₂. The calculations show that the Ti–N bond strength is weak; however, coordination of the amine to the metal center is supported by a N–H···O hydrogen bond of the amine with the neighboring alkoxo ligand. The Ti–N interaction is purely σ in nature, while the Ti–O interactions include both σ and π contributions. The lowest unoccupied molecular orbitals are mainly localized on Ti t_2g-like orbitals.     

pKa calculations in solution and proteins with QM/MM free energy perturbation simulations: a quantitative test of QM/MM protocols

The accuracy of biological simulations depends, in large part, on the treatment of electrostatics. Due to the availability of accurate experimental values, calculation of pKa provides stringent evaluation of computational methods. The generalized solvent boundary potential (GSBP) and Ewald summation electrostatic treatments were recently implemented for combined quantum mechanical and molecular mechanics (QM/MM) simulations by our group. These approaches were tested by calculating pKa shifts due to differences in electronic structure and electrostatic environment; the shifts were determined for a series of small molecules in solution, using various electrostatic treatments, and two residues (His 31, Lys 102) in the M102K T4-lysozyme mutant with large pKa shifts, using the GSBP approach. The calculations utilized a free energy perturbation scheme with the QM/MM potential function involving the self-consistent charge density functional tight binding (SCC-DFTB) and CHARMM as the QM and MM methods, respectively. The study of small molecules demonstrated that inconsistent electrostatic models produced results that were difficult to correct in a robust manner; by contrast, extended electrostatics, GSBP, and Ewald simulations produced consistent results once a bulk solvation contribution was carefully chosen. In addition to the electrostatic treatment, the pKa shifts were also sensitive to the level of the QM method and the scheme of treating QM/MM Coulombic interactions; however, simple perturbative corrections based on SCC-DFTB/CHARMM trajectories and higher level single point energy calculations were found to give satisfactory results. Combining all factors gave a root-mean-square difference of 0.7 pKa units for the relative pKa values of the small molecules compared to experiment. For the residues in the lysozyme, an accurate pKa shift was obtained for His 31 with multiple nanosecond simulations. For Lys 102, however, the pKa shift was estimated to be too large, even after more than 10 nanosecond simulations for each lambda window; the difficulty was due to the significant, but slow, reorganization of the protein and water structure when Lys 102 was protonated. The simulations support that Lys 102 is deprotonated in the X-ray structure and the protein is highly destabilized when this residue is protonated.     

Computational studies of the 3-Dimensional structure of cyclopenta polycyclic aromatic hydrocarbons containing a gulf region

Recently, some cyclopenta-fused polyaromatic hydrocarbons, an environmentally relevant subclass of chemicals, have been shown to have carcinogenic activity in animals. It has been suggested that benz[l] aceanthrylene ( I ), an active member of this subclass with a gulf region, has a trans dihydrodiol metabolite that is nonplanar and has two distinct spatial configurations. We have used MMP 2(85) and AM 1 to investigate the three-dimensional structure of this dihydrodiol and other similar derivatives of ( I ) and have found that although ( I ) is somewhat nonplanar the relevant derivatives are all nearly planar. Further, we have computed potential functions for the bending of the angular ring in the gulf region using MMP 2(85), AM 1, and ab initio computed energies for AM 1 spatial configurations and find that these molecules all have only a single potential minimum. We have performed the same calculations for benzo[c]phenanthren and its 1,12 dimethyl derivative, molecules with a similar gulf region for which crystallographic data exists. In agreement with that data, we find that two distinct spatial configurations exist separated by significant barries. The differences between the results generated by the three different methods of computation will be discussed.     

Occurrence of Neighboring Group Participation Reactions in Amide-N and Amidine Complexes Derived from Pentaammine(dinitrile)cobalt(III) Ions

N-Bonded pentaamminecobalt(III) complexes of 2-cyanobenzamide, 2-cyanoacetamide, and fumaric, succinic, glutaric, and adipic amide-nitriles have been prepared. The kinetics of the base hydrolysis of (succinonitrile)pentaamminecobalt(III) have been measured: k(obsd) = k(OH) [OH(-)]; k(OH) = 1.23 x 10(3) {I = 1.00 M (NaCH(3)COO), 25 degrees C}. Amido-N-coordinated 2-cyanobenzamide cyclized in aqueous base, and it forms [(1-oxo-3-iminoisoindolino-endo-N)pentaamminecobalt(III). In aqueous acid it protonates on the exo-imine and solvolyzes (k(H) = 7.9 x 10(-)(5) s(-)(1)), forming the pentaammineaquacobalt(III) complex and 1-oxo-3-iminoisoindoline. In aqueous acid the amido-N complexes are protonated on the amide oxygen. The 2-cyanobenzamide species rearranges to form the nitrile-bonded linkage isomer in aqueous acid and also in Me(2)SO-d(6), while the succinic amide nitrile complex rearranges more slowly in aqueous acid to form solely the nitrile-bonded linkage isomer. The kinetics of the reaction were k(obsd) = f(k(H)[H(+)]/(K(a) + [H(+)])) where k(H) = 3.4 x 10(-)(4) M(-)(1) s(-)(1) and K(a) = 6.76 x 10(-)(2) M, pK(a) 1.2; pK(a) 1.3 (spectrophotometric) {I = 1.00 M (LiClO(4).3H(2)O), 25 degrees C}. In Me(2)SO-d(6) this amide-N complex reacts by three pathways: solvolysis, amide-N to -O isomerization, and amide-N to nitrile-bonded rearrangement (10%). The conjugate acid of the 2-cyanoacetamido-N complex reacted in both aqueous acid and acidified Me(2)SO-d(6) by solvolysis, amide N to O isomerization, and amide-N to nitrile-bonded rearrangement (17% in each solvent). The fumaric, glutaric, and adipic amide-nitrile complexes bonded through the amide nitrogen react only by solvolysis and amide-N to -O isomerization. Pentaamminecobalt(III) complexes of 2-cyanobenzamidine and succinic, glutaric, and adipic amidine-nitriles bonded through the amidine secondary nitrogen have been prepared. The 2-cyanobenzamidine complex undergoes rapid ligand cyclization to form the corresponding complex of 1,3-diiminoisoindoline bonded through the deprotonated endocyclic nitrogen. In aqueous acid the complex is protonated on one of the exo-imines, and this solvolyzes to form the pentaammineaquacobalt(III) complex and 1,3-diiminoisoindoline {k(H) = 1.7 x 10(-)(3) s(-)(1) (0.5 M HCl, 25 degrees C). Coordinated succinic amidine-nitrile also cyclizes in liquid ammonia to yield the complex of 2,5-diiminopyrrolidine bonded through the deprotonated endocyclic nitrogen. This is stable in aqueous base but solvolyzes rapidly (t(1/2) (s)) in aqueous acid to the aqua complex and succinimide; the latter is formed by hydrolysis of the free 2,5-diiminopyrrolidine. The dinuclear complex &mgr;-decaammine(succinonitrile)dicobalt(III) was synthesized; in aqueous base it forms &mgr;-(succinamido-N)decaamminecobalt(III). The dinuclear dinitrile complex reacts in liquid ammonia to form the corresponding succinic amidine-nitrile species which cyclizes rapidly to form &mgr;-decaammine(2,5-diiminopyrrolidino)cobalt(III) in which the ligand is bonded to cobalt(III) through the exo-imines.     

Global and local electrophilicity patterns of diazonium ions and their reactivity toward pi-nucleophiles

The global and local electrophilicity patterns of a series of 15 diazonium ions have been evaluated using the absolute scale of electrophilicity proposed by Parr et al. (J. Am. Chem. Soc. 1999, 121, 1922). The predicted global electrophilicity pattern of the whole series of diazonium ions correctly compares with the experimental electrophilicity recently determined for these charged electrophiles. The global electrophilicity is then projected into the different potential active sites of the molecular ions using the electrophilic Fukui function. The highest regional electrophilicity power is found at the terminal nitrogen atom of the arenediazonium ions, yet the highest positive charge is located on the nitrogen atom directly attached to the aromatic ring. This result is consistent with the observed reactivity displayed by diazonium ions toward substituted alkenes, thereby suggesting that the formation of the azocarbocation intermediate is mostly orbital rather than charge controlled.     

Übergangsmetall-silyl-komplexe: IV. Bildung eines ring-substituierten wolframocen dihydrid derivats durch reaktion von Cp2WCl2 MIT Li[Si(SiMe3)3]

In the reaction of Cp₂WGl₂ with Li[Si(SiMe₃)₃] the dihydrid tungstenocene derivative [(Me₃Si)₃SiC₅H₄]WH₂ (3) is formed with a 56% yield. 3 crystallizes in space group P$\text{1}$, with a 918.0(4), b 1580.9(4), c 1621.2(7) pm, α 117.63(2), β 89.95(3), γ 94.39(3)° at ?40° C. The dihedral angle between the Cp planes is 140.9°.     

Selex: An expert system for evaluating published data on selenium in foods

Providing consistent and objective evaluation of published data on nutrient composition is critical for planning future analytical studies and for effective use of data. Based on a commercial expert system shell, a computer system of approximately 200 rules has been created to evaluate and ratre quantitatively published data on selenium in foods. The evaluation scheme uses five general categories for its rule-making process: number of samples, analytical method, sample handling, sampling plan, and analytical quality control. For each selenium value to be evaluted, rating ar assigned in each category by the expert system based on input which is derived from the information reportedin a given paper. A quality index, which is derived from the ratings, is a measure of the reliability of a given selenium value over all categories for a given study. The concepts used in developing SELEX have the potential of establishing criteria for evaluation of proposed analytical methods prior to their publication.     

Two new marine sediment standard reference materials (SRMs) for the determination of organic contaminants

Two new marine sediment standard reference materials (SRMs), SRM 1941b Organics in Marine Sediment and SRM 1944 New York/New Jersey Waterway Sediment, have been recently issued by the National Institute of Standards and Technology (NIST) for the determination of organic contaminants including polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyl (PCB) congeners, and chlorinated pesticides. Both sediment SRMs were analyzed using multiple analytical methods including gas chromatography/mass spectrometry (GC/MS) on columns with different selectivity, reversed-phase liquid chromatography with fluorescence detection (for PAHs only), and GC with electron capture detection (for PCBs and pesticides only). SRM 1941b has certified concentrations for 24 PAHs, 29 PCB congeners, and 7 pesticides, and SRM 1944 has certified concentrations for 24 PAHs, 29 PCB congeners, and 4 pesticides. Reference concentrations are also provided for an additional 58 (SRM 1941b) and 39 (SRM 1944) PAHs, PCB congeners, and pesticides. SRM 1944, which was collected from multiple sites within New York/New Jersey coastal waterways, has contaminant concentrations that are generally a factor of 10–20 greater than SRM 1941b, which was collected in the Baltimore (Maryland) harbor. These two SRMs represent the most extensively characterized marine sediment certified reference materials available for the determination of organic contaminants.Electronic Supplementary Material Supplementary material is available in the online version of this article at . A link in the frame on the left on that page takes you directly to the supplementary material.