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A distributed point polarizable model potential for carbon dioxide, with explicit terms for charge penetration and induction, is introduced. This model potential accurately describes the structures and interaction energies of small (CO2)n clusters and also gives the second virial coefficients and radial distribution functions of supercritical CO2 in excellent agreement with experiment. © 2012 Springer-Verlag.

State-of-the-art ADC(2), EOM-EA-CCSD, and EOM-EA-CCSD(2) many-body methods are used to calculate the energies for binding an excess electron to selected water clusters up to (H 2O) 24 in size. The systems chosen for study include several clusters for which the Hartree-Fock method either fails to bind the excess electron or binds it only very weakly. The three theoretical methods are found to give similar values of the electron binding energies. The reported electron binding energies are the most accurate to date for such systems, and these results should prove especially valuable as benchmarks for testing model potential approaches for describing the interactions of excess electrons with water clusters and bulk water. © 2012 American Chemical Society.

The Tkatchenko-Scheffler vdW-TS method [Phys. Rev. Lett. 2009, 102, 073005] has been implemented in a plane-wave DFT code and used to characterize several dispersion-dominated systems, including layered materials, noble-gas solids, and molecular crystals. Full optimizations of the structures, including relaxation of the stresses on the unit cells, were carried out. Internal geometrical parameters, lattice constants, bulk moduli, and cohesive energies are reported and compared to experimental results. © 2012 American Chemical Society.

Adsorption and reactions of CO 2 in the presence of H 2O and OH species on the TiO 2 rutile (110)-(1×1) surface were investigated using dispersion-corrected density functional theory and scanning tunneling microscopy. The coadsorbed H 2O (OH) species slightly increase the CO 2 adsorption energies, primarily through formation of hydrogen bonds, and create new binding configurations that are not present on the anhydrous surface. Proton transfer reactions to CO 2 with formation of bicarbonate and carbonic acid species were investigated and found to have barriers in the range 6.1-12.8 kcalmol, with reactions involving participation of two or more water molecules or OH groups having lower barriers than reactions involving a single adsorbed water molecule or OH group. The reactions to form the most stable adsorbed formate and bicarbonate species are exothermic relative to the unreacted adsorbed CO 2 and H 2O (OH) species, with formation of the bicarbonate species being favored. These results are consistent with single crystal measurements which have identified formation of bicarbonate-type species following coadsorption of CO 2 and water on rutile (110). © 2012 American Institute of Physics.

The (HCOOH) 2 anion, formed by electron attachment to the formic acid dimer (FA 2), is an archetypal system for exploring the mechanics of the electron-induced proton transfer motif that is purported to occur when neutral nucleic acid base-pairs accommodate an excess electron K. Aflatooni, G. A. Gallup, and P. D. Burrow, J. Phys. Chem. A 102, 6205 (1998)10.1021/jp980865n; J. H. Hendricks, S. A. Lyapustina, H. L. de Clercq, J. T. Snodgrass, and K. H. Bowen, J. Chem Phys. 104, 7788 (1996)10.1063/1.471484; C. Desfrancois, H. Abdoul-Carime, and J. P. Schermann, J. Chem Phys. 104, 7792 (1996). The FA 2 anion and several of its HD isotopologues were isolated in the gas phase and characterized using Ar-tagged vibrational predissociation and electron autodetachment spectroscopies. The photoelectron spectrum of the FA 2 anion was also recorded using velocity-map imaging. The resulting spectroscopic information verifies the equilibrium FA 2- geometry predicted by theory which features a symmetrical, double H-bonded bridge effectively linking together constituents that most closely resemble the formate ion and a dihydroxymethyl radical. The spectroscopic signatures of this ion were analyzed with the aid of calculated anharmonic vibrational band patterns. © 2012 American Institute of Physics.

The transition states of a chemical reaction in solution are generally accessed through exchange of thermal energy between the solvent and the reactants. As such, an ensemble of reacting systems approaches the transition state configuration of reactant and surrounding solvent in an incoherent manner that does not lend itself to direct experimental observation. Here we describe how gas-phase cluster chemistry can provide a detailed picture of the microscopic mechanics at play when a network of six water molecules mediates the trapping of a highly reactive "hydrated electron" onto a neutral CO 2 molecule to form a radical anion. The exothermic reaction is triggered from a metastable intermediate by selective excitation of either the reactant CO 2 or the water network, which is evidenced by the evaporative decomposition of the product cluster. Ab initio molecular dynamics simulations of energized CO 2•(H 2O) 6- clusters are used to elucidate the nature of the network deformations that mediate intracluster electron capture, thus revealing the detailed solvent fluctuations implicit in the Marcus theory for electron-transfer kinetics in solution. © 2011 American Chemical Society.

The interaction of a water molecule with the (100) surface of MgO as described by cluster models is studied using MP2, coupled MP2 (MP2C) and symmetry-adapted perturbation theory (SAPT) methods. In addition, diffusion Monte Carlo (DMC) results are presented for several slab models as well as for the smallest, 2X2 cluster model. For the 2X2 model it is found that the MP2C, DMC, and CCSD(T) methods give nearly the same potential energy curve for the water-cluster interaction, whereas the potential energy curve from the SAPT calculations differs slightly from those of the other methods. The interaction of the water molecule with the cluster models of the MgO(100) surface is weakened upon expanding the number of layers from one to two and also upon expanding the description of the layers from 2X2 to 4X4 to 6X6. The SAPT calculations reveal that both these expansions of the cluster model are accompanied by reductions in the magnitudes of the induction and dispersion constributions. The best estimate of the energy for binding an isolated water molecule to the surface obtained from the cluster model calculations is in good agreement with that obtained from the DMC calculations using a 2-layer slab model with periodic boundary conditions. © 2012 the Owner Societies.

The diffusion of CO 2 molecules on a reduced rutile TiO 2(110)-(1×1) surface has been investigated using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. The STM feature associated with a CO 2 molecule at an oxygen vacancy (VO) becomes increasingly streaky with increasing temperature, indicating thermally activated CO 2 diffusion from the VO site. From temperaturedependent tunneling current measurements, the barrier for diffusion of CO 2 from the VO site is estimated to be 3.31 ± 0.23 kcal/mol. The corresponding value from the DFT calculations is 3.80 kcal/mol. In addition, the DFT calculations give a barrier for diffusion of CO 2 along Ti rows of only 1.33 kcal/mol. © 2011 American Chemical Society.

Adsorption of CO2 on the rutile(110) surface was investigated using dispersion-corrected density functional theory and scanning tunneling microscopy (STM). On the oxidized surface the CO2 molecules are found to bind most strongly at the five-fold coordinated Ti sites adopting tilted or flat configurations. The presence of bridging oxygen defects introduces two new adsorption structures, the most stable of which involves CO2 molecules bound in tilted configurations at the defect sites. Inclusion of dispersion corrections in the density functional theory calculations leads to large increases in the calculated adsorption energies bringing these quantities into good agreement with experimental data. The STM measurements confirm two of the calculated adsorption configurations. © 2011 American Institute of Physics.

Adsorption, diffusion, and dissociation of CO2 on the anatase (101) surface were investigated using dispersion-corrected density functional theory. On the oxidized surface several different local minima were identified of which the most stable corresponds to a CO2 molecule adsorbed at a five-fold coordinated Ti site in a tilted configuration. Surface diffusion is characterized by relatively small activation barriers. Preferential diffusion takes place along Ti rows and involves a cartwheel type of motion. The presence of a bridging oxygen defect or a surface interstitial Ti atom allows creation of several new strong binding configurations the most stable of which have bent CO2 structures with simultaneous bonding to two surface Ti atoms. Subsurface oxygen vacancy or interstitial Ti defects are found to enhance the bonding of CO2 molecules to the surface. CO2 dissociation from these defect sites is calculated to be exothermic with barriers less than 21 kcalmol. The use of such defects for catalytic activation of CO2 on anatase (101) surface would require a mechanism for their regeneration. © 2011 American Institute of Physics.

The stacking parameters, lattice constants, bond lengths, and bulk moduli of pyrophyllite and montmorillonites (MMTs), with alkali and alkali earth metal ions, are investigated using density functional theory with and without dispersion corrections. For pyrophyllite, it is found that the inclusion of the dispersion corrections significantly improves the agreement of the calculated values of the lattice parameters and bulk modulus with the experimental values. For the MMTs, the calculations predict that the interlayer spacing varies approximately linearly with the cation radius. The inclusion of dispersion corrections leads to sizable shifts of the interlayer spacings to shorter values. In Li-MMT, compaction of the interlayer distance triggers migration of the Li ion into the tetrahedral sheet and close coordination with basal oxygen atoms. Analysis of electron density distributions shows that the isomorphic octahedral Al3+/Mg2+ substitution in MMT causes an increase of electron density on the basal oxygen atoms of the tetrahedral sheets. © 2011 American Chemical Society.

The interaction of a water monomer with a series of linear acenes (benzene, anthracene, pentacene, heptacene, and nonacene) is investigated using a wide range of electronic structure methods, including several "dispersion"- corrected density functional theory (DFT) methods, several variants of the random phase approximation (RPA), DFT-based symmetry-adapted perturbation theory with density fitting (DF-DFT-SAPT), MP2, and coupled-cluster methods. The DF-DFT-SAPT calculations are used to monitor the evolution of the electrostatics, exchange-repulsion, induction, and dispersion contributions to the interaction energies with increasing acene size and also provide the benchmark data against which the other methods are assessed. © 2010 American Chemical Society.

We report the spectral signatures of water molecules occupying individual sites in an extended H-bonding network using mass-selective, double-resonance vibrational spectroscopy of isotopomers. The scheme is demonstrated on the water heptamer anion, (H2O)7̄ where we first randomly incorporate a single, intact D2O molecule to create an ensemble of isotopomers. The correlation between the two OD stretching frequencies and that of the intramolecular DOD bending transition is then revealed by photochemical modulation of the isotopomer population responsible for particular features in the vibrational spectrum. The observed patterns confirm the assignment of the dominant doublet, appearing most red-shifted from the free OD stretch, to a single water molecule attached to the network in a double H-bond acceptor (AA) arrangement. The data also reveal the unanticipated role of accidentally overlapping transitions, where the highest-energy OD stretch, for example, occurs with its companion OD stretch obscured by the much stronger AA feature. © 2010 American Chemical Society.

A discrete variable representation (DVR) implementation of an one-electron polarization model (OPEM) for characterizing (H2O)n- clusters is described. For the (H2O)90- cluster, evaluation of the energy and gradient using a suitable DVR basis sets is about a 2 orders of magnitude faster than corresponding calculations using a Gaussian orbital basis set. The DVR version of the code has been parallelized using OpenMP to enable molecular dynamics (MD) simulations of large (H 2O)n- clusters. © 2010 American Chemical Society.

Molecular dynamics simulations are used to characterize the hydrates of Xe, methane, and CO2, allowing for a systematic comparison of the structural and dynamical properties for these three hydrates. Although the host-guest interaction energy for the T = 0 K structures is most attractive in the case of Xe, other structural and dynamical properties from the simulations indicate that, in fact, host-guest coupling is most important for the CO 2 hydrate. Specifically, the host lattice of CO2 hydrate expands more with increasing temperature than do the lattices of the xenon and methane hydrates, and the translational and rotational dynamics of the water molecules are predicted to be most perturbed in the CO2 hydrate. The simulations predict that the CO2 and xenon hydrates have lower speed of sound values and lower themal conductivities than methane hydrate or the empty lattice. © 2010 American Chemical Society.

Many chemical reactions in atmospheric aerosols and bulk aqueous environments are influenced by the surrounding solvation shell, but the precise molecular interactions underlying such effects have rarely been elucidated. We exploited recent advances in isomer-specific cluster vibrational spectroscopy to explore the fundamental relation between the hydrogen (H)-bonding arrangement of a set of ion-solvating water molecules and the chemical activity of this ensemble. We find that the extent to which the nitrosonium ion (NO+) and water form nitrous acid (HONO) and a hydrated proton cluster in the critical trihydrate depends sensitively on the geometrical arrangement of the water molecules in the network. Theoretical analysis of these data details the role of the water network in promoting charge derealization.

The diffusion Monte Carlo (DMC) method is used to calculate the electron binding energies of two forms of (H2O)6-. It is found that the DMC method, when using either Hartree-Fock or density functional theory trial wave functions, gives electron binding energies in excellent agreement with the results of large basis set CCSD(T) calculations. This demonstrates that the DMC method will be a viable method for characterizing larger (H 2O)n- ions for which CCSD(T) calculations are not feasible. © 2010 American Chemical Society.

Infrared photodissociation spectroscopy is reported for mass-selected H+(H2O)n complexes and their deuterated analogues with and without argon "tagging." H+(H 2O)nArm and D+(D2O) nArm complexes are studied in the O-H (O-D) stretching region for clusters in the small size range (n = 2-5). Upon infrared excitation, these clusters fragment by the loss of either argon atoms or one or more intact water molecules. Their excitation spectra show distinct bands in the region of the symmetric and asymmetric stretches of water and in the hydrogen bonding region. Experimental studies are complemented by computational work that explores the isomeric structures, their energetics and vibrational spectra. The addition of an argon atom is essential to obtain photodissociation for the n = 2-3 complexes, and specific inclusion of the argon in calculations is necessary to reproduce the measured spectra. For n = 3-5, spectra are obtained both with and without argon. The added argon atom allows selection of a subset of colder clusters and it increases the photodissociation yield. Although most of these clusters have more than one possible isomeric structure, the spectra measured correspond to a single isomer that is computed to be the most stable. Deuteration in these small cluster sizes leads to expected lowering of frequencies, but the spectra indicate the presence of the same single most-stable isomer for each cluster size. © 2010 American Chemical Society.

A new model for free electrons in water suggests that they are localized in regions of enhanced rather than depleted water density.

The low-lying potential energy minima of the H+(H2O) n, n = 6, 21, and 22, protonated water clusters have been investigated using two versions of the self-consistent-charge density-functional tight-binding plus dispersion (SCC-DFTB+D) electronic structure methods. The relative energies of different isomers calculated using the SCC-DFTB+D methods are compared with the results of DFT and MP2 calculations. This comparison reveals that for H+(H2O)6 the SCC-DFTB+D method with H-bonding and third-order corrections more closely reproduces the results of the MP2 calculations, whereas for the n = 21 and 22 clusters, the uncorrected SCC-DFTB+D method performs better. Both versions of the SCC-DFTB+D method are found to be biased toward Zundel structures. © 2010 American Chemical Society.

Localized molecular orbital energy decomposition analysis and symmetry-adapted perturbation theory (SAPT) calculations are used to analyze the two- and three-body interaction energies of four low-energy isomers of (H 2 O)6 in order to gain insight into the performance of several popular density functionals for describing the electrostatic, exchange-repulsion, induction, and short-range dispersion interactions between water molecules. The energy decomposition analyses indicate that all density functionals considered significantly overestimate the contributions of charge transfer to the interaction energies. Moreover, in contrast to some studies that state that density functional theory (DFT) does not include dispersion interactions, we adopt a broader definition and conclude that for (H2 O)6 the short-range dispersion interactions recovered in the DFT calculations account about 75% or more of the net (short-range plus long-range) dispersion energies obtained from the SAPT calculations. © 2010 American Institute of Physics.

We have explored the room temperature response of metal nanoparticle decorated single-walled carbon nanotubes (NPSWNTs) using a combination of electrical transport, optical spectroscopy, and electronic structure calculations. We have found that upon the electrochemical growth of Au NPs on SWNTs, there is a transfer of electron density from the SWNT to the NP species, and that adsorption of CO molecules on the NP surface is accompanied by transfer of electronic density back into the SWNT. Moreover, the electronic structure calculations indicate dramatic variations in the charge density at the NP-SWNT interface, which supports our previous observation that interfacial potential barriers dominate the electrical behavior of NP-SWNT systems. © 2010 American Chemical Society.

In a previous study (J. Phys. Chem. C, 2009, 113, 10242-10248) we used density functional theory based symmetry-adapted perturbation theory (DFT-SAPT) calculations of water interacting with benzene (C6H6), coronene (C24H12), and circumcoronene (C54H18) to estimate the interaction energy between a water molecule and a graphene sheet. The present study extends this earlier work by use of a more realistic geometry with the water molecule oriented perpendicular to the acene with both hydrogen atoms pointing down. We also include results for an intermediate C48H18 acene. Extrapolation of the water-acene results gives a value of -3.0 ± 0.15 kcal mol-1 for the binding of a water molecule to graphene. Several popular dispersion-corrected DFT methods are applied to the water-acene systems and the resulting interacting energies are compared to results of the DFT-SAPT calculations in order to assess their performance. © the Owner Societies.

In the present study we revisit the problem of the interaction of a water molecule with a single graphite sheet. The density fitting-density functional theory-symmetry-adapted perturbation theory (DF-DFT-SAPT; J. Chem. Phys. 2005, 122, 014103) method is used to calculate the individual contributions arising from the interaction of a water molecule with various acenes, including benzene, coronene, and dodecabenzocoronene. These results are combined with calculations of the electrostatic interactions with water and a C 216H 36 acene to extrapolate to the limit of an infinite graphite sheet, giving a interaction energy of -2.2 kcal/mol for the water-graphite system, with the assumed geometrical structure with one hydrogen atom pointed down toward the ring system. The structure with two hydrogens pointed down is predicted to be more stable, with a net interaction energy of -2.7 kcal/mol. © 2009 American Chemical Society.

We introduce a method based on sequential application of vibrational predissociation spectroscopy to explore the high-amplitude rearrangements available in a small H-bonded complex that is vibrationally excited within a larger Ar cluster. The weakly bound Ar atoms play the role of a solvent in mediating the energy content of the embedded system, ultimately quenching it into local minima through evaporation. We demonstrate the approach on the NO 2-·H 2O binary hydrate, which is known to occur in two nearly isoenergetic isomeric forms. The scheme involves three stages of mass separation to select a particular NO 2-·H 2O·Ar m parent ion cluster prior to vibrational excitation and then isolate the NO 2-·H 2O·Ar fragment ions for interrogation using resonant vibrational predissociation with a second infrared laser. The initial vibrational excitation selectively energizes one of the isomers through one of its characteristic resonances while the predissociation spectrum of the NO 2-·H 2O· Ar fragment encodes the distribution of isomers present after Ar evaporation. Isomerization from the front- to backside form is found to occur upon excitation of the NO stretch near 1200 cm -1; although the reverse reaction is not observed upon excitation of the NO stretch, it is observed upon excitation of the higher-energy OH stretching fundamental near 3000 cm -1. We discuss these observations in the context of the calculated isomerization energetics, which focus on the minimum energy structures for the isomers as well as the transition states for their interconversion. © 2009 American Chemical Society.

Molecular dynamics simulations have been carried out to study decomposition of methane hydrate at different cage occupancies. The decomposition rate is found to depend sensitively on the hydration number. The rate of the destruction of the cages displays Arrhenius behavior, consistent with an activated mechanism. During the simulations, reversible formation of partial water cages around methane molecules in the liquid was observed at the interface at temperatures above the computed hydrate decomposition temperature. © 2009 American Chemical Society.

In this work, we present infrared spectra of H 5O 2+ and its D 5O 2+, D 4HO 2+, and DH 4O 2+ isotopologues calculated by classical molecular dynamics simulations on an accurate potential energy surface generated from CCSD(T) calculations, as well as on the BLYP DFT potential energy surface sampled by means of the Car-Parrinello algorithm. The calculated spectra obtained with internal energies corresponding to a temperature of about 30 K are in overall good agreement with those from experimental measurements and from quantum dynamical simulations. © 2009 American Chemical Society.

A multistate empirical valence bond (MSEVB) model for protonated water clusters, which incorporates the TIP4P water model, is presented. This model which is designated MSEVB4P represents a significant improvement over the original model of Voth et al. (J. Phys. Chem. B 1998, 102, 5547) which was based on the TIP3P water model and a smaller improvement over the recently introduced MSEVB3 model (J. Phys. Chem. B 2008, 112, 467) which is based on the SPC/Fw (J. Chem. Phys. 2006, 124, 024503) water model. ©2009 American Chemical Society.

A polarizable force field that explicitly includes contributions from exchange repulsion, dispersion, charge penetration, and multipole electrostatics was developed to describe the interaction between bromine and water. This force field was combined with a polarizable force field for water and used in molecular dynamics simulations to calculate the relative energetics of three bromine clathrate hydrates. The simulations predict the tetragonal structure (Allen, K. W.; Jeffrey, G. A. J. Chem. Phys. 1963, 38, 2304) to be the most stable, with the CS-I and CS-II cubic structures being less stable. Although the CS-II species is not the most stable energetically, we argue that it could be formed under conditions of low bromine concentration, in agreement with recent measurements (Goldschleger, I. U.; Kerenskaya, G.; Janda, K. C.; Apkarian, V. A. J. Phys. Chem. A 2008, 112, 787) that provide evidence for three different bromine hydrate crystal types. © 2009 American Chemical Society.

Si2H4 and Si7H8 cluster models are constructed for studying the role of high-order electron correlation effects on the buckling of SiSi dimers on the Si(100) surface. CASPT3, multi-reference coupled-pair functional, and the diffusion Monte Carlo methods are used to examine whether correlation effects not recovered in CASPT2 calculations are important for the buckling of the dimers. The calculations show that such high-order correlation effects are indeed important for determining the relative stability of the buckled and unbuckled structures.

Expressions for the analytical gradients of the PM1 polarization model for describing the interaction of excess electrons with water clusters are reported. For the (H2 O)45- cluster, evaluation of the gradient by use of the analytical expressions lead to a factor of five speedup over numerical evaluation of the gradient. © 2008 Elsevier B.V. All rights reserved.

Parallel tempering Monte Carlo simulations of the water heptamer anion have been performed for temperatures ranging from 42 to 200 K. At low temperatures, a single peak near 250 meV in the electron binding energy distribution is obtained, while at high temperatures a second, weak peak near 450 meV is observed, in good agreement with those observed experimentally. It is further confirmed that the high electron binding energies are due to hydrogen bonding networks with large net dipole moments and, in most cases, also containing a single double-acceptor monomer, while weak electron binding arises from configurations with smaller dipoles. © 2008 Elsevier B.V. All rights reserved.

The formation of hydroxyl chains from water dissociation on the Cu(110) surface has been studied by using a combination of scanning tunneling microscopy (STM), electron stimulated desorption ion angular distribution (ESDIAD), temperature programmed desorption (TPD), and density functional theory (DFT) calculations. Annealing the D2O-covered surface to a temperature of ̃200 K leads to desorption of D2O molecules and produces a zigzag structure due to adsorbed OD groups with a periodicity of 5 Å along the <110> direction in the STM image. Coadsorption of O2 promotes the water dissociation reaction and produces hydroxyl chains with much higher coverage. ESDIAD measurements show a two-beam pattern consistent with OD(a) species inclined ̃40° with respect to the surface normal and orientated along the <001> azimuth. The calculations reveal the existence of stable chain structures comprised solely of hydroxyl groups as well as of interacting water and hydroxyl groups that are consistent with the observed STM image. © 2008 American Chemical Society.

Nonequilibrium molecular dynamics simulations with the nonpolarizable SPC/E (Berendsen et al., J. Phys. Chem. 1987, 91, 6269) and the polarizable COS/G2 (Yu and van Gunsteren, J. Chem. Phys. 2004, 121, 9549) force fields have been employed to calculate the thermal conductivity and other associated properties of methane hydrate over a temperature range from 30 to 260 K. The calculated results are compared to experimental data over this same range. The values of the thermal conductivity calculated with the COS/G2 model are closer to the experimental values than are those calculated with the nonpolarizable SPC/E model. The calculations match the temperature trend in the experimental data at temperatures below 50 K; however, they exhibit a slight decrease in thermal conductivity at higher temperatures in comparison to an opposite trend in the experimental data. The calculated thermal conductivity values are found to be relatively insensitive to the occupancy of the cages except at low (T < 50 K) temperatures, which indicates that the differences between the two lattice structures may have a more dominant role than generally thought in explaining the low thermal conductivity of methane hydrate compared to ice Ih. The introduction of defects into the water lattice is found to cause a reduction in the thermal conductivity but to have a negligible impact on its temperature dependence. © 2008 American Chemical Society.

Resonant two-photon ionization (R2PI), UV hole-burning (UVHB), and resonant ion-dip infrared (RIDIR) spectroscopy have been used to study the single-conformation infrared and ultraviolet spectroscopy of 3-(4-hydroxyphenyl)-N-benzylpropionamide (HNBPA, HOC6H 5CH2CH2(C=O)NHCH2C6H 5) cooled in a supersonic expansion. UVHB determines the presence of three conformers, two of which dominate the spectrum. RIDIR spectra in the OH stretch (3600-3700 cm-1), amide NH stretch (3450-3500 cm -1), and C=O stretch (1700-1750 cm-1) regions reveal the presence of small shifts in these fundamentals that are characteristic of the folding of the flexible chain and the ring-ring and ring-chain interactions. On the basis of a comparison of the experimental frequency shifts with calculations, the two major experimentally observed conformers are assigned to two folded structures in which the two aromatic rings are (nominally) face-to-face and perpendicular to one another. The perpendicular structure has a transition assignable to the S0-S2 origin, while the face-to-face structure does not, consistent with a faster nonradiative process in the latter case. The calculated structures and vibrational frequencies are quite sensitive to the level of theory due to the flexibility of the interconnecting chain and the importance of dispersive interactions between the two aromatic rings. © 2008 American Chemical Society.

We report a combined photoelectron and vibrational spectroscopy study of the (H2 O) 7- cluster anions in order to correlate structural changes with the observed differences in electron binding energies of the various isomers. Photoelectron spectra of the (H2 O) 7- Arm clusters are obtained over the range of m=0-10. These spectra reveal the formation of a new isomer (I′) for m<5, the electron binding energy of which is about 0.15 eV higher than that of the type I form previously reported to be the highest binding energy species [Coe, J. Chem. Phys. 92, 3980 (1990)]. Isomer-selective vibrational predissociation spectra are obtained using both the Ar dependence of the isomer distribution and photochemical depopulation of the more weakly (electron) binding isomers. The likely structures of the isomers at play are identified with the aid of electronic structure calculations, and the electron binding energies, as well as harmonic vibrational spectra, are calculated for 28 low-lying forms for comparison with the experimental results. The HOH bending spectrum of the low binding type II form is dominated by a band that is moderately redshifted relative to the bending origin of the bare water molecule. Calculations trace this feature primarily to the bending vibration localized on a water molecule in which a dangling H atom points toward the electron cloud. Both higher binding forms (I and I′) display the characteristic patterns in the bending and OH stretching regions signaling electron attachment primarily to a water molecule in an AA binding site, a persistent motif found in non-isomer-selective spectra of the clusters up to (H2 O) 50-. © 2008 American Institute of Physics.

In this work we focus on the binding of excess electrons to water clusters, a problem for which dispersion interactions, which originate from long-range correlation effects, are especially important. Two different model potential approaches, one using quantum Drude oscillators and the other using polarization potentials, are investigated for describing the long-range correlation effects between the weakly bound excess electron and the more tightly bound electrons of the monomers. We show that these two approaches are related in that the polarization potential models can be derived from the quantum Drude model approach by use of an adiabatic separation between the excess electron and the Drude oscillators. The model potential approaches are applied to clusters containing up to 45 water monomers. Where possible, comparison is made with the results of ab initio electronic structure calculations. Overall, the polarization potential approach is found to give electron binding energies in good agreement with those from the Drude model and ab initio calculations, with the greatest discrepancies being found for "cavity-bound" anion states. © 2008 American Chemical Society.

Self-assembly of molecules on surfaces is a route toward not only creating structures, but also engineering chemical reactivity afforded by the intermolecular interactions. Dimethyldisulfide (CH3SSCH3) molecules self-assemble into linear chains on single-crystal gold surfaces. Injecting low-energy electrons into individual molecules in the self-assembled structures with the tip of a scanning tunneling microscope led to a propagating chemical reaction along the molecular chain as sulfur-sulfur bonds were broken and then reformed to produce new CH3SSCH3 molecules. Theoretical and experimental evidence supports a mechanism involving electron attachment followed by dissociation of a CH3SSCH3 molecule and initiation of a chain reaction by one or both of the resulting CH 3S intermediates.

In this work we examine three water models, including one developed by us, with distributed polarizable sites. These models are assessed by comparison with the results of MP2 calculations. The use of distributed polarizable sites is found to be especially important for describing the 3-body interaction energies of water clusters. It is also shown that all-atom repulsion terms are necessary to accurately describe cluster geometries.

Resonant two-photon ionization (R2PI), resonant ion-dip infrared (RIDIR), and UV-UV hole-burning spectroscopies have been employed to obtain conformation-specific infrared and ultraviolet spectra under supersonic expansion conditions for O -(2-acetamidoethyl)- N -acetyltyramine (OANAT), a doubly substituted aromatic in which amide-containing alkyl and alkoxy side chains are located in para positions on a phenyl ring. For comparison, three single-chain analogs were also studied: (i) N -phenethyl-acetamide (NPEA), (ii) N - (p -methoxyphenethyl-acetamide) (NMPEA), and (iii) N -(2-phenoxyethyl)- acetamide (NPOEA). Six conformations of OANAT have been resolved, with S0 - S1 origins ranging from 34 536 to 35 711 cm-1, denoted A-F, respectively. RIDIR spectra show that conformers A-C each possess an intense, broadened amide NH stretch fundamental shifted below 3400 cm-1, indicative of the presence of an interchain H bond, while conformers D-F have both amide NH stretch fundamentals in the 3480-3495 cm-1 region, consistent with independent-chain structures with two free NH groups. NPEA has a single conformer with S0 - S1 origin at 37 618 cm-1. NMPEA has three conformers, two that dominate the R2P1 spectrum, with origin transitions between 35 580 and 35 632 cm-1. Four conformations, one dominate and three minor, of NPOEA have been resolved with origins between 35 654 and 36 423 cm-1. To aid the making of conformational assignments, the geometries of low-lying structures of all four molecules have been optimized and the associated harmonic vibrational frequencies calculated using density functional theory (DFT) and RIMP2 methods. The S0 - S1 adiabatic excitation energies have been calculated using the RICC2 method and vertical excitation energies using single-point time-dependent DFT. The sensitivity of the S0 - S1 energy separation in OANAT and NPOEA primarily arises from different orientations of the chain attached to the phenoxy group. Using the results of the single-chain

The infrared spectra of the O2- benzene and O4- benzene complexes are determined by means of Ar predissociation spectroscopy. Several transitions due to CH stretch fundamentals and various combination bands are observed in the 2700-3100 cm-1 region. The experimental results are interpreted with the aid of electronic structure calculations. A comparison of the calculated and experimental spectra reveals that the spectrum of O2- benzene most likely arises from an isomer where the superoxide molecule binds preferentially to one CH group of benzene. In contrast, the spectrum of O4- benzene yields a CH pattern remarkably similar to that displayed by the C2 X- benzene (X=halogen) complexes, consistent with a structure with two CH groups equally involved in the bonding. The lower energy vibrational fundamental transitions of the O4- anion are recovered with a slight redshift in the O4- benzene spectrum, establishing that this charge-delocalized dimer ion retains its identity upon complexation. © 2007 American Institute of Physics.

Molecular dynamics simulations of methane hydrate have been carried out using the polarizable AMOEBA and COS/G2 force fields. Properties calculated include the temperature dependence of the lattice constant, the OC and OO radial distribution functions, and the vibrational spectra. Both the AMOEBA and COS/G2 force fields are found to successfully account for the available experimental data, with overall somewhat better agreement with experiment being found for the AMOEBA model. Comparison is made with previous results obtained using TIP4P and SPC/E effective two-body force fields and the polarizable TIP4P-FQ force field, which allows for in-plane polarization only. Significant differences are found between the properties calculated using the TIP4P-FQ model and those obtained using the other models, indicating an inadequacy of restricting explicit polarization to in-plane only. © 2007 American Chemical Society.

A potential energy surface for trans-formanilide (TFA)- H2 O is calculated and applied to study energy flow in the complex as well as the kinetics of water shuttling between hydrogen bonding sites on TFA. In addition to the previously identified H2 O-TFA (CO) and H2 O-TFA (NH) minima, with the water monomer bound to the CO and NH groups, respectively, the new surface reveals a second local minimum with the water bound to the CO group, and which lies energetically 310 cm-1 above the previously identified H2 O-TFA (CO) global minimum. On this surface, the energy barrier for water shuttling from H2 O-TFA (CO) global minimum to H2 O-TFA (N-H) is 984 cm-1, consistent with the experimental bounds of 796 and 988 cm-1 [J. R. Clarkson Science 307, 1443 (2005)]. The ergodicity threshold of TFA is calculated to be 1450 cm-1; for the TFA- H2 O complex, the coupling to the water molecule is found to lower the ergodicity threshold to below the isomerization barrier. Energy transfer between the activated complex and the vibrational modes of TFA is calculated to be sufficiently rapid that the Rice-Ramsperger-Kassel-Marcus (RRKM) theory does not overestimate the rate of water shuttling. The possibility that the rate constant for water shuttling is higher than the RRKM theory estimate is discussed in light of the relatively high energy of the ergodicity threshold calculated for TFA. © 2007 American Institute of Physics.

As a step toward a first principles characterization of the optical properties of chlorine hydrate, we have calculated the electronic absorption spectrum of a chlorine molecule trapped in dodecahedral (H2O) 20 and hexakaidodecahedral (H2O)24 cages. For comparison, spectra were also calculated for an isolated Cl2 molecule as well as for selected Cl2(H2O)n, n ≤ 8, clusters cut out of the Cl2(H2O)2O cluster, allowing us to follow the evolution of the low-lying excited states with increasing number of surrounding water molecules. Although encapsulation of a chlorine molecule within the water cages has relatively little effect on its low-lying valence transitions, it does result in a large number of solvent-to-solute charge-transfer transitions at energies starting near 48 000 cm-1. © 2007 American Chemical Society.

Rhodium- and molybdenum-catalyzed allenic [2 + 2 + 1] cycloaddition reactions give 4-alkylidene and α-alkylidene cylopentenones, respectively. The selective reaction of one double bond of the allene over another is controlled by the transition metal and not the substrate structure. Calculations were performed to explain this unique control element using the B3LYP functional as implemented in Gaussian 03. The 6-31G(d) basis set was applied to all elements except rhodium, which is described with the LANL2 effective core potential and the LANL2DZ basis set. The product-determining step for both reaction pathways is oxidative addition of the metal to the alkynyl allene to form the corresponding metallocycles B and B'. The calculations strongly suggest that geometric constraints imposed by the metal in the transition state are the key controlling factor of the double-bond selectivity. The transition state structure of rhodium-catalyzed oxidative addition has a distorted square planar geometry that affords a lower transition state energy when coordinated to the distal double bond of the allene. In turn, the distorted trigonal bipyramidal geometry of molybdenum in the transition state structure imposes conformational constraints upon binding to the distal double bond on the allene and thus leads to the energetically preferred complexation and reaction with the proximal double bond. © 2006 American Chemical Society.

In order to gain insight into the factors responsible for the different behavior of H2O and H2S with respect to hydrate formation, we have studied theoretically the (H2O)21 and (H2O)20 · H2S cluster systems. For each cluster, two isomers resulting from insertion of an H2O or H2S molecule into a dodecahedral (H2O)20 cage were examined. The calculations were performed using the density functional and RIMP2 methods. © 2006 Elsevier B.V. All rights reserved.

We present density functional theory calculations of partially hydrated or "wet"-electron states on H2 O and H-covered Ti O2 (110) surfaces. Based on the investigation of different coverages and structures, we determine different patterns of hydrogen bonding for the chemisorbed H2 O and H on Ti O2 (110). We find that a network of dangling H atoms can stabilize photoexcited electrons, in so-called wet-electron states. The energies of the wet-electron states correlate closely with the number and configuration of the dangling H atoms. The effect of the adsorbate chemisorption on the surface charge distribution is also discussed. The calculated energies of the wet-electron states are in good agreement with those measured in two-photon photoemission experiments. © 2006 The American Physical Society.

Two low-energy minima of (H2O)21 with very different H-bonding arrangements have been investigated with the B3LYP density functional and RIMP2 methods, as well as with the TIP4P, Dang - Chang, AMOEBA, and TTM2-F force fields. The AMOEBA and TTM2-F model potentials give an energy ordering that agrees with the results of the electronic structure calculations, while the TIP4P and Dang - Chang models give the opposite ordering. Insight into the role of many-body polarization for establishing the relative stability of the two isomers is provided by an n-body decomposition of the energies calculated using the various theoretical methods. © 2006 American Chemical Society.

It is has been established that the excess electrons in small (i.e., n ≤ 7) (H2O)n- clusters are bound in the dipole field of the neutral cluster and, thus, exist as surface states. However, the motifs for the binding of an excess electron to larger water clusters remain the subject of considerable debate. The prevailing view is that electrostatic interactions with the "free" OH bonds of the cluster dominate the binding of the excess electron in both small and large clusters. In the present study, a quantum Drude model is used to study selected (H2O) n- clusters in the n = 12-24 size range with the goal of elucidating different possible binding motifs. In addition to the known surface and cavity states, we identify a new binding motif, where the excess electron permeates the hydrogen-bonding network. It is found that electrostatic interactions dominate the binding of the excess electron only for isomers with large dipole moments, whereas in isomers without large dipole moments polarization and correlation effects dominate. Remarkably, for the network-permeating states, the excess electron binds even in the absence of electrostatic interactions. © 2006 American Chemical Society.

Coronene (C24 H12) adsorption on the clean Si(001)- 2×1 surface was investigated by scanning tunneling microscopy and by density-functional calculations. The coronene adsorbed randomly at 25 °C on the surface and did not form two-dimensional islands. The scanning tunneling microscopy measurements revealed three adsorption sites for the coronene molecule on the Si(001) surface at low coverage. The major adsorption configuration involves coronene bonding to four underlying Si atoms spaced two lattice spacings apart in a dimer row. The two minor adsorption configurations involve asymmetrical bonding of a coronene molecule between Si dimer rows and form surface species with a mirror plane symmetry to their chiral neighbor species. The two minor bonding arrangements are stabilized by a type- C defect on the Si(001) surface. © 2006 American Institute of Physics.