Mapping the interaction energy surfaces of cyclic alkanes: evaluating the transferability of an ab initio based potential model

Detailed interaction energy maps are computed for symmetric cyclopropane and tetrahedrane dimer systems using ab initio methods. Interaction energies of cubane and cyclohexane dimers are also reported. The global minimum energy structures of cyclopropane and tetrahedrane systems are both D(3d) structures with energies of -1.850 and -2.171 kcal mol(-1). The ability of NIPE potential model, based on ab initio nonbonding data of neopentane (N), isobutane (I), propane (P), ethane (E) and all their combinations to predict the pair interaction energies of these strained cyclic hydrocarbons is also investigated. The difference between the energies predicted by NIPE and those obtained from the ab initio calculations increases with ring strain In general, NIPE values are in close agreement with the ab initio results for alkane ring structures having low ring strain.     

Icosahedral and ring-shaped allotropes of arsenic.

We predict the existence of two novel families of arsenic nanostructures: icosahedral cages and ring-shaped chains. Quantum chemical calculations on the cages, rings, and the experimentally known allotropes of arsenic suggest the nanostructures to be thermodynamically stable. The icosahedral cages are modifications of the gray allotrope of arsenic, while the ring-shaped chains are structurally related to the red allotrope of phosphorus. Comparisons between the analogous allotropes of arsenic and phosphorus show distinct differences. While phosphorus favors the ring-shaped chains over the icosahedral cages, large cages become favorable for arsenic. From the thermodynamical point of view, experimental preparation of the proposed families of arsenic nanostructures is expected to be viable.     

Influence of the ligand structure of hafnocene polymerization catalysts: A theoretical study on chain termination reactions in ethene polymerization

Ligand effects on chain termination reactions in hafnocene-catalyzed ethene polymerization process have been systematically studied by quantum chemical methods. β-hydrogen transfer to metal, β-hydrogen transfer to monomer and hydrogenolysis were studied for 27 hafnocenes, initiating the chain termination reactions after insertion of the second ethene monomer. The results of the calculations were studied as a function of the ligand structure, focusing on the effects of various ancillary ligands, ligand substituents and bridging units. The ligand effects on chain termination reactions are strongly affected by combined effects of various structural units, in particular, in the cases of β-hydrogen transfer to monomer and hydrogenolysis. The results are expected to aid in design and development of new hafnocene polymerization catalysts.     

Modification of surface properties of polyethylene by perfluoropolyether blending

The preparation of a stabile blend from thermoplastic polymer and lubricating additive was studied with high density polyethylene (HDPE) and perfluoropolyether (PFPE). PFPE was melt blended within HDPE by injection molding. The chemical composition of the mixtures, the relative amount of PFPE on the surface, and the nature of the surface were studied by three surface sensitive methods: attenuated total reflectance infrared (ATR‐IR) spectroscopy, secondary ion mass spectroscopy (SIMS), and contact angle (CA) measurement. All the blends exhibited improved hydrophobicity. CA and SIMS gave a maximum response when about 2.0 wt % PFPE was added, whereas ATR‐IR spectroscopy gave maximum response for an addition of about 3.0 wt %. No changes in surface properties were observed when samples were reanalyzed about 1–4 months after preparation. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2252–2258, 2005     

Global forecast model to predict the daily dose of the solar erythemally effective UV radiation

A worldwide forecast of the erythemally effective ultraviolet (UV) radiation is presented. The forecast was established to inform the public about the expected amount of erythemally effective UV radiation for the next day. Besides the irradiance, the daily dose is forecasted to enable people to choose the appropriate sun protection tools. Following the UV Index as the measure of global erythemally effective irradiance, the daily dose is expressed in units of UV Index hours. In this study, we have validated the model and the forecast against measurements from broadband UV radiometers of the Robertson-Berger type. The measurements were made at four continents ranging from the northern polar circle (67.4 degrees N) to the Antarctic coast (61.1 degrees S). As additional quality criteria the frequency of underestimation was taken into account because the forecast is a tool of radiation protection and made to avoid overexposure. A value closer than one minimal erythemal dose for the most sensitive skin type 1 to the observed value was counted as hit and greater deviations as underestimation or overestimation. The Austrian forecast model underestimates the daily dose in 3.7% of all cases, whereas 1.7% results from the model and 2.0% from the assumed total ozone content. The hit rate could be found in the order of 40%.     

Ab initio studies on nanoscale friction between graphite layers: effect of model size and level of theory

Ab initio methods were used to investigate the nanoscale friction between two graphite layers placed in contact. The interaction energies were calculated for four two-layer models in series, C(6(n+1))2H(6n+1))-C(6)(n)2H(6)(n) with n = 1, 2, 3, and 4, and additionally for C(54)H(18)-C(6)H(6) and C(150)H(30)-C(6)H(6). The study was done with the Hartree-Fock method using basis sets 3-21G and 6-31G and with the second-order M?ller-Plesset theory using basis set 6-31G. A density functional method (B3PW91) was also tested for reference purposes. The main interest was how the model size and level of theory affect the nanoscale friction coefficient. Most of the calculated friction coefficients fell within the range of values of 0.07-0.14.     

Metal-metal bonds between group 12 metals and tin: structural characterization of the complete series of Sn-M-Sn (M=Zn,Cd, Hg) heterodimetallic complexes

Reaction of the lithium triamidostannate [MeSi[SiMe(2)N(p-Tol)](3)SnLi(OEt(2))] (1) with 0.5 molar equivalents of MCl(2) (M=Zn, Cd, Hg) in toluene afforded the corresponding heterodimetallic complexes [MeSi[SiMe(2)N(p-Tol)](3)Sn](2)M [M=Hg (2), Cd (3), and Zn (4)]. The molecular structures of the mercury and cadmium complexes were determined by X-ray diffraction and found to adopt a linear Sn-M-Sn metal-metal bonded array (d(Sn-Hg) 2.6495(2), d(Sn-Cd) 2.6758(1) A), these being the first Hg-Sn and Cd-Sn bonds to be characterized by X-ray diffraction. That the Hg-Sn bonds are shorter than the Cd-Sn bonds in the isomorphous complexes is attributed to relativistic effects in the mercury system. In contrast, the structure of the Zn analogue is unsymmetrical with one stannate unit being Sn-Zn bonded (d(Sn(1)-Zn) 2.5782(4) A), while the Zn(II) atom bridges two amido functions of the second stannate cage, thus representing a second isomeric form of these complexes. The different degree of metal-metal bond polarity is reflected in the (119)Sn NMR chemical shifts of the three complexes. Variable-temperature NMR studies and a series of (1)H ROESY experiments of the cadmium complex 3 in solution revealed a dynamic exchange between the two isomers.     

ON NON-DETERMINED EHRENFEUCHT-FRAÏSSÉ GAMES AND UNSTABLE THEORIES

In this paper we prove under some set theoretical assumptions that if T is a countable unstable theory then there is a pair of models of T such that Ehrenfeucht-Fraïssé games between these models of large variety of lengths are non-determined.     

Ab initio study on adsorption of hydrated Na+ and Cu+ cations on the Cu(111) surface

The interactions of Na+ and Cu+ cations with a Cu(111) surface in the presence and absence of water molecules were investigated using cluster models and ab initio methods. Adsorption in aqueous solution was modeled with one to five water molecules around the adsorbing cation. The Cu surface was described with Cu10 and Cu18 cluster models and the computational method was MP2/RECP/6-31+G. The effect of the basis set superposition error (BSSE) was taken into account with counterpoise (CP) correction, and the accuracy of HF-level results was examined. The interactions between Na+ and the Cu surface were found to be primarily electrostatic, and the energy differences among the different adsorption sites were small. The largest CP-corrected MP2 adsorption energy for the Cu18 cluster was -188 kJ/mol. When water molecules were added around it, Na+ receded from the Cu surface and finally was surrounded totally by the water molecules. The interactions between Cu+ and the Cu surface were dominated by orbital interactions, and Cu+ preferred to adsorb on sites where it could bind to more than one surface atom. The largest CP-corrected MP2 adsorption energy for the Cu18 cluster was -447 kJ/mol. Adding water molecules around it did not cause Cu+ to draw away from the surface, but instead the water molecules began to form hydrogen bonds with one another. The magnitude of BSSE was substantial in most cases. CP corrections did not, however, have a significant impact on the relative trends among the interaction energies.