Adsorption, desorption, and clustering of H2O on Pt111

The adsorption, desorption, and clustering behavior of H2O on Pt111 has been investigated by specular He scattering. The data show that water adsorbed on a clean Pt111 surface undergoes a structural transition from a random distribution to clustered islands near 60 K. The initial helium scattering cross sections as a function of temperature are found to be insensitive to the incident H2O flux over a range of 0.005 monolayers (ML)/s-0.55 ML/s indicating that the clustering process is more complex than simple surface diffusion. The coarsening process of an initially random distribution of water deposited at 25 K is found to occur over a broad temperature range, 60相似文献   

Additions of enantiopure alpha-sulfinyl carbanions to (S)-N-sulfinimines: asymmetric synthesis of beta-amino sulfoxides and beta-alphamino alcohols

The addition of the lithium anions derived from (R)- and (S)-methyl and -ethyl p-tolyl sulfoxides to (S)-N-benzylidene-p-toluenesulfinamide provides an easy access route to enantiomerically pure beta-(N-sulfinyl)amino sulfoxides. Stereoselectivity can be achieved when the configurations at the sulfur atoms of the two reagents are opposite (matched pair), thus resulting in only one diastereoisomer, even for the case in which two new chiral centers are created. The N-sulfinyl group primarily controls the configuration of the carbon bonded to the nitrogen, whereas the configuration of the alpha-sulfinyl carbanion seems to be responsible for the level of asymmetric induction, as well as for the configuration of the new stereogenic C-SO carbon in the reactions with ethyl p-tolyl sulfoxides. An efficient method for transforming the obtained beta-(N-sulfinyl)amino sulfoxides into optically pure beta-amino alcohols, based on the stereoselective non-oxidative Pummerer reaction, is also reported.     

Water adsorption, desorption, and clustering on FeO(111)

The adsorption of water on FeO(111) is investigated using temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRAS). Well-ordered 2 ML thick FeO(111) films are grown epitaxially on a Pt(111) substrate. Water adsorbs molecularly on FeO(111) and desorbs with a well resolved monolayer peak. IRAS measurements as a function of coverage are performed for water deposited at 30 and 135 K. For all coverages (0.2 ML and greater), the adsorbed water exhibits significant hydrogen bonding. Differences in IRAS spectra for water adsorbed at 30 and 135 K are subtle but suggest that water adsorbed at 135 K is well ordered. Monolayer nitrogen TPD spectra from water covered FeO(111) surfaces are used to investigate the clustering of the water as a function of deposition or annealing temperature. Temperature dependent water overlayer structures result from differences in water diffusion rates on bare FeO(111) and on water adsorbed on FeO(111). Features in the nitrogen TPD spectra allow the monolayer wetting and 2-dimensional (2D) ordering of water on FeO(111) to be followed. Voids in a partially disordered first water layer exist for water deposited below 120 K and ordered 2D islands are found when depositing water above 120 K.     

(Z)-3-p-tolylsulfinylacrylonitrile as a chiral dienophile: diels-alder reactions with furan and acyclic dienes

The behavior of (Z)-3-p-tolylsulfinylacrylonitrile (1) as a chiral dienophile has been evaluated from its reactions with furan and acyclic dienes. Electrostatic interactions of the cyano group with the sulfinyl one restrict the conformational mobility around the C-S bond, thus controlling the pi-facial selectivity, which is almost complete in all cases, the approach of the diene from the less-hindered face of the dienophile (that bearing the lone electron pair) in the predominant rotamer being the favored one. The regioselectivity is also completely controlled by the cyano group. Additionally, the reactivity of compound 1 as well as its endo-selectivity are both higher than those observed for the corresponding (Z)-3-sulfinylacrylates, thus proving the potential of sulfinylnitriles as chiral dienophiles.     

Spin-singlet clusters in the ladder compound NaV2O5

The space group of alpha(')-NaV2O5 turns below T(c) = 34 K from Pmmn with all V sites equivalent, into Fmm2 with three independent vanadium sites per layer. This is incompatible with models of charge ordering into V4+ and V5+. Our structure determination indicates that the phase transition consists of a charge ordering with three distinct valence states, formally V4+, V4.5+, and V5+. The singlet formation is not associated with dimerization on the spin ladder, but with the formation of spin clusters. Finally, we ascribe the quadrupling of the c axis to the large polarizability of the V2O5 skeleton.     

Dependence of pulser driving responses on electrical and motional characteristics of NDE ultrasonic probes

Acoustic performance in ultrasonic transmitters can be improved by means of a suitable electrical driving response and matching/tuning networks. It is important to predict this electrical response, but doing so is not easy because it departs notably from the nominal pattern with the loading probes. In practice, the analysis of HV pulser spikes in NDE applications requires fairly complex models in the transient regime and, in addition, non-linear problems could arise, especially in the case of tuned transmitters. In this paper, the most relevant influences of loading characteristics of NDT ultrasonic probes on the pulser electrical driving responses are evaluated in time and frequency domains. Conventional pulse generators and typical NDE pulsers are considered. Driving responses are analysed across commercial ultrasonic probes and, alternatively, across similar purely electrical loads. Distinct influences on pulser responses from electrical and motional sections of the probes are identified. All these aspects are studied on the basis of experimental and computer results.     

Aggregation behavior and dynamics of synthetic amphiphiles that self-assemble to anion transporters

The amphiphilic heptapeptides-referred to as synthetic anion transporters (SATs)-mediate chloride transport in planar lipid bilayer membranes, synthetic liposomes, and mammalian cells. The SATs described have the general formula R1(2)NCOCH2OCH2CO-(Gly)3-Pro-(Gly)3-OR2. Substitution at R1 and R2 with various aliphatic or aromatic groups alters the ability of SATs to transport chloride through a phospholipid bilayer membrane. Despite extensive structure-activity relationship studies concerning Cl(-)-mediated transport by SATs, relatively little was known about the mechanism of insertion and pore-formation in the membrane. In the current study, the mechanistic behavior of SATs was investigated in aqueous solution and at the air-water interface. In the latter case, Langmuir trough studies and Brewster angle microscopy (BAM) revealed the extent of monolayer stability and organization for SATs. Dynamic light scattering and transmission electron microscopy (TEM) confirmed these results and defined the aggregation behavior of SATs in solution. SAT derivatives that showed low chloride transport activity organized into stable monolayers at the air-water interface, while more active SATs formed less stable monolayers. The relationship between intermolecular organization of SATs and pore-formation in the membrane is discussed along with its implications for chloride transport.     

Aggregation and supramolecular membrane interactions that influence anion transport in tryptophan-containing synthetic peptides

Self-assembly is a desired property in supramolecular chemistry, but extensive aggregation may be counterproductive. Rigid systems typically have better organization, but are inherently less dynamic. This work shows that ion transport by amphiphilic heptapeptides (synthetic anion transporters or SATs) is affected by aggregation of the monomers in the bulk aqueous phase to which they are added and within the bilayer. Ion transport was assessed for all compounds by assay of Cl(-) release from liposomes. The mechanism of ion transport was confirmed by planar bilayer conductance studies for two compounds at opposite ends of the efficacy scale. Dynamic light scattering, the Langmuir trough, transmission electron microscopy, ion release from liposomes, and planar bilayer conductance studies were used to assess the importance of self-assembly versus aggregation in ion transport. Generally, greater aggregation was has an adverse effect on the transport, although at least dimerization is required for amphiphilic heptapeptides to readily transport Cl(-). Anion transport in these systems was found to be sensitive to changes in the C-terminal portion of the (Gly)(3)Pro(Gly)(3) sequence. Moreover, a significant difference in transport efficacy was apparent when L-Trp was replaced by D-Trp in the same position.