James V. Coe

Fundamental properties of bulk water from cluster ion data

Several collaborative efforts involving the Coe group are described that connect the properties of aqueous clusters to bulk. Success in connecting cluster properties to bulk has led to new insights and reassessed properties of bulk water including, firstly, the determination of the absolute bulk hydration enthalpy and free energy of the proton using experimental clustering data on A

Using cluster studies to approach the electronic structure of bulk water: Reassessing the vacuum level, conduction band edge, and band gap of water

Aqueous cluster studies have lead to a reassessment of the electronic properties of bulk water, such as band gap, conduction band edge, and vacuum level. Using results from experimental hydrated electron cluster studies, the location of the conduction band edge relative to the vacuum level (often called the V0 value) in water has been determined to be −0.12 eV⩽V0⩽0.0 eV, which is an order of magnitude smaller than most experimental values in the literature. With V0=−0.12 eV and making use of the calculated solvation energy of OH in water, the band gap of water is determined to be 6.9 eV. Again, this is smaller than many literature estimates. In the course of this work, it is shown that due to water’s ability to reorganize about charge (1) photoemission thresholds of water or anionic defects in water do not determine the vacuum level, and (2) there is almost no probability of accessing the bottom of the conduction band of water with a vertical/optical process from water’s valence band. The results are pres...

On the use of graph invariants for efficiently generating hydrogen bond topologies and predicting physical properties of water clusters and ice

Water clusters and some phases of ice are characterized by many isomers with similar oxygen positions, but which differ in direction of hydrogen bonds. A relationship between physical properties, like energy or magnitude of the dipole moment, and hydrogen bond arrangements has long been conjectured. The topology of the hydrogen bond network can be summarized by oriented graphs. Since scalar physical properties like the energy are invariant to symmetry operations, graphical invariants are the proper features of the hydrogen bond network which can be used to discover the correlation with physical properties. We demonstrate how graph invariants are generated and illustrate some of their formal properties. It is shown that invariants can be used to change the enumeration of symmetry-distinct hydrogen bond topologies, nominally a task whose computational cost scales like N2, where N is the number of configurations, into an N ln N process. The utility of graph invariants is confirmed by considering two water clusters, the (H2O)6 cage and (H2O)20 dodecahedron, which, respectively, possess 27 and 30 026 symmetry-distinct hydrogen bond topologies associated with roughly the same oxygen atom arrangements. Physical properties of these clusters are successfully fit to a handful of graph invariants. Using a small number of isomers as a training set, the energy of other isomers of the (H2O)20 dodecahedron can even be estimated well enough to locate phase transitions. Some preliminary results for unit cells of ice-Ih are given to illustrate the application of our results to periodic systems.

Photoelectron spectra of hydrated electron clusters vs. cluster size: connecting to bulk

Connecting cluster properties to bulk can result in new insights, both for the bulk and for clusters, and this is especially the case for the hydrated electron, e–(aq), and its cluster counterparts. In bulk, e–(aq) can be viewed as an anionic defect state of pure water, which is itself a large-band-gap semiconductor. Thus, when properly extrapolated to bulk, the properties of clusters must reflect important energetic properties of bulk water. Several of these have been extracted and are presented here. As for the nature of clusters themselves, the topic has now inspired spirited debate for over two decades. Here, we offer our perspective on these intriguing entities. Among several isomeric groups of and clusters, the one having the highest electron binding energies (herein called bulk embryonts) has been extrapolated to deduce the implied photoelectron spectrum of the bulk hydrated electron. The legitimacy of the lineshape fitting procedures used here is supported by the successful extrapolation of cluster absorption data to its known bulk benchmarks.

Fullerene (C60) Films for Solid Lubrication

The advent of techniques for producing gram quantities of a new form of stable, pure, solid carbon, designated as fullerene, opens a profusion of possibilities to be explored in many disciplines including tribology. Fullerenes take the form of hollow, geodesic domes, which are formed from a network of pentagons and hexagons with covalently bonded carbon atoms. The C60 molecule has the highest possible symmetry (icosaliedral) and assumes the shape of a soccer ball. At room temperature, fullerene molecules pack in a face centered cubic (fee) lattice bonded with weak van der Waals attractions. Fullerenes can be dissolved in solvents such as toluene and benzene and are easily sublimed. The low surface energy, high chemical stability, spherical shape, weak intermolecular bonding, and high load bearing capacity of C60 molecules offer potential for various mechanical and tribological applications. This paper describes the crystal structure and properties of fullerenes and proposes a mechanism for self-lubricatin...

Sublimed C60 films for tribology

Fullerenes take the form of hollow, geodesic domes, which are formed from a network of pentagons and hexagons. The C60 molecule has the highest possible symmetry (icosahedral) and assumes the shape of a soccer ball. At room temperature, fullerene molecules pack in a face‐centered‐cubic lattice bonded with weak van der Waals attractions. Fullerenes can be dissolved in solvents such as toluene and benzene and easily sublimed. The resilience, high load bearing capacity, low surface energy, high chemical stability, and spherical shape of C60 molecules and weak intermolecular bonding offer great potential for various mechanical and tribological applications. Sublimed films of C60 have been produced and friction and wear performance of these films in various operating environments are the subject of this letter.

Enhanced infrared absorption spectra of self-assembled alkanethiol monolayers using the extraordinary infrared transmission of metallic arrays of subwavelength apertures.

The surface-plasmon-mediated, extraordinary transmission of metallic arrays of subwavelength apertures has been used as the light source for absorption studies of self-assembled monolayers on metal. Enhanced infrared absorption spectra of a sequence of alkanethiol self-assembled monolayers on copper were recorded for carbon chain lengths varying from 8 to 18 atoms. Transition positions and intensities are presented over a large range of the infrared region. The connection between the vibrational modes of the CH(2) wagging progression and the infinite methylene chain is explored using a traditional coupled oscillator approach and a new cluster perspective.

Extraordinary infrared transmission of Cu-coated arrays with subwavelength apertures: Hole size and the transition from surface plasmonto waveguide transmission

The zero-order, infrared transmission spectra were recorded and studied at varying angles of incidence to study the dispersion of the resonances in both the Γ−X and Γ−M reciprocal lattice directions. Reduction of hole size shows dramatic effects on the intensity of transmission, the width of the resonances, the identity of the most prominent resonances, and the dispersion behavior of resonances.

Connecting cluster ions and bulk aqueous solvation: A new determination of bulk single ion solvation enthalpies

Abstract A new least-squares method based on cluster ion data and well-known bulk constraints has been used to determine the singleion bulk solvation enthalpies of a set of monovalent ions. A value of − 1153 (6) kJ mol has been determined for the standard solvation enthalpy of the proton. A value of 683 (9) kJ mol at 298 K has also been determined for the proton affinity of water. A simple framework has been presented for estimating the solvation enthalpy of any size cluster ion which provides further evidence for the view that large halide ions are not solvated internally by small water clusters.

Photoelectron spectra of hydrated electron clusters: Fitting line shapes and grouping isomers

The photoelectron spectra of (H2O)(n = 2-69) - and (D2O)(n = 2-23) - are presented, and their spectral line shapes are analyzed in detail. This analysis revealed the presence of three different groupings of species, each of which are seen over the range, n = 11-16. These three groups are designated as dipole boundlike states, seen from n = 2-16, intermediate states, found from n = 6-16, and bulk embryonts, starting at n = 11 and continuing up through the largest sizes studied. Almost two decades ago [J. V. Coe et al., J. Chem. Phys. 92, 3980 (1990)], before the present comprehensive analysis, we concluded that the latter category of species were embryonic hydrated electrons with internalizing excess electrons (thus the term embryonts). Recent experiments with colder expansion (high stagnation chamber pressures) conditions by Neumark and coworkers [J. R. R. Verlet et al., Science 307, 93 (2005)] have also found three groups of isomers including the long-sought-after surface states of large water cluster anions. This work confirms that the species here designated as embryonts are in the process of internalizing the excess electron states as the cluster size increases (for n > or = 11).