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15 Jun 2002

Volume 116, Issue 23, pp. 10041-10557

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back to top Theoretical Methods and Algorithms

Quasi-isotropic single-transition cross-polarization in nuclear magnetic resonance

Thomas R. Eykyn, Fabien Ferrage, and Geoffrey Bodenhausen

J. Chem. Phys. 116, 10041 (2002); http://dx.doi.org/10.1063/1.1477176 (10 pages) | Cited 4 times

Online Publication Date: 30 May 2002

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The theory of single-transition cross-polarization in nuclear magnetic resonance is presented and verified by experimental evidence. In comparison to conventional cross-polarization a qualitative change in the mechanism is observed. Under the influence of matched radio-frequency fields with amplitudes that are smaller than the scalar coupling constant JIS for a two-spin system with I = ½ and S = ½ in isotropic solution, two simultaneous coherence transfer processes are observed between single-transition coherences which have phases that are parallel to those of the radio-frequency fields, an on-resonance transfer from SxIα to SαIx and an off-resonance transfer from SxIβ to SβIx, without mixing between the two pathways. Coherence transfer is also observed between single-transition coherences with phases that are perpendicular to the radio-frequency fields, from SyIα to SαIy and from SyIβ to SβIy, as well as between longitudinal components, from SzIα to SαIz and from SzIβ to SβIz. The transfer may therefore be considered quasi-isotropic. We consider the conditions under which such transfer processes can be observed. Coherence transfer is affected by differential relaxation due to cross-correlation effects. © 2002 American Institute of Physics.
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76.60.-k Nuclear magnetic resonance and relaxation
33.25.+k Nuclear resonance and relaxation

Quantum-classical description of rotational diffractive scattering using Bohmian trajectories: Comparison with full quantum wave packet results

E. Gindensperger, C. Meier, J. A. Beswick, and M-C. Heitz

J. Chem. Phys. 116, 10051 (2002); http://dx.doi.org/10.1063/1.1471904 (9 pages) | Cited 15 times

Online Publication Date: 30 May 2002

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We apply the mixed quantum/classical method based on the Bohmian formulation of quantum mechanics [E. Gindensperger, C. Meier, and J. A. Beswick, J. Chem. Phys. 113, 9369 (2000)] to the case of rotational diffractive surface scattering of a diatomic molecule. The rotation as well as the normal translational degree of freedom are treated classically while the two parallel degrees of freedom that account for the diffraction are treated quantum mechanically. The effects of treating some degrees of freedom classically are discussed in detail by comparing our novel approximate method to quantum wave packet results obtained by the multiconfiguration time-dependent Hartree method. © 2002 American Institute of Physics.
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34.50.Ez Rotational and vibrational energy transfer
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces
34.35.+a Interactions of atoms and molecules with surfaces
68.49.Df Molecule scattering from surfaces (energy transfer, resonances, trapping)

Direct generation of local orbitals for multireference treatment and subsequent uses for the calculation of the correlation energy

Daniel Maynau, Stefano Evangelisti, Nathalie Guihéry, Carmen J. Calzado, and Jean-Paul Malrieu

J. Chem. Phys. 116, 10060 (2002); http://dx.doi.org/10.1063/1.1476312 (9 pages) | Cited 49 times

Online Publication Date: 30 May 2002

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We present a method that uses the one-particle density matrix to generate directly localized orbitals dedicated to multireference wave functions. On one hand, it is shown that the definition of local orbitals making possible physically justified truncations of the CAS (complete active space) is particularly adequate for the treatment of multireference problems. On the other hand, as it will be shown in the case of bond breaking, the control of the spatial location of the active orbitals may permit description of the desired physics with a smaller number of active orbitals than when starting from canonical molecular orbitals. The subsequent calculation of the dynamical correlation energy can be achieved with a lower computational effort either due to this reduction of the active space, or by truncation of the CAS to a shorter set of references. The ground- and excited-state energies are very close to the current complete active space self-consistent field ones and several examples of multireference singles and doubles calculations illustrate the interest of the procedure. © 2002 American Institute of Physics.
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31.15.xr Self-consistent-field methods

Quantum-phase dynamics of dimer systems interacting with a two-mode squeezed coherent field

Masayoshi Nakano and Kizashi Yamaguchi

J. Chem. Phys. 116, 10069 (2002); http://dx.doi.org/10.1063/1.1471906 (14 pages) | Cited 6 times

Online Publication Date: 30 May 2002

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It is well-known that the interaction among atoms/molecules and quantized electromagnetic fields with a small number of photons provides a peculiar quantum nature, i.e., collapses and revivals in the Rabi oscillations. In this study, we investigate the intermolecular interaction (dipole–dipole interaction) effect on the collapse-revival behavior using several dimer models (composed of two kinds of two-state monomers with slightly different excitation energies) with different intermolecular distances in the presence of a two-mode squeezed coherent field, in which each mode is initially correlated. It is found that although the collapse-revival behavior is fairly overlapped and indistinct in the case of a noninteracting dimer under the present two-mode squeezed coherent field, the decrease in the intermolecular distance (the increase in the intermolecular interaction) resurrects relatively distinct collapse-revival behavior with longer collapse and revival times. By analyzing the quantum behavior from the viewpoint of the dynamics of two-mode Pegg–Barnett photon-phase distributions and off-diagonal dimer density matrices, this feature is found to closely relate to a significant change in the degree of contribution between one- and two-photon processes caused by the variation in the intermolecular interaction. © 2002 American Institute of Physics.
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42.50.Dv Quantum state engineering and measurements
33.80.-b Photon interactions with molecules

“Coarse” stability and bifurcation analysis using stochastic simulators: Kinetic Monte Carlo examples

Alexei G. Makeev, Dimitrios Maroudas, and Ioannis G. Kevrekidis

J. Chem. Phys. 116, 10083 (2002); http://dx.doi.org/10.1063/1.1476929 (9 pages) | Cited 54 times

Online Publication Date: 30 May 2002

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We implement a computer-assisted approach that, under appropriate conditions, allows the bifurcation analysis of the “coarse” dynamic behavior of microscopic simulators without requiring the explicit derivation of closed macroscopic equations for this behavior. The approach is inspired by the so-called time-stepper based numerical bifurcation theory. We illustrate the approach through the computation of both stable and unstable coarsely invariant states for kinetic Monte Carlo models of three simple surface reaction schemes. We quantify the linearized stability of these coarsely invariant states, perform pseudoarclength continuation, detect coarse limit point and coarse Hopf bifurcations, and construct two-parameter bifurcation diagrams. © 2002 American Institute of Physics.
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05.40.-a Fluctuation phenomena, random processes, noise, and Brownian motion
02.50.Ey Stochastic processes
05.45.-a Nonlinear dynamics and chaos

Association–dissociation in solution/Long-time relaxation prediction by a mode coupling approach

Wolfgang Naumann

J. Chem. Phys. 116, 10092 (2002); http://dx.doi.org/10.1063/1.1477929 (7 pages) | Cited 1 time

Online Publication Date: 30 May 2002

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It is shown that the correct power-law decay predictions for the long-time relaxation of reversible diffusion-influenced reactions, which were recently derived by Gopich, Ovchinnikov and Szabo with a hydrodynamic approach [Phys. Rev. Lett. 79, 922 (2001)] can also be obtained with an appropriate mode-coupling approximation after rephrasing the problem into a memory function approach. This is explicitly demonstrated for association–dissociation in the extreme situations of the target and trapping model. By constructing the memory function approximations via the solution of self-consistent equations for the relevant time-correlation functions, this method paves the way for a systematic study of higher reactant concentration effects. Moreover, the fact that the correlation decay equations derived have the typical form for stochastic approaches facilitates comparison with other theoretical concepts. © 2002 American Institute of Physics.
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82.30.Nr Association, addition, insertion, cluster formation
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)

The influence of finite bandwidth excitation sources in degenerate four-wave mixing spectroscopy

Thierry A. W. Wasserman, Patrick H. Vaccaro, and Bruce R. Johnson

J. Chem. Phys. 116, 10099 (2002); http://dx.doi.org/10.1063/1.1476936 (23 pages) | Cited 6 times

Online Publication Date: 30 May 2002

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The effects of finite-bandwidth excitation are incorporated into the theoretical treatment of degenerate four-wave mixing (DFWM) spectroscopy by employing a general description of applied electromagnetic fields, thereby permitting the transient nature of typical laser sources to be taken explicitly into account. A computationally efficient perturbative (weak-field) formalism is developed by exploiting exponentially-decaying wave forms as a flexible basis for expanding the temporal envelope functions of incident quasimonochromatic pulses. This approach has the distinct advantage of yielding time-domain response integrals that can be evaluated analytically since they are almost as simple in form as their monochromatic counterparts. The resulting frequency-domain expressions for the induced DFWM signal polarization reflect the pronounced influence of finite-bandwidth excitation without needing to cobble such behavior into phenomenological rates for population decay and coherence dissipation. Spectral line shapes are examined for both stationary and nonstationary ensembles of isolated (gas-phase) target molecules, with the latter analyses demonstrating the systematic degradation of spatial discrimination introduced by nonmonochromatic implementations of the sub-Doppler (phase-conjugate) four-wave mixing scheme. In keeping with recent experimental findings, the precipitous drop in signal intensity accompanying the action of collisional dephasing processes is predicted to be tempered greatly by the use of short-duration excitation pulses. © 2002 American Institute of Physics.
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42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation

A highly efficient algorithm for electron repulsion integrals over relativistic four-component Gaussian-type spinors

Takeshi Yanai, Takahito Nakajima, Yasuyuki Ishikawa, and Kimihiko Hirao

J. Chem. Phys. 116, 10122 (2002); http://dx.doi.org/10.1063/1.1479351 (7 pages) | Cited 11 times

Online Publication Date: 30 May 2002

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In the previous studies, a highly efficient computational scheme has been proposed for the Dirac–Hartree–Fock and the Dirac–Kohn–Sham solutions using the generally contracted kinetically balanced Gaussian-type spinors (GTSs). Nevertheless, the calculations based on the full Dirac Hamiltonian are limited to small systems if they contain heavy elements. The bottleneck is the calculation of the two-electron repulsions over the four-component GTSs. The present paper presents an improved algorithm for evaluation of the four-component relativistic integrals. The new algorithm fully exploits the transfer relation of Head-Gordon and Pople (HGP) and the accompanying coordinate expansion (ACE) formulas of Ishida. The HGP transfer relation can reduce the four-component integrals into several common two-center integrals (p0∣q0), which can be computed rapidly using the ACE method. The algorithm is implemented into the four-component program system REL4D. Benchmark calculations demonstrate that a good performance is achieved, particularly for the calculation of the (SS∣SS) integrals. © 2002 American Institute of Physics.
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31.15.xr Self-consistent-field methods
31.30.J- Relativistic and quantum electrodynamic (QED) effects in atoms, molecules, and ions

Energy correction to simulation of volume polarization in reaction field theory

Daniel M. Chipman

J. Chem. Phys. 116, 10129 (2002); http://dx.doi.org/10.1063/1.1477928 (10 pages) | Cited 9 times

Online Publication Date: 30 May 2002

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Reaction field theory is useful in electronic structure calculations of solute properties to evaluate the effects of long-range electrostatic interactions with solvent. The reaction field is affected by quantum mechanical penetration of solute charge density outside the surface of the solvent cavity that nominally encloses it. Exact treatment of the effect of this charge penetration is possible, within the context of a simple dielectric continuum model, by invoking a certain volume polarization in addition to the usually treated surface polarization. However, with general nonspherical molecular cavities this is difficult and computationally demanding to handle, so more tractable means to optimally simulate or otherwise approximate the effect of volume polarization in terms of modified surface polarization are useful in practice. This work proposes and tests a simple formula for a correction term to estimate the energy difference between exact treatment of volume polarization and its optimal surface simulation. © 2002 American Institute of Physics.
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82.20.Wt Computational modeling; simulation

Varying the fraction of orbital exchange in density functional theory: Influence on nuclear magnetic resonance shielding constants

Philip J. Wilson and David J. Tozer

J. Chem. Phys. 116, 10139 (2002); http://dx.doi.org/10.1063/1.1477926 (9 pages) | Cited 14 times

Online Publication Date: 30 May 2002

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A series of hybrid exchange-correlation functionals containing varying fractions of orbital exchange ξ = 0.0, 0.1, 0.2, … 1.0, are determined using conventional molecular thermochemical fits. The functionals are used to determine Kohn–Sham nuclear magnetic resonance shielding tensors for a series of small molecules involving first- and second-row atoms; results are compared with experimental values. On average, isotropic and anisotropic shieldings determined using the conventional coupled approach become progressively less accurate as ξ increases from 0.0 to 1.0. By contrast, isotropic and anisotropic shieldings determined from the hybrid Kohn–Sham densities using the uncoupled multiplicative Kohn–Sham (MKS) approach [Chem. Phys. Lett. 337, 341 (2001)] improve significantly as ξ increases from 0.0 to 0.2–0.3; optimal results are more than three times as accurate as the corresponding coupled results. As ξ is further increased, the MKS results degrade. The quality of the Kohn–Sham highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) eigenvalue difference in the MKS calculations is investigated by comparing it with values determined from coupled cluster Brueckner doubles densities. In line with the shielding observations, optimal HOMO–LUMO differences are obtained near ξ = 0.3. © 2002 American Institute of Physics.
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33.25.+k Nuclear resonance and relaxation
31.15.E- Density-functional theory
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Spectroscopic determination of the water dimer intermolecular potential-energy surface

N. Goldman, R. S. Fellers, M. G. Brown, L. B. Braly, C. J. Keoshian, C. Leforestier, and R. J. Saykally

J. Chem. Phys. 116, 10148 (2002); http://dx.doi.org/10.1063/1.1476932 (16 pages) | Cited 43 times

Online Publication Date: 30 May 2002

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Two polarizable six-dimensional water dimer intermolecular potential surfaces have been determined by fitting the distributed multipole ASP (anisotropic site potential) potential form to microwave, terahertz, and midinfrared cavity ringdown (D2O)2 spectra via a rigorous calculation of the water dimer eigenstates with the PSSH (pseudo-spectral split Hamiltonian) method. The fitted potentials accurately reproduce most ground-state vibration-rotation-tunneling spectra and yield excellent second virial coefficients for both H2O and D2O. The calculated dimer structure and dipole moment are close to those determined from microwave spectroscopy and high level ab initio calculations, except that the O–O distance (2.952 Å) is significantly shorter than the currently accepted experimental value. The dimer binding energy (4.85 kcal/mol) is considerably smaller than the accepted experimental result, but in excellent agreement with recent theoretical results, as are the acceptor switching and donor–acceptor interchange tunneling barriers and the cyclic water trimer and tetramer structures and binding energies. © 2002 American Institute of Physics.
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34.20.Gj Intermolecular and atom-molecule potentials and forces
33.20.Bx Radio-frequency and microwave spectra
33.20.Ea Infrared spectra
33.20.Vq Vibration-rotation analysis

Formation of anion fragments from gas-phase glycine by low energy (0–15 eV) electron impact

Sascha Gohlke, Andrzej Rosa, Eugen Illenberger, Frank Brüning, and Michael A. Huels

J. Chem. Phys. 116, 10164 (2002); http://dx.doi.org/10.1063/1.1479348 (6 pages) | Cited 51 times

Online Publication Date: 30 May 2002

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We have measured the formation of anion fragments in gas phase glycine (H2NCH2COOH) via dissociative electron attachment (DEA) reactions in the 0–15 eV electron energy range, using a monochromatic electron beam and mass spectrometric detection of the negative ions. By far the most intense product observed is the closed shell glycine anion (H2NCH2COO)− which appears from a low-energy resonance with a peak located at 1.4 eV and a cross section in the range 10−16 cm2. The corresponding precursor ion can be characterized by electron attachment into the empty π∗ orbital of the −COOH group as recently assigned from electron transmission experiments and ab initio self-consistent field calculations [Aflatooni, Hitt, Gallup, and Burrow, J. Chem. Phys. 115, 6489 (2001)]. This precursor state is also observed to decompose (with much lower intensity) yielding a negative ion fragment with 58 amu, which is attributed to anions of the stoichiometric composition H2C2O2− or H4C2NO−. A further prominent DEA peak is observed at 6 eV, which is likely associated with a core excited resonance, and leads to formation of at least six different negative ion fragment species with the following mass numbers: 16 amu (O−/NH2−), 17 amu (OH−), 26 amu (CN−), 28 amu (H2CN−), 45 amu (HCO2−), 56 amu (H2C2NO−). © 2002 American Institute of Physics.
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34.80.Gs Molecular excitation and ionization
34.80.Lx Recombination, attachment, and positronium formation

Vibrational effects in a weakly-interacting quantum solvent: The CO molecule in 4He gas and in 4He droplets

F. Paesani and F. A. Gianturco

J. Chem. Phys. 116, 10170 (2002); http://dx.doi.org/10.1063/1.1478690 (13 pages) | Cited 26 times

Online Publication Date: 30 May 2002

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The coupling between the intermolecular motion and the internal vibrational coordinate in the He–CO system is computed at the post-Hartree–Fock level using the DFT+DISP model already employed by us for similar systems and reviewed here in the main text. The quality of the computation of such weak effects is compared with other, earlier model calculations and then used for the evaluation of the vibrational relaxation cross sections of the CO molecule diluted in 4He gas. A further assessment of the vibrational coupling is carried out by computing, with a stochastic approach that employs the Diffusion Monte Carlo method, the effects on the vibrational frequency of the CO impurity from its immersion in 4He droplets of variable size. Both sets of results are analyzed and discussed to gauge the reliability of the computed coupling vis-à-vis one of those suggested by earlier calculations. This study provides further evidence on the difficulty of quantitatively obtaining from calculations the extremely small effects connected with molecular vibrational features in this system and caused by the weak interaction between the title molecule and a quantum solvent like 4He. © 2002 American Institute of Physics.
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33.20.Tp Vibrational analysis
31.15.xr Self-consistent-field methods
31.15.E- Density-functional theory

Dynamically localized wave packets as a tool to study the dynamics of the LiNC⇌LiCN isomerization reaction

E. Martín-Fierro, F. Borondo, J. M. Gomez Llorente, and R. M. Benito

J. Chem. Phys. 116, 10183 (2002); http://dx.doi.org/10.1063/1.1476935 (14 pages)

Online Publication Date: 30 May 2002

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The dynamics of the isomerization reaction LiNC⇌LiCN is considered at a quantum level. This study is performed with the aid of dynamically localized wave packets, which provide a wealth of information about the relevant parts of the molecular phase space. Three cases are considered, corresponding to energies close to the threshold for isomerization. The initial position of the packet is always localized in the LiNC well (which is the most stable), and different energies and distributions among the vibrational modes of the molecule are used. The packet will then explore phase space and, as a result of these dynamics, different time scales (including those from intramolecular energy transfer processes) exist, which appear in the low resolution features of the corresponding spectra. The characteristics of these spectra are discussed and analyzed using a variety of techniques, making extensive use of the ideas of nonlinear dynamics, and the conditions for the applicability of statistical theories to evaluate reaction rates in this case are considered. © 2002 American Institute of Physics.
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82.30.Qt Isomerization and rearrangement
82.20.Db Transition state theory and statistical theories of rate constants
82.20.Pm Rate constants, reaction cross sections, and activation energies

SVRT calculation for bond-selective reaction H+HOD→H2+OD, HD+OH

Xin Zhang, KeLi Han, and John Z. H. Zhang

J. Chem. Phys. 116, 10197 (2002); http://dx.doi.org/10.1063/1.1478691 (4 pages) | Cited 7 times

Online Publication Date: 30 May 2002

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The semirigid vibrating rotor target (SVRT) model is applied to study bond-selective branching reaction H+HOD→H2+OD, HD+OH on the Schatz–Elgersma potential energy surface when one of the stretching modes of HOD is excited. Using the SVRT model, the time-dependent wavepacket calculation is carried out in four-mathematical dimensions with the remaining two internal coordinates fixed. The reaction probabilities for producing two product branches are calculated from two separate dynamics calculations. The results show that for reaction H+HOD(100)→HD+OH when O–D stretching mode is excited, the SVRT calculation gives excellent results. The SVRT result is slightly worse for reaction H+HOD(001)→H2+OD when the O–H stretching mode is excited. The current study demonstrates that the SVRT model is also applicable for giving accurate results for polyatomic reactions when the chemical bond that is broken is vibrationally excited. © 2002 American Institute of Physics.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.-w Chemical kinetics and dynamics

Gas phase trapped ion studies of collisionally formed MgC60+ complexes

R. I. Thompson, M. Welling, H. A. Schuessler, and H. Walther

J. Chem. Phys. 116, 10201 (2002); http://dx.doi.org/10.1063/1.1478698 (11 pages) | Cited 2 times

Online Publication Date: 30 May 2002

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A series of experiments were performed to study the formation of MgC60+ complexes via Mg++C60 collisions, and to study the structure and stability of the resulting complexes via optical spectroscopy and mass spectrometry. Collision experiments were carried out in a linear ion trap apparatus under conditions of controlled collision energies, reactant densities, and buffer gas pressure. Ion trap mass spectrometry was used to measure the relative reaction rates for complex formation, charge transfer, and fragmentation reactions in Mg++C60 collisions. Laser-induced photodissociation was then used to study the MgC60+ complexes in order to measure their stability and identify their molecular structure. Absolute photodissociation cross sections were determined for complexes generated at low collision energies and reactant densities over the wavelength range of 1300–280 nm and were in the range from 2×10−20 cm2 to 5×10−17 cm2 in magnitude. The shape of the generated cross-section curve indicates that at low collision energies most of these complexes are exohedrals. More refined measurements of samples generated at optimum buffer gas pressures and higher collision energies demonstrated the existence of a second more slowly dissociating fraction that apparently results from the presence of endohedral complexes. © 2002 American Institute of Physics.
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36.40.Mr Spectroscopy and geometrical structure of clusters
36.40.Jn Reactivity of clusters
82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
82.30.Nr Association, addition, insertion, cluster formation
33.15.Ta Mass spectra
33.80.Gj Diffuse spectra; predissociation, photodissociation

Molecules in high spin states: The millimeter and submillimeter spectrum of the MnS radical (X 6Σ+)

J. M. Thompsen, M. A. Brewster, and L. M. Ziurys

J. Chem. Phys. 116, 10212 (2002); http://dx.doi.org/10.1063/1.1476931 (9 pages) | Cited 9 times

Online Publication Date: 30 May 2002

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The pure rotational spectrum of MnS (v = 0) in its X 6Σ+ ground state has been recorded using millimeter and submillimeter direct absorption techniques in the range 160–502 GHz. MnS was synthesized in the gas phase by the reaction of manganese vapor and CS2 in a high-temperature Broida-type oven. Fourteen rotational transitions for this radical were measured, each consisting of six fine-structure components. In the lower rotational lines, hyperfine structure, arising from the 55Mn nuclear spin of 5/2, was also resolved in each spin component. These data were analyzed using a case (b) Hamiltonian, and rotational, fine structure, and hyperfine parameters determined for MnS. In the analysis, the third-order correction to the spin-rotation interaction, γS, and the fourth-order spin–spin coupling term, θ, were found necessary for an acceptable fit. The hyperfine constants determined suggest that MnS is more covalent than MnO, but more ionic than MnH. There additionally appears to be considerable sdσ hybridization in molecular orbital formation for this molecule. Bond lengths of the 3d transition-metal sulfides were compared as well, and those of MnS, CuS, and TiS do not follow the trend of their oxide analogs. This result indicates that there are significant bonding differences between transition-metal sulfides and transition-metal oxides. © 2002 American Institute of Physics.
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33.20.Bx Radio-frequency and microwave spectra
33.20.Sn Rotational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
31.30.Gs Hyperfine interactions and isotope effects
33.15.Pw Fine and hyperfine structure

Carbon 1s photoelectron spectroscopy of CF4 and CO: Search for chemical effects on the carbon 1s hole-state lifetime

Thomas X. Carroll, Knut J. Børve, Leif J. Sæthre, John D. Bozek, Edwin Kukk, Jeffrey A. Hahne, and T. Darrah Thomas

J. Chem. Phys. 116, 10221 (2002); http://dx.doi.org/10.1063/1.1476933 (8 pages) | Cited 39 times

Online Publication Date: 30 May 2002

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Carbon 1s photoelectron spectra for CF4 and CO have been measured at several photon energies near the carbon 1s threshold. The spectra have been analyzed in terms of the vibrational structure and the natural linewidth. For CO the vibrational structure shows evidence for anharmonicity in both the energy spacing and the intensity. Analysis of the results using an anharmonic model gives an equilibrium bond length for core-ionized CO that is 4.85 pm shorter than that of neutral CO. For CF4, the vibrational structure is very weak, and the analysis shows that the change in equilibrium CF bond length upon ionization is no more than 0.54 pm. Ab initio theoretical calculations give results in accord with these bond-length changes. The unusually small bond-length contraction in CF4 can be understood in terms of nonbonded fluorine–fluorine repulsion. The natural linewidth for core-ionized CO, 95±5 meV, is essentially the same as that of CH4. This result is in contrast with expectations based on the one-center model of Auger decay and earlier predictions based on semiempirical molecular orbital theory. More recent calculations indicate, however, that there is only a small difference between CO and CH4, in agreement with the observed result. For CF4, the natural linewidth is 77±6 meV. This value differs from that for CH4 in the direction expected from the electronegativities of hydrogen and fluorine, but is greater than the prediction based on semiempirical theory. The natural linewidth for CO with a carbon 1s electron excited to the 2π resonance is 83 meV, which is significantly less than is found for core-ionized CO. Although this difference is supported by theoretical calculations, the direction of the difference is counterintuitive. An overview is presented of the current state of experimental and theoretical knowledge on carbon 1s linewidths. © 2002 American Institute of Physics.
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33.60.+q Photoelectron spectra
33.20.Tp Vibrational analysis
31.15.A- Ab initio calculations
31.15.bu Semi-empirical and empirical calculations (differential overlap, Hückel, PPP methods, etc.)
33.15.Dj Interatomic distances and angles

Fragmentation path for hydrogen atom dissociation from methoxy radical

Nicholas D. K. Petraco, Wesley D. Allen, and Henry F. Schaefer

J. Chem. Phys. 116, 10229 (2002); http://dx.doi.org/10.1063/1.1477180 (9 pages) | Cited 19 times

Online Publication Date: 30 May 2002

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Salient features of the potential surface for hydrogen atom dissociation from the methoxy radical (CH3O) have been investigated via high-level coupled-cluster methods using a TZ2P(f,d) basis set for geometry optimization and harmonic vibrational analyses and the correlation-consistent cc-pVXZ (X = 2–6) series for final energetic determinations and extrapolations. Of central concern for continuing photofragmentation dynamics experiments is the Cs-symmetry 2A′ transition state for dissociation, which TZ2P(f,d) RCCSD(T) theory locates at a critical C–H distance of 1.79 Å with a barrier frequency of 947i cm−1. Our zero-point-corrected focal-point extrapolations place this transition state 4.7 kcal mol−1 above the CH2O+H products and yield a dissociation energy of 20.1 kcal mol−1; the latter differs from the most reliable experimental values by only 0.2–0.3 kcal mol−1. A revised enthalpy of formation, ΔHf,0°(CH3O) = 6.5 kcal mol−1, is proposed. Disappointingly, TZ2P(f,d) UB3LYP theory underestimates the CH2O+H association barrier by 2.3 kcal mol−1, missing about half the barrier height. The complete set of TZ2P(f,d) RCCSD(T) data for structures and frequencies coupled with final focal-point energetics provides definitive values for parameters essential to the analysis of experimental photofragmentation rate profiles. © 2002 American Institute of Physics.
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82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.20.Bc State selected dynamics and product distribution

Electronic structure and spectroscopic properties of electronic states of ScC3 and ScC3−

S. Roszak, D. Majumdar, and K. Balasubramanian

J. Chem. Phys. 116, 10238 (2002); http://dx.doi.org/10.1063/1.1477181 (9 pages) | Cited 8 times

Online Publication Date: 30 May 2002

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Theoretical studies on the electronic and thermodynamic properties of several electronic states are presented for the ScC3 molecule and the ScC3− anion employing state-of-the-art techniques that included up to 13 million configurations. The ground and the low-lying electronic states of these two species have been predicted to have C2v ring structures. On the basis of our computed results on the low-lying excited electronic states, we have suggested assignment of the observed anion photodetachment spectra of ScC3−. Thermodynamic properties of reactions involving ScC3 are corrected using the computed gas-phase properties of the molecule and the partition functions. Thermodynamic functions of ScC3− are also determined by fitting the available experimental data with thermodynamic energy cycles. The nature of bonding in the neutral and the anionic carbides has been considered. © 2002 American Institute of Physics.
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33.20.Tp Vibrational analysis
33.80.Eh Autoionization, photoionization, and photodetachment
31.15.xr Self-consistent-field methods
31.15.vq Electron correlation calculations for polyatomic molecules
31.15.xp Perturbation theory
31.15.E- Density-functional theory

Cobalt–benzene cluster anions: Mass spectrometry and negative ion photoelectron spectroscopy

M. Gerhards, O. C. Thomas, J. M. Nilles, W.-J. Zheng, and K. H. Bowen

J. Chem. Phys. 116, 10247 (2002); http://dx.doi.org/10.1063/1.1477924 (6 pages) | Cited 51 times

Online Publication Date: 30 May 2002

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(Cobalt)n(benzene)m− cluster anions, (n,m) were generated by laser vaporization and studied by both mass spectrometry and anion photoelectron spectroscopy. Our assignment of the photoelectron spectrum of the (1,2) cluster anion suggests that it possesses a sandwich structure with the cobalt atom located between two parallel benzene rings, that the ground state of this anion is a singlet, and that the ground state of its corresponding neutral is a doublet. The photoelectron spectra of cobalt-rich cluster anions of the form (n,1) are interpreted as cobalt metal cluster anions which have been solvent-stabilized by their interaction with, in each case, a single benzene molecule. The photoelectron spectra of the benzene-rich cluster anions, (2,3), (2,2), and (3,3), are tentatively interpreted as suggesting extended sandwich structures for these anion complexes. © 2002 American Institute of Physics.
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36.40.Mr Spectroscopy and geometrical structure of clusters
33.15.Ta Mass spectra
33.60.+q Photoelectron spectra

Experimental and theoretical study of benzene (acetonitrile)n clusters, n = 1–4

M. Samy El-Shall, George M. Daly, and Douglas Wright

J. Chem. Phys. 116, 10253 (2002); http://dx.doi.org/10.1063/1.1476317 (14 pages) | Cited 5 times

Online Publication Date: 30 May 2002

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Well-resolved spectra of benzene–acetonitrile binary clusters BAn, with n = 1–4 have been obtained by the (one-color) resonant two-photon ionization technique using the benzene’s B2u←A1g 000 and 601 resonances. The spectra reveal a rapid increase in complexity with the number of acetonitrile molecules in the cluster, associated with van der Waal modes and isomeric forms. While only single cluster origins are found for the benzene–acetonitrile (BA) and the BA2 clusters, two and four distinct isomers are identified for the BA3 and BA4 clusters, respectively. The origins of the BA and BA2 clusters are blueshifted with respect to the free benzene molecule by 38 cm−1 and 26 cm−1, respectively. Monte Carlo (MC) simulations reveal two types of isomeric structures of the BAn clusters. The clusters containing an even number of the acetonitrile molecules (BA2, BA4, and BA6) are dominated by acetonitrile anti-parallel paired dimers. The BA3 cluster consists of a cyclic acetonitrile trimer parallel to the benzene ring. In the BA5 clusters, the acetonitrile molecules are assembled in a cyclic trimer + a paired dimer configuration or in two paired dimers + a single monomer structure. The R2PI spectra, in conjunction with the MC structural models and simple energetic arguments, provide a reasonably compelling picture of the spectroscopic and dynamical phenomena associated with dipole pairing molecular cluster systems. © 2002 American Institute of Physics.
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36.40.Mr Spectroscopy and geometrical structure of clusters
33.80.Wz Other multiphoton processes
33.80.Eh Autoionization, photoionization, and photodetachment

Double-resonance overtone photofragment spectroscopy of trans-HONO. II. State- and time-resolved dissociation and OH-product state distributions

F. Reiche, B. Abel, R. D. Beck, and T. R. Rizzo

J. Chem. Phys. 116, 10267 (2002); http://dx.doi.org/10.1063/1.1471236 (10 pages) | Cited 11 times

Online Publication Date: 30 May 2002

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Linewidths, unimolecular dissociation rates and product state distributions (PSDs) have been measured for single rovibratational states of the ν1 = 5–7 levels of gas-phase trans-nitrous acid (HONO) by double-resonance overtone photofragment spectroscopy (DROPS). The linewidth measurements, together with the unimolecular dissociation rates in 5ν1, suggest that the intramolecular dynamics are not statistical but rather depend sensitively upon specific intramolecular couplings and the vibrational character of the initial state. Comparison with calculated rate constants from statistical unimolecular rate theory reveals that intramolecular vibrational energy redistribution (IVR) is the rate determining step in the dissociation of HONO subsequent to vibrational overtone excitation. Despite this, we find the measured product state distributions to be close to the predictions of statistical theory. We explain these observations in terms of a simple tier model incorporating hierarchical IVR. The experimental findings underscore the importance of the preparation technique, and hence the nature of the initially excited state, in determining the subsequent intramolecular dynamics. © 2002 American Institute of Physics.
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82.50.-m Photochemistry
33.80.Gj Diffuse spectra; predissociation, photodissociation
07.57.-c Infrared, submillimeter wave, microwave and radiowave instruments and equipment
07.60.-j Optical instruments and equipment
82.20.Pm Rate constants, reaction cross sections, and activation energies
33.70.Jg Line and band widths, shapes, and shifts

Binding to gold(0): Accurate computational methods with application to AuNH3

Nicholas A. Lambropoulos, Jeffrey R. Reimers, and Noel S. Hush

J. Chem. Phys. 116, 10277 (2002); http://dx.doi.org/10.1063/1.1473197 (10 pages) | Cited 18 times

Online Publication Date: 30 May 2002

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The nature of the bonding of molecules to neutral gold atoms or surfaces is of wide interest, particularly with regard to recent molecular electronics experiments involving molecules linked to gold electrodes and nanoclusters. Here, the fundamental problem of accurate calculation of gold atom–ligand interactions is addressed, and a best-possible estimate for the binding energy of AuNH3 is obtained via coupled-cluster and density-functional calculations using series of Gaussian, Slater, and plane-wave basis sets. Poor convergence of both coupled-cluster and density-functional calculations toward the infinite basis-set limit is obtained from the Gaussian basis sets; using Slater basis sets, convergence is more rapid while plane-wave basis sets easily reached convergence. A total of 24 Gaussian basis sets are examined, and a method is introduced for determining if a particular basis set is sufficiently balanced in its treatment of the metal and its ligand. For balanced basis sets, better estimates of the binding energy are obtained neglecting corrections for basis-set superposition error. Various treatment of relativistic effects are examined including the use of relativistic effective core potentials (RECPs), ultrasoft pseudopotentials, and all electron scalar and full spin–orbit zero-order regular approximation calculations. While the use of RECPs has minimal affect, use of ultrasoft pseudopotentials and neglect of spin–orbit coupling both result in underestimation of the binding energy by 2–3 kcal mol−1 (15%–20%), as does the neglect of triples excitations in coupled-cluster theory. The PW91, B3LYP, BLYP, and LDA density functionals were investigated and of these only PW91 predicted binding energies and geometries in qualitative agreement with the coupled-cluster results. The AuNH3 complex is found to be a realistic model for the bonding of NH3 to a gold (111) surface, the primary differences being the prediction of charge transfer within the complex and associated significantly stronger binding. This may have profound implications for molecular electronics applications in which small gold clusters are used to represent macroscopic electrodes. © 2002 American Institute of Physics.
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31.15.bw Coupled-cluster theory
31.15.E- Density-functional theory
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy

Spectroscopic properties of lead hexamer and its ions (Pb6, Pb6+, Pb6−)

Cunyuan Zhao and K. Balasubramanian

J. Chem. Phys. 116, 10287 (2002); http://dx.doi.org/10.1063/1.1476311 (10 pages) | Cited 11 times

Online Publication Date: 30 May 2002

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We have computed the optimized geometries and energy separations of low-lying electronic states of the lead hexamer (Pb6) and its positive and negative ions. Our techniques have included high level relativistic electron correlation techniques such as complete active space multiconfiguration self-consistent field (CAS-MCSCF) method followed by large scale multireference singles plus doubles configuration interaction (MRSDCI) and relativistic configuration interaction (RCI) computations that included up to 16 million configurations. Our computed results have facilitated the assignment of the anion photodetachment spectra of Pb6− and also in the prediction of the properties of yet to be observed electronic states. A  1A1g tetragonal bipyramid structure (D4h symmetry) is found as the ground state for Pb6. The excitation energy, atomization energies, ionization potentials, and vertical and adiabatic electron affinities are computed and compared with the experimental results. We have assigned the observed X, A, B, C, D, and E states of the anion photoelectron spectra of Pb6−, and discuss spin–orbit versus Jahn-Teller effects. © 2002 American Institute of Physics.
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31.30.J- Relativistic and quantum electrodynamic (QED) effects in atoms, molecules, and ions
31.15.V- Electron correlation calculations for atoms, ions and molecules
33.15.Bh General molecular conformation and symmetry; stereochemistry
31.15.xr Self-consistent-field methods
33.80.Eh Autoionization, photoionization, and photodetachment
33.15.Fm Bond strengths, dissociation energies
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