Molecule-metal potentials, molecular

As briefly stated in the introduction, we may consider one-dimensional cross sections through the zero-order potential energy surfaces for the two spin states, cf. Fig. 9, in order to illustrate the spin interconversion process and the accompanying modification of molecular structure. The potential energy of the complex in the particular spin state is thus plotted as a function of the vibrational coordinate that is most active in the process, i.e., the metal-ligand bond distance, R. These potential curves may be taken to represent a suitable cross section of the metal 3N-6 dimensional potential energy hypersurface of the molecule. Each potential curve has a minimum corresponding to the stable... 

The metal-oxo molecular models outlined above have a quite remarkable potential for studying the metal activity in a quite unusual environment. Some of the possibilities could be (1) the generation and the chemistry of M—C, M=C, M=C functionalities (2) the interaction with alkenes, alkynes, hydrocarbons, and hydrogen (3) the activation of small molecules like N26 and CO (4) the support of metal-metal bonded functionalities and (5) the generation of highly reactive low-valent metals. 

Similarly, bioemulsifiers, such as emulsan produced by Acinetobacter calcoaceticus, have been shown to aid in removal of metals. Potential for remediation of soils using bacterial exopolymers is indicated by a study which showed that purified exopolymers from 13 bacterial isolates removed cadmium and lead from an aquifer sand with efficiencies ranging from 12 to 91% (Chen et al., 1995). Although such molecules have much larger molecular weights ( 106) than biosurfactants, this study showed that sorption by the aquifer sand was low, suggesting that in a porous medium with a sufficiently. large mean pore size, use of exopolymers may be feasible. 

Since mesoporous materials contain pores from 2 nm upwards, these materials are not restricted to the catalysis of small molecules only, as is the case for zeolites. Therefore, mesoporous materials have great potential in catalytic/separation technology applications in the fine chemical and pharmaceutical industries. The first mesoporous materials were pure silicates and aluminosilicates. More recently, the addition of key metallic or molecular species into or onto the siliceous mesoporous framework, and the synthesis of various other mesoporous transition metal oxide materials, has extended their applications to very diverse areas of technology. Potential uses for mesoporous smart materials in sensors, solar cells, nanoelectrodes, optical devices, batteries, fuel cells and electrochromic devices, amongst other applications, have been suggested in the literature.11 51... 

Fig. 17.6 Different scenarios for the way the potential bias is distributed (dotted hue) along a molecular conductor, (a) An unbiased junction (b) the potential drops linearly along the molecular bridge (c) the potential drops only at the molecule-metal contacts.

The importance of the electrostatic potential profile on the molecular bridge in determining the conduction properties of a metal-molecule-metal junction was recently discussed by Tian et al [72] in conjunction with the current-voltage characteristics of a junction comprised of an STM tip, a gold substrate and a molecule with two bonding sites (e.g. a, a xylyl dithiol) connecting the two. 

Molecular self-assembly, the spontaneous formation of molecules into covalently bonded, well-defined, stable structures, is a very important concept in biological systems and has increasingly become the focus of synthetic sophisticated research [56]. In 1946, Zisman published the preparation of a monomolecular layer by adsorption of a surfactant onto a clean metal surface [57]. However, the potential of self-assembly was not recognized at that time, and the techniques for surface analysis were limited, so the development of self-assanbly was slow. In 1980, Sagiv reported SAMs with the adsorption of octadecyltrichlorosilane (OTS-CigHjTSiClj) on a silicon substrate [58]. In 1983, Nuzzo and Allara [59] successfully prepared SAMs of alkanethiolates on gold. As a potential molecular-level lubricant, SAMs have attracted much attention [60-62] and have been demonstrated to be capable of effectively reducing the friction and adhesion in MEMS [3,61,63,64]. 

To our knowledge, the only investigation into metal/SAM interactions for OPEs was carried out by Haynie et al. [6]. They studied the interactions between PPB-SH assembled on Au(l 11) and a series of vapor-deposited metals Au, Ti, Fe, Cr, Cu, Ag and Al, via TOF-SIMS. No evidence of any chemical interactions or penetration was found in the case of Au, Fe, Cr, Ag and Al. Tliis was attributed to the high packing density of these monolayers and the consequent steric hindrance toward penetration. In the case of Ti, the vapor-deposition destroyed the molecular array this is important, since, in many molecular electronics experiments, Ti is often us as a "bonding" metal for the formation of molecule-metal connections [2, 23]. In the case of Cu, they found several peaks in the mass spectrum, indicating a potential reaction between the metal and the benzene rings. 

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