Mark W. Severson

Dipole–dipole coupling effects upon infrared spectroscopy of compressed electrochemical adlayers: Application to the Pt(111)/CO system

Experimental infrared spectra for CO adlayers on Pt(111) electrodes having known real‐space structures as deduced by scanning tunneling microscopy are compared with predictions extracted from conventional dipole–dipole coupling models in order to test the validity of such treatments for compressed electrochemical adlayers, especially with regard to band‐intensity transfer effects. The specific structures considered are (2×2)–3CO and (√19×√19)R23.4°–13CO hexagonal adlayers; the former is especially close packed (θCO=0.75) with a pair of threefold hollow and one atop CO per unit cell, while the latter has a lower coverage (θCO=13/19) and involves largely asymmetric binding sites. The comparisons between dipole‐coupling theory and experiment include infrared spectra for various 13CO/12CO mixtures, thereby exploiting the well‐known systematic alterations which are induced in the degree of coupling for a given adlayer. Consistent with an earlier assessment (Ref. ) the conventional dipole–dipole treatment can a...

Ab initio study of the van der Waals interaction of NH(X 3Σ−) with Ar(1S)

The potential energy surface for the Ar(1S)+NH(Xu20093Σ−) interaction is calculated using the supermolecular unrestricted Mo/ller–Plesset (UMP) perturbation theory approach and analyzed via the perturbation theory of intermolecular forces. The global minimum occurs for the approximate T-shaped geometry with Ar skewed toward the H atom at about Θ=67° and R=6.75u2009a0. Our UMP4 estimate of the well depth of the global minimum is De=100.3u2009cm−1 and the related ground state dissociation energy obtained by rigid-body diffusion quantum Monte Carlo calculations (RBDQMC) is D0=71.5±0.1u2009cm−1. These values are expected to be accurate to within a few percent. The potential energy surface also features a wide plateau in the proximity of Ar-N-H collinear geometry, at ca. 7.0u2009a0. RBDQMC calculations reveal nearly a free rotation of the NH subunit in the complex.

Applications of infrared spectroscopy in the study of catalytic reactions and related adsorption phenomena on single crystal electrodes: connections between electrochemical and ultra high vacuum surface science

Abstract The aim of this review is to describe applications of infrared spectroscopy in the study of catalytic reactions and related adsorption phenomena at single crystal electrodes. The majority of work in this area has focused on reactions that form adsorbed CO as an intermediate, such as the electrochemical oxidation pathways of CO and several small organic molecules. In particular, studies that probed CO adsorption processes at a variety of single crystal electrode substrates are emphasized, as this work has been of fundamental importance for defining specific structural features of CO adlayers that form in the electrochemical environment. Efforts to derive CO adlayer structural information through comparisons with infrared spectroscopic studies of corresponding metal-adsorbate systems in ultra high vacuum are described, and both experimental and computational work is discussed. Applications of infrared spectroscopy in single crystal electrochemistry not discussed here include work on ionic adsorption [50, 152–156] and bimetallic interfaces (cf. Refs [157–159], as well as Refs [77, 78]). The reader is referred to the recent papers on these topics for a complete discussion of these areas. In addition, important work on polycrystalline materials continues to appear, and several reviews on these related topics are cited throughout the text.

Librational modes of ice. I

A joint spectroscopic–computational study was carried out of molecular librations in ice. The measured Fourier transform infrared spectra included H2O and HDO isolated in D2O ice and D2O and HDO isolated in H2O ice. Isotopic isolation greatly simplifies the spectra. The observed bands were assigned to hindered rotations around different molecular principal axes. Intermolecular coupling results in remarkable broadening of the librational spectrum, however hindered rotations around the respective axes appear to retain their frequency ordering. Diffusion Monte Carlo investigation was carried out of anharmonic effects. Anharmonic shifts of up to several tens of inverse centimeters are negative for high frequency modes and positive for low frequency ones (with the exception of one HDO frequency in H2O), and affect the ordering of the frequencies.

Ab initio study of the O2(X 3Σg−)+Ar(1S) van der Waals interaction

A potential energy surface for the Ar(1S)+O2(Xu20093Σg−) interaction is calculated using the supermolecular unrestricted Mo/ller–Plesset (UMP) perturbation theory and analyzed via the perturbation theory of intermolecular forces. The global minimum occurs for the T-shaped geometry, around 6.7u2009a0. Our UMP4 estimate of the well depth of the global minimum is De=117u2009cm−1 and the related ground state dissociation energy obtained by diffusion Monte Carlo calculations is 88u2009cm−1. These values are expected to be accurate to within a few percent. The potential energy surface also reveals a local minimum for the collinear geometry at ca∼7.6u2009a0. The well depth for the secondary minimum at the UMP4 level is estimated at De=104u2009cm−1. The minima are separated by a barrier of 23u2009cm−1. The global minimum is determined by the minimum in the exchange repulsion in the direction perpendicular to the O–O bond. The secondary, linear minimum is enhanced by a slight flattening of the electron density near the ends of the interoxyge...

Quantum Monte Carlo simulations of Arn–CO2 clusters

Potential-energy functions for CO2–Arn clusters are constructed using the pairwise-additive approximation from the Ar–Ar potential of Aziz [J. Chem. Phys. 99, 4518 (1993)] and three different CO2–Ar potentials which have been reported recently. These are used to find minimum-energy structures and to carry out rigid-body diffusion Monte Carlo simulations of the ground vibrational state for CO2–Arn clusters with n up to 30, as well as the first excited state for n=1. From these results, the CO2 ν3 redshift is estimated. For all values of n, the Ar atoms tend to surround the CO2 molecule. A complete first solvation shell is first found for n=14, and the largest complete first solvation shell is found for n=17. Although the most recent semiempirical CO2–Ar potential function of Hutson et al. [J. Chem. Phys. 105, 9130 (1996)] gives more accurate predictions of spectroscopic properties for n=1 than the best available ab initio potential function [Marshall et al., J. Chem. Phys. 104, 6569 (1996)], both potential...

ArC2H2: a challenging system for ab initio calculations

Abstract The Arue5f8HCCH interaction is investigated by the supermolecular Moller-Plesset perturbation theory and coupled-cluster theory in conjunction with the perturbation theory of intermolecular forces. The interaction energy in this cluster is dissected into the fundamental components such as exchange, induction, and dispersion, and the anisotropy of the overall surface is analysed in terms of the angular behaviors of these components. The shape of the PES of Arue5f8HCCH is very sensitive to the level of ab initio theory. The coupled-cluster approach which included the single, double, and approximate triple excitations combined with the aug-cc-pvtz basis set supplemented by bond functions was necessary to obtain the best estimates of D e (within ± 5% error) for two minima: the global one for a skew T-configuration (118 cm −1 ), and a secondary one for the collinear arrangement (115 cm −1 ). The PES generated at a lower level of calculation was used in diffusion quantum Monte Carlo (DMC) calculations of the ground and first excited bending vibrational state of the cluster. The expectation values of the coordinates and rotational constants are provided for the ground state. DMC calculations are relatively unsensitive to the details of the PES. The nature and decomposition of three-body interactions in the related cluster Ar 2 ue5f8HCCH were also investigated.

Characterization of ArnO− clusters from ab initio and diffusion Monte Carlo calculations

The structure and energetics of the ArnO− clusters for n=1,…,13 have been modeled in the framework of Diffusion Monte Carlo (DMC), using two- and three-body ab initio determined potentials derived previously by Buchachenko et al. [J. Chem. Phys. 112, 5852 (2000)], and Jakowski et al. [preceding paper, J. Chem. Phys. 118, 2731 (2003)], respectively. The anion cluster structures are largely determined by the two-body potential since the dominant contribution to the stabilization energy is due to pair interactions. However, the three-body effects are important since their role grows with n, from a few percent for n=2 to ca. 30% for n=12. The three-body effects are well approximated by the induction component only. The exchange and dispersion three-body and the induction many-body effects were found to be much less important. The effect of the spin–orbit coupling on the stabilization energies is small and almost independent of the size of a cluster. Specifically, it amounts to about 5% for ArO−, and to 0.1% f...

DIRECT OBSERVATION OF INFRARED BAND INTENSITY TRANSFER BETWEEN COADSORBATES HAVING WIDELY SEPARATED OSCILLATOR FREQUENCIES : INTERMIXED NO/CO ADLAYERS ON ORDERED IRIDIUM ELECTRODES

The occurrence of substantial (two–threefold) transfer of infrared band intensity between juxtaposed coadsorbates having widely separated (200–250 cm−1) oscillator frequencies is demonstrated directly for intermixed NO/CO adlayers on ordered Ir(111) and (110) electrodes by selectively removing CO or NO by electrochemical oxidation and reduction, respectively. The surprisingly large effect is nevertheless semiquantitatively consistent with the predictions of theoretical dipole-coupling models.

Structure and energetics of ArnNO− clusters from ab initio calculations

The potential energy surface for the ground state of the Ar–NO−(3Σ−) complex has been calculated at the unrestricted Mo/ller–Plesset perturbation theory through the fourth order (UMP4). Calculations have been performed using the augmented correlation-consistent polarized triple zeta basis set supplemented with bond functions (aug-cc-pVTZ+bfu200a). The global minimum with a well depth of approximately 651 cm−1 has been found for an approximate T-shaped structure (R=6.28 ao and Θ=83.62°). Two other, local minima correspond to two collinear forms, Ar–N–O and N–O–Ar. All these minima are located in the regions of local reduction of exchange repulsion. The ground vibrational state dissociation energy D0 determined by the collocation method has been found to be 587 cm−1. Several rovibrational states have been predicted and characterized. The ground-state complex assumes a fairly rigid T-shaped structure. The trimer, Ar2NO−(3Σ−), is also of the T-shaped-cross structure, and the pairwise equilibrium and dissociation ...