Formation of supramolecular cavitands on copper electrode surfaces

The adsorption of 1,1'-dibenzyl-4,4'-bipyridinium molecules (dibenzyl-viologen or DBV(2+) for the sake of simplicity) on chloride precovered Cu(100) has been studied in an electrochemical environment by means of cyclic voltammetry and in situ scanning tunneling microscopy. DBV(2+) spontaneously forms a highly ordered phase on the chloride c(2 x 2) adlayer at potentials close to the onset of the copper dissolution reaction when the pure supporting electrolyte (10 mM HCl/5 mM KCl) is exchanged by one also containing DBV(2+). This ordered phase can be described by a ( radical 53 x radical 53)R15.9 degrees unit cell relating the organic adlayer to the chloride c(2 x 2) structure underneath or alternatively by a ( radical 106 x radical 106)R29.05 degrees unit cell relating the organic layer to the Cu(1 x 1) substrate structure. Thus, the negatively charged chloride layer acts as a template for the adsorption and phase formation of DBV(2+). Compared to the copper-chloride interaction, the DBV(2+)-chloride interaction appears to be weaker since the organic layer can be easily removed from the surface by the tunneling tip when drastic tunneling conditions (low bias voltage, high tunneling current) are applied. A key structural element of the DBV(2+) adlayer is an assembly of four individual DBV(2+) molecules forming square-shaped supramolecular units with pronounced cavities in their center. Characteristically, the supramolecular assemblies reveal a preferential rotational orientation resulting in the appearance of two chiral forms of these assemblies. Furthermore, these two chiral supramolecular assemblies occur in two mirrored domains of the ( radical 53 x radical 53)R15.9 degrees structure. It can be assumed that these viologen-based supramolecular architectures can be used as potential host cavitands for the inclusion of smaller organic molecules.     

High-pressure STM of the interaction of oxygen with Ag(111)

To identify surface phases that could play a role for the epoxidation of ethylene on Ag catalysts we have studied the interaction of Ag(111) with O(2) at elevated pressures. Experiments were performed using high-pressure scanning tunneling microscopy (STM) at temperatures between 450 and 480 K and O(2) pressures in the mbar range. Below p(O(2)) approximately 1 mbar the surface largely showed the structure of bare Ag(111). At p(O(2)) above approximately 1 mbar the (4 x 4)O structure and the closely related (4 x 5 radical 3)rect structure were observed. The findings confirm theoretical predictions that the (4 x 4)O structure is thermodynamically stable at the oxygen partial pressure of the industrial ethylene oxide synthesis. However, in other experiments only a rough, disordered structure was observed. The difference is caused by the chemical state of the STM cell that depends on the pretreatment and on previous experiments. The surface was further analyzed by X-ray photoelectron spectroscopy (XPS). Although these measurements were performed after sample transfer to ultra-high vacuum (UHV), so that the surface composition was modified, the two surface states could still be identified by the presence of carbonate or a carbonaceous species, and by the absence or presence of a high-binding energy oxygen species, respectively. It turns out that the (4 x 4)O structure only forms under extremely clean conditions, indicating that the (4 x 4)O phase and similar oxygen-induced reconstructions of the Ag(111) surface are chemically unstable. Chemical reactions at the inner surfaces of the STM cell also complicate the detection of the catalytic formation of ethylene oxide.     

(Zr6B)Cl11-xI2+x] (0< or = x < or = 6): a new mixed-halide structure with zigzag cains of clusters in multiply twinned crystals

The new [(Zr6B)Cl11-xI2+x] phase (with 0 < or = x < or = 6) is obtained from reactions of ZrI4, ZrCl4, and elemental Zr and B for 2-4 weeks in sealed Ta tubing at 800-850 degrees C. Single crystals of [(Zr6B)Cl6.44(7)I6.56] have been characterized by X-ray diffraction at room temperature (orthorhombic Pbcn, Z = 4, a = 12.365(2) A, b = 15.485(3) A, c = 13.405(2) A). This structure contains zigzag chains of boron-centered (Zr6B) octahedra that are interconnected by Cl(i-i) halides. Further three-dimensional connectivity is achieved by I(a-a-a) bridges. The noncluster interconnecting two-bonded X(i) sites are occupied statistically by a mixture of Cl and I. For each site both positions were resolved. This structure forms within a phase width of 0 < or = x < or = 6 at temperatures between 800 and 850 degrees C. Crystals of this phase appear to be always multiply twinned.     

Self-assembled monolayers of benzylmercaptan and p-cyanobenzylmercaptan on Au(111) surfaces: structural and spectroscopic characterization

The formation of self-assembled monolayers of benzylmercaptan (BM) and p-cyanobenzylmercaptan (pCBM) on Au(111) surfaces is investigated by a combination of X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), and scanning tunneling microscopy (STM). The NEXAFS results of pCBM are supported by ab initio calculations. It is found that BM as well as pCBM form well-ordered monolayers with the molecules oriented almost perpendicular to the surface. BM forms a ( radical 3 x radical 3)R30 degrees structure whereas pCBM forms a slightly different c(7 x 7) hexagonal structure. No phase separation is detected for the adsorption of a 1:1 mixture of the two molecules. The implications of the results for the covalent attachment of transition-metal complexes to thiol-functionalized surfaces are discussed.     

Interaction of SO3 with c-ZrO2(111) films on Pt(111)

Single-crystalline sulfated c-ZrO2(111) films of the cubic (c) type have been prepared by reactive deposition of Zr onto Pt(111) in an O2 atmosphere and subsequent exposition to a SO3 atmosphere. The morphology, atomic structure, and composition have been examined by scanning tunneling microscopy, low-energy electron diffraction (LEED), Auger electron spectroscopy, and density functional theory (DFT) calculations. The clean c-ZrO2(111) films display a (2x2) surface structure. During SO3 exposure at room temperature, a clear (radical3xradical3)R30 degrees structure develops. At about 700 K, the SO3-induced (radical3xradical3)R30 degrees structure disappears and the bright (2x2) LEED pattern of the clean ZrO2 films reappears. The energies of plausible c-ZrO2(111)/SO3 structures have been examined by DFT. The (radical3xradical3)R30 degrees structure found in the experiments turned out to be the most stable one for temperatures below 700 K. At temperatures around 700 K, a disordered low coverage structure may exist, which can not be observed by conventional LEED. A comparison of cubic zirconia surfaces with the alternative tetragonal system yields similar results for the SO3 adsorption in the DFT calculations and shows that c-ZrO2 surfaces are good models for the industrial used tetragonal ZrO2 supports.     

The First Mixed-Halide Zirconium Cluster Compounds: Zr(6)Cl(1.6)I(10.4)Be, Zr(6)Cl(1.3)I(10.7)B, and Zr(6)Cl(11.5)I(1.5)B. Matrix Effects and Halogen Substitution in Compact Network Structures

Investigations of the effect of halogen size on structure stability have been conducted in well-reduced and heavily interbridged zirconium chloride-iodide cluster systems. The title compounds are obtained in good yields from reactions of Zr, ZrCl(4), ZrI(4), and B or Be in sealed Ta tubes for approximately 4 weeks at 850 degrees C. Single-crystal diffraction at room temperature established these as Zr(6)Cl(1.65(4))I(10.35(4))Be and Zr(6)Cl(1.27(3))I(10.73(3))B [R&thremacr;, Z = 3, a = 14.3508(8), 14.389(1) ?, c = 9.8777(9), 9.915(2) ?, respectively] and Zr(6)Cl(11.47(2))I(1.53(2))B [P4(2)/mnm, Z = 2, a = 12.030(1) ?, c = 7.4991(8) ?]. These are derivatives of the Zr(6)I(12)C and orthorhombic Zr(6)Cl(13)B structures, respectively, the latter containing unusual linear chains of clusters interbridged by Cl(i-i) that are in turn interconnected by three-bonded Cl(a-a-a) atoms. The random substitution of fractional Cl at specific I sites in the first two, and I for certain Cl in the third, was positionally resolved in all cases. The replacement always occurs at two-bonded X(i), so that single types of halogen are left in sites that interconnect clusters and generate the three-dimensional array. Structural changes seen in both structures are specifically related to relief of X.X crowding in the parent structure (matrix effects). Substitution of Cl for I(i) in the Zr(6)I(12)C type greatly reduces intercluster I.I repulsions and allows, among other things, a 0.20 ? (5.8%) reduction in Zr-I(a-i) intercluster bond lengths. Increased Cl.I repulsions caused by I substitution in orthorhombic Zr(6)Cl(13)B (Pnnm) convert the twisted chains and angular Cl(a-a-a) interchain bridges to planarity in tetragonal Zr(6)Cl(11.5)I(1.5)B. Phase widths found are 0 相似文献   

Bimetallic cyanide-bridged complexes based on the photochromic nitroprusside anion and paramagnetic metal complexes. Syntheses, structures, and physical characterization of the coordination compounds [Ni(en)2]4[Fe(CN)5NO]2[Fe(CN)6]x5H2O, [Ni(en)2][Fe(CN)5NO]x3H2O, [Mn(3-MeOsalen)(H2O)]2[Fe(CN)5NO], and [Mn(5-Brsalen)]2[Fe(CN)5NO

The synthesis, crystal structure, and physical characterization of the coordination compounds [Ni(en)2]4[Fe(CN)5NO]2[Fe(CN)6]x5H2O (1), [Ni(en)2][Fe(CN)5NO]x3H2O (2), [Mn(3-MeOsalen)(H2O)]2[Fe(CN)5NO] (3), and [Mn(5-Brsalen)]2[Fe(CN)5NO] (4) are presented. 1 crystallizes in the monoclinic space group P2(1)/n (a = 7.407(4) A, b = 28.963(6) A, c = 14.744(5) A, alpha = 90 degrees, beta = 103.26(4) degrees, gamma = 90 degrees, Z = 2). Its structure consists of branched linear chains formed by cis-[Ni(en)2]2+ cations and ferrocyanide and nitroprusside anions. The presence of two kinds of iron(II) sites has been demonstrated by M?ssbauer spectroscopy. 2 crystallizes in the monoclinic space group P2(1)/c (a = 11.076(3) A, b = 10.983(2) A, c = 17.018(5) A, alpha = 90 degrees, beta = 107.25(2) degrees, gamma = 90 degrees, Z = 4). Its structure consists of zigzag chains formed by an alternated array of cis-[Ni(en)2]2+ cations and nitroprusside anions. 3 crystallizes in the triclinic space group P1 (a = 8.896(5) A, b = 10.430(5) A, c = 12.699(5) A, alpha = 71.110(5) degrees, beta = 79.990(5) degrees, gamma = 89.470(5) degrees, Z = 1). Its structure comprises neutral trinuclear bimetallic complexes in which a central [Fe(CN)5NO]2- anion is linked to two [Mn(3-MeOsalen)]+ cations. 4 crystallizes in the tetragonal space group P4/ncc (a = 13.630(5) A, c = 21.420(8) A, Z = 4). Its structure shows an extended 2D neutral network formed by cyclic octameric [-Mn-NC-Fe-CN-]4 units. The magnetic properties of these compounds indicate the presence of quasi-isolated paramagnetic Ni2+ and Mn3+. Irradiated samples of the four compounds have been studied by differential scanning calorimetry to detect the existence of the long-lived metastable states of nitroprusside.     

Self-assembled monolayers of alkanethiols on Au(111): surface structures, defects and dynamics

The surface structures, defects and dynamics of self-assembled monolayers (SAMs) on Au(111) are reviewed. In the case of the well-known c(4 x 2) and radical 3 x radical 3 R30 degrees surface structures, the present discussion is centered on the determination of the adsorption sites. A more complex scenario emerges for the striped phases, where a variety of surface structures that depends on surface coverage are described. Recently reported surface structures at non-saturation coverage show the richness of the self-assembly process. The study of surface dynamics sheds light on the relative stability of some of these surface structures. Typical defects at the alkanethiol monolayer are shown and discussed in relation to SAMs applications.     

A novel open framework uranyl molybdate: synthesis and structure of (NH4)4[(UO2)5(MoO4)7](H2O)5

Single crystals of (NH(4))(4)[(UO(2))(5)(MoO(4))(7)](H(2)O)(5) have been synthesized hydrothermally using (NH(4))(6)Mo(7)O(24), (UO(2))(CH(3)COO)(2).2H(2)O, and H(2)O at 180 degrees C. The phase has been characterized by single-crystal X-ray diffraction using a merohedrally twinned single crystal: it is hexagonal, P6(1), a = 11.4067(5) A, c = 70.659(5) A, V = 7961.9(7) A(3), and Z = 6. The structure is based upon an open framework with composition [(UO(2))(5)(MoO(4))(7)](4-) that is composed of UO(7) pentagonal bipyramids that share vertexes with MoO(4) tetrahedra. The framework has large channels (effective pore size: 4.8 x 4.8 A(2)) parallel to the c axis and a system of smaller channels (effective pore size: 2.5 x 3.6 A(2)) parallel to [100], [110], [010], [110], [110], and [110]. The channels are occupied by NH(4)(+) cations and H(2)O molecules. The topological structure of the uranyl molybdate framework can be described either in terms of fundamental chains of UO(7) pentagonal bipyramids and MoO(4) tetrahedra or in terms of tubular building units parallel to the c axis.     

Rectangular (3 x 2 square root of 3) superlattice of a dodecanethiol self-assembled monolayer on Au(111) observed by ultra-high-vacuum scanning tunneling microscopy

A rectangular (3 x 2 radical3) surface lattice for long-term-annealed dodecanethiol self-assembled monolayers on Au(111) is observed by ultra-high-vacuum scanning tunneling microscopy. The new lattice has the same density and a unit cell of the same size as the well-known c(4 x 2) reconstruction. In contrast, it does not show hexagonal symmetry but rather a sort of thiol pairing, leading to a shift in the binding position of every second molecule. The described structure is believed to be an intermediate phase close to desorption.     

Scanning tunneling microscopy investigation of the structure of methanethiolate on Ag(111)

Scanning tunneling microscopy (STM) has been used to investigate the structure of the ordered methanethiolate overlayer formed on Ag(111) by reaction at room temperature with dimethyl disulfide. High-resolution images show an ordered structure with three inequivalent atomic-scale protrusions within each ( radical7 x radical7)R19 degrees surface unit mesh which can be reconciled with methanethiolate species on a regular lateral submesh, similar to that proposed in the multilayer ( radical7 x radical7)R19 degrees -S sulfide phase previously reported. STM imaging during dynamic dosing also provides evidence for a significant change in the outermost layer Ag atom density, consistent with a reconstructed surface model. Possible models for this reconstruction are presented and discussed in the light of available information.     

The growth of perylene on Ru(0001)

The structure of perylene adsorbed on Ru(0001) surface has been studied by ultraviolet photoemission spectroscopy (UPS) and low-energy electron diffraction. An ordered p(4x4) structure is observed from a monolayer (about 4 A thickness) of the perylene on Ru(0001) surface. UPS measurements show the molecular features, from the perylene multiplayer, between 2 and 10 eV below the Fermi level. Angle-resolved ultraviolet photoemission spectroscopy measurements suggest that the perylene molecular plane is parallel to the substrate. Temperature dependent UPS measurements show that the perylene multilayer is stable on Ru(0001) surface up to 125 degrees C. The desorption of the multilayer and the decomposition of the monolayer are observed above 125 degrees C.     

Hydrothermal syntheses, crystal structures, and magnetic properties of inorganic-organic hybrid vanadium selenites with zero- to three-dimensional structures: (1,10-phenanthroline)(2)V(2)SeO(7), (2,2'-bipyridine)VSeO(4), (4,4'-bipyridine)V(2)Se(2)O(8), and (4,4'-bipyridine)(2)V(4)Se(3)O(15).H(2)O

A family of inorganic-organic hybrid vanadium selenites with zero-, one-, two-, and three-dimensional structures, (1,10-phen)(2)V(2)SeO(7), (2,2'-bipy)VSeO(4), (4,4'-bipy)V(2)Se(2)O(8), and (4,4'-bipy)(2)V(4)Se(3)O(15).H(2)O (where phen = phenanthroline and bipy = bipyridine), were hydrothermally synthesized and characterized by single-crystal X-ray diffraction. Different bidentate organodiamine ligands and reactant concentrations were used in the four reaction systems, which are responsible for the variety of structural dimensions of the compounds. (1,10-phen)(2)V(2)SeO(7) crystallizes in a monoclinic system with space group P2(1)/n and cell parameters a = 8.6509(3) A,( )b = 7.8379(2) A, c = 34.0998(13) A, beta = 91.503(2) degrees, and Z = 4. (2,2'-bipy)VSeO(4) crystallizes in a monoclinic system with space group C2/c and cell parameters a = 17.0895(12) A, b = 14.7707(10) A, c = 11.7657(8) A, beta = 131.354(3) degrees, and Z = 8. (4,4'-bipy)V(2)Se(2)O(8) crystallizes in a triclinic system with space group Ponemacr; and cell parameters a = 7.1810(10) A, b = 10.8937(13) A, c = 11.1811(15) A, alpha = 115.455(3) degrees, beta = 107.582(3) degrees, gamma = 91.957(4) degrees, and Z = 2. (4,4'-bipy)(2)V(4)Se(3)O(15).H(2)O crystallizes in a monoclinic system with space group Pc and cell parameters a = 7.9889(9) A, b = 7.8448 A, c = 23.048(3) A, beta = 99.389(4) degrees, and Z = 2. (1,10-phen)(2)V(2)SeO(7) has an isolated structure, (2,2'-bipy)VSeO(4) has a chain structure, (4,4'-bipy)V(2)Se(2)O(8) has a layered structure, and (4,4'-bipy)(2)V(4)Se(3)O(15).H(2)O has a framework structure. The chains are constructed from VO(4)N(2) octahedra and SeO(3) pyramids, laced by organic ligands (2,2'-bipy). The layers consist of vanadium selenite chains [(VO)(2)(SeO(3))(2)]( infinity ), linked by 4,4'-bipy molecules. The framework is composed of vanadium selenite sheets [V(4)Se(3)O(16)]( infinity ), pillared by 4,4'-bipy molecules. All of the compounds are thermally stable to 300 degrees C, and the magnetic susceptibilities confirm the existence of tetravalent V atoms in the antiferromagnetic (4,4'-bipy)V(2)Se(2)O(8) complex and mixed tetravalent and pentavalent V atoms in the paramagnetic complex (4,4'-bipy)(2)V(4)Se(3)O(15).H(2)O.     

Two organically templated niobium and zinconiobium fluorophosphates: low temperature hydrothermal syntheses of NbOF(PO4)2(C2H10N2)2 and Zn3(NbOF)(PO4)4(C2H10N2)2

Two new niobium and zinconiobium fluorophosphates, NbOF(PO4)2(C2H10N2)2 (1) and Zn3(NbOF)(PO4)4-(C2H10N2)2 (2), have been prepared under hydrothermal conditions using ethylenediamine as a template. The structures were determined by single crystal diffraction to be triclinic, space group P1 (No. 2), a = 8.1075 (6) A, b = 9.8961 (7) A, c = 10.1420(8) A, alpha = 111.655(1) degrees, beta = 111.51(1) degrees, gamma = 93.206(1) degrees, V = 686.19(9) A3, and Z = 2 for 1 and orthorhombic, space group Fddd (No. 70), a = 9.1928(2) A, b = 14.2090(10) A, c = 32.2971 (6) A, V = 4218.66(12) A3, and Z = 8 for 2, respectively. Compound 1 is an infinite linear chain consisting of corner-sharing [Nb2P2] 4-MRs bridged at the Nb centers with organic amines situated between chains, and compound 2, containing the chains similar to that in 1, forms a zeotype framework with organic amines situated in the gismondine-type [4684] cavities. The topology of 2 was previously unknown with vertex symbol 4 x 4 x 4 x 4 x 8 x 8 (vertex 1), 4 x 4 x 4 x 82 x 8 x 8 (vertex 2), 4 x 4 x 8 x 8 x 82 x 82 (vertex 3), and 4 x 4 x 4 x 82 x 8 x 8 (vertex 4). The topological relationships between the 4-connected network of 2 and several reported (3,4)-connected networks were discussed.     

Tuning intermolecular magnetic exchange interactions in the solids C(x)F(2)(x)(CNSSS)(2)(AsF(6))(2): structural, EPR, and magnetic characterization of dimeric (x = 2, 4) diradicals

A series of diradical containing salts CxF2x(CNSSS)2(**2+0(AsF6-)2 {x = 2, 1[AsF6]2; x = 3, 3[AsF6]2; x = 4, 2[AsF6]2} have been prepared. 1[AsF6]2 and 2[AsF6]2 were fully characterized by X-ray, variable-temperature magnetic susceptibility, and solid-state EPR measurements, further allowing us to extend the number of examples of the family of rare 7pi RCNSSS(*+) radical cations. 1[AsF6]2: a = 6.5314(7) A, b = 7.5658(9) A, c = 9.6048(11) A, alpha = 100.962(2) degrees , beta = 96.885(2) degrees , gamma = 107.436(2) degrees , triclinic, space group P, Z = 1, T = 173 K. 2[AsF6]2: a = 10.6398(16) A, b = 7.9680(11) A, c = 12.7468(19) A, beta = 99.758(2) degrees , monoclinic, space group P21/c, Z = 2, T = 173 K. In the solid-state, CxF2x(CNSSS)2(**2+) (x = 2, 4) formed one-dimensional polymeric chains of dications containing discrete centrosymmetric radical pairs in which radicals were linked by four centered two-electron pi*-pi* bonds [12+, d(S...S) = 3.455(1) A; 22+, d(S...S) = 3.306(2) A]. The exchange interactions in these bonds were determined to be -500 +/- 30 and -900 +/- 90 cm-1, by variable temperature magnetic susceptibility measurements, respectively, providing rare experimental data on the singlet-triplet gaps in the field of thiazyl radicals. For 2[AsF6]2, the thermally excited triplet state was unambiguously characterized by EPR techniques [/D/ = 0.0254(8) cm(-1), /E/ = 0.0013(8) cm(-1)]. These experimental data implied a weakly associated nature of the radical moieties contained in the solids 1[AsF6]2 and 2[AsF6]2. Computational analysis of the dimerization process is presented, and we show that the 2c 4 electron pi*-pi* bonds in 1[AsF6]2 and 2[AsF6]2 have ca. 50% and 40% diradical character, respectively. In contrast, 3[AsF6]2.SO2, containing diradical C3F6(CNSSS)2(**2+) with an odd number of CF2 spacers, showed magnetic behavior that was consistent with the presence of monomeric radical centers in the solid state.     

The structures of Langmuir-Blodgett films of fatty acids and their salts

Recent advances in several experimental techniques have enabled detailed structural information to be obtained for floating (Langmuir) monolayers and Langmuir-Blodgett films. These techniques are described briefly and their application to the study of films of fatty acids and their salts is discussed. Floating monolayers on aqueous subphases have been shown to possess a complex polymorphism with phases whose structures may be compared to those of smectic mesophases. However, only those phases that exist at high surface pressures are normally used in Langmuir-Blodgett (LB) deposition. In single LB monolayers of fatty acids and fatty acid salts the acyl chains are in the all-trans conformation with their long axes normal to the substrate. The in-plane molecular packing is hexagonal with long-range bond orientational order and short-range positional order: known as the hexatic-B structure. This structure is found irrespective of the phase of the parent floating monolayer. The structures of multilayer LB films are similar to the structures of their bulk crystals, consisting of stacked bilayer lamellae. Each lamella is formed from two monolayers of fatty acid molecules or ions arranged head to head and held together by hydrogen bonding between pairs of acids or ionic bonding through the divalent cations. With acids the acyl chains are tilted with respect to the substrate normal and have a monoclinic structure, whereas the salts with divalent cations may have the chains normal to the substrate or tilted. The in-plane structures are usually centred rectangular with the chains in the trans conformation and packed in a herringbone pattern. Multilayer films of the acids show only a single-step order-disorder transition at the melting point. This temperature tends to rise as the number of layers increases. Complex changes occur when multilayer films of the salts are heated. Disorder of the chains begins at low temperatures but the arrangement of the head groups does not alter until the melting temperature is reached. Slow heating to a temperature just below the melting temperature gives, with some salts, a radical change in phase. The lamellar structure disappears and a new phase consisting of cylindrical rods lying parallel to the substrate surface and stacked in a hexagonal pattern is formed. In each rod the cations are aligned along the central axis surrounded by the disordered acyl chains.     

Surface reorganization accompanying the formation of sulfite and sulfate by reaction of sulfur dioxide with oxygen on Ag111

On the Ag(111)-p(4x4)-O surface SO2(g) reacts with oxygen according to SO2(g)+O(a)-->SO3(a). Sulfite forms in a (2 radical3x2 radical3)R30 degrees structure. The restructuring of the surface atoms during sulfite formation is indicative of the deconstruction of the p(4x4)-O structure. Heating the sulfite-covered surface to 700 K affects the disproportionation of SO3 to SO4 in a (4 square root of 3 x square root of 3)R30 degrees structure accompanied by the desorption of SO2(g) and smoothing of the surface. Continued heating beyond 700 K affects the complete decomposition of sulfate to SO2(g) and O2(g).     

Surface stress, kinetics, and structure of alkanethiol self-assembled monolayers

The surface stress induced during the formation of alkanethiol self-assembled monolayers (SAMs) on gold from the vapor phase was measured using a micromechanical cantilever-based chemical sensor. Simultaneous in situ thickness measurements were carried out using ellipsometry. Ex situ scanning tunneling microscopy was performed in air to ascertain the final monolayer structure. The evolution of the surface stress induced during coverage-dependent structural phase transitions reveals features not apparent in average ellipsometric thickness measurements. These results show that both the kinetics of SAM formation and the resulting SAM structure are strongly influenced both by the surface structure of the underlying gold substrate and by the impingement rate of the alkanethiol onto the gold surface. In particular, the adsorption onto gold surfaces having large, flat grains produces high-quality self-assembled monolayers. An induced compressive surface stress of 15.9 +/- 0.6 N/m results when a c(4x2) dodecanethiol SAM forms on gold. However, the SAMs formed on small-grained gold are incomplete and an induced surface stress of only 0.51 +/- 0.02 N/m results. The progression to a fully formed SAM whose alkyl chains adopt a vertical (standing-up) orientation is clearly inhibited in the case of a small-grained gold substrate and is promoted in the case of a large-grained gold substrate.     

Sum frequency generation vibrational spectroscopic and high-pressure scanning tunneling microscopic studies of benzene hydrogenation on Pt(111)

Sum frequency generation (SFG) vibrational spectroscopy and high-pressure scanning tunneling microscopy (HP-STM) have been used in combination for the first time to study a catalytic reaction. These techniques have been able to identify surface intermediates in situ during benzene hydrogenation on a Pt(111) single-crystal surface at Torr pressures. In a background of 10 Torr of benzene, STM is able to image small ordered regions corresponding to the c(2 radical3 x 3)rect structure in which each molecule is chemisorbed at a bridge site. In addition, individual benzene molecules are also observed between the ordered regions. These individual molecules are assumed to be physisorbed benzene on the basis of the SFG results showing both chemisorbed and physisorbed molecules. The surface becomes too mobile to image upon addition of hydrogen but is determined to have physisorbed and chemisorbed benzene present by SFG. It was spectroscopically determined that heating the platinum surface after poisoning with CO displaces benzene molecules. The high-coverage pure CO structure of (radical19 x radical19)R23.4 degrees imaged with STM is a verification of spectroscopic measurements.