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8 Jul 2004

Volume 121, Issue 2, pp. 625-1168

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Evidence for a bound HeH2 halo molecule by diffraction from a transmission grating

Anton Kalinin, Oleg Kornilov, Lev. Yu Rusin, and J. Peter Toennies

J. Chem. Phys. 121, 625 (2004); http://dx.doi.org/10.1063/1.1768935 (3 pages) | Cited 11 times

Online Publication Date: 24 June 2004

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The HeH2 van der Waals complex has been identified in a molecular beam produced by a cryogenic (T0 = 24.7 K) free jet expansion of a 1% H2 mixture in 99% 4He gas. The weakly bound HeH2 complexes in the beam are identified via their first order diffraction angles after passing through a 100 nm period transmission grating. An electron impact mass spectrometer analysis of the diffraction patterns is used to discriminate against ion fragments of the constituent gas clusters. © 2004 American Institute of Physics.
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33.15.Ta Mass spectra
37.20.+j Atomic and molecular beam sources and techniques
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
34.80.-i Electron and positron scattering

Exploring molecular complexity: Conical intersections and NH3 photodissociation

David R. Yarkony

J. Chem. Phys. 121, 628 (2004); http://dx.doi.org/10.1063/1.1772351 (4 pages) | Cited 10 times

Online Publication Date: 24 June 2004

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The role of conical intersections in the photodissociation of the  1A2″ state of NH3 is investigated using extended atomic basis sets and a configuration state function expansion of approximately 8.5 million terms. A previously unknown portion of the 1 1A–2 1A seam of conical intersections with only Cs symmetry is located. This portion of the seam is readily accessible from the equilibrium geometry of the  1A2″ state. These conical intersections are expected to play a role in the competition between adiabatic and nonadiabatic pathways for NH3( 1A2″) photodissociation. © 2004 American Institute of Physics.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
33.15.Bh General molecular conformation and symmetry; stereochemistry

Tetrasulfur, S4: Rotational spectrum, interchange tunneling, and geometrical structure

M. C. McCarthy, S. Thorwirth, C. A. Gottlieb, and P. Thaddeus

J. Chem. Phys. 121, 632 (2004); http://dx.doi.org/10.1063/1.1769372 (4 pages) | Cited 10 times

Online Publication Date: 24 June 2004

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The rotational spectrum of S4 has been observed for the first time in an electrical discharge through sulfur vapor. Two techniques have been used: Fourier transform microwave spectroscopy and long-path millimeter-wave absorption spectroscopy. Small, but systematic shifts of the measured transition frequencies of the normal isotopic species indicate that S4 has C2v symmetry but with a low-lying transition state of D2h symmetry, yielding interchange tunneling at 14.1(2) kHz in its ground vibrational state. From the rotational constants of the normal and the single 34S isotopic species, an experimental (r0) structure has been derived: S4 is a singlet planar trapezoid with a terminal bond length of 1.899(7) Å, a central bond of 2.173(32) Å, and an S-S-S angle of 103.9(8)°. Like thiozone (S3), S4 is a candidate for detection in the atmosphere of the Jovian moon Io and in other astronomical sources. © 2004 American Institute of Physics.
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33.20.Sn Rotational analysis
33.20.Bx Radio-frequency and microwave spectra
33.70.Jg Line and band widths, shapes, and shifts
33.20.Tp Vibrational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.15.Dj Interatomic distances and angles
33.15.Bh General molecular conformation and symmetry; stereochemistry
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Higher order and infinite Trotter-number extrapolations in path integral Monte Carlo

L. Brualla, K. Sakkos, J. Boronat, and J. Casulleras

J. Chem. Phys. 121, 636 (2004); http://dx.doi.org/10.1063/1.1760512 (8 pages) | Cited 15 times

Online Publication Date: 24 June 2004

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Improvements beyond the primitive approximation in the path integral Monte Carlo method are explored both in a model problem and in real systems. Two different strategies are studied: The Richardson extrapolation on top of the path integral Monte Carlo data and the Takahashi-Imada action. The Richardson extrapolation, mainly combined with the primitive action, always reduces the number-of-beads dependence, helps in determining the approach to the dominant power law behavior, and all without additional computational cost. The Takahashi-Imada action has been tested in two hard-core interacting quantum liquids at low temperature. The results obtained show that the fourth-order behavior near the asymptote is conserved, and that the use of this improved action reduces the computing time with respect to the primitive approximation. © 2004 American Institute of Physics.
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61.20.Ja Computer simulation of liquid structure
02.60.Ed Interpolation; curve fitting
02.70.Uu Applications of Monte Carlo methods

Cumulative isomerization probability studied by various transition state wave packet methods including the MCTDH algorithm. Benchmark: HCN→CNH isomerization

B. Lasorne, F. Gatti, E. Baloitcha, H.-D. Meyer, and M. Desouter-Lecomte

J. Chem. Phys. 121, 644 (2004); http://dx.doi.org/10.1063/1.1760713 (11 pages) | Cited 7 times

Online Publication Date: 24 June 2004

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The 3D cumulative isomerization probability N(E) for the transfer of a light particle between two atoms is computed by one time-independent and two time-dependent versions of the transition state wave packet (TSWP) method. The time-independent method is based on the direct expansion of the microcanonical projection operator on Chebyshev polynomials. In the time-dependent TSWP methods, the propagations are carried out by the split operator scheme and the multiconfiguration time-dependent Hartree (MCTDH) algorithm. This is the very first implementation of the TSWP method in the Heidelberg MCTDH package [G. W. Worth, M. H. Beck, A. Jäckle, and H.-D. Meyer, The MCDTH package, Version 8.2 (2000); H.-D Meyer, Version 8.3 (2002). See http://www.pci.uni-heidelberg.de/tc/usr/mctdh/]. The benchmark is the HCN→CNH isomerization for zero total angular momentum. Particular insights are given into the tunneling region. In larger systems, the time-dependent version of TSWP making use of the MCTDH algorithm will permit to treat more and more modes quantum mechanically, for very accurate results. Therefore, it was important to calibrate the implementation. Besides, we also assess the efficiency of a reduced dimensionality approach by comparing the new exact 3D calculations of N(E) for the HCN→CNH isomerization with results obtained via 1D or 2D active subspaces. This suggests that, it should be possible to take directly benefit of the present 3D approaches, adapted for triatomic Jacobi coordinates to compute N(E) for H-transfer in larger systems, via three active coordinates. The prerequisite is then the simplification of the reduced 3D kinetic energy operator with rigid constraint to take the form corresponding to a pseudo triatomic system in Jacobi coordinates with effective masses. This last step is checked in the methoxy radical and malonaldehyde. Finally, different ways to obtain reliable eigenvectors of the flux operator associated with a dividing surface are revisited. © 2004 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

Asymptotic correction of the exchange–correlation kernel of time-dependent density functional theory for long-range charge-transfer excitations

Oleg Gritsenko and Evert Jan Baerends

J. Chem. Phys. 121, 655 (2004); http://dx.doi.org/10.1063/1.1759320 (6 pages) | Cited 42 times

Online Publication Date: 24 June 2004

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Time-dependent density functional theory (TDDFT) calculations of charge-transfer excitation energies ωCT are significantly in error when the adiabatic local density approximation (ALDA) is employed for the exchange–correlation kernel fxc. We relate the error to the physical meaning of the orbital energy of the Kohn–Sham lowest unoccupied molecular orbital (LUMO). The LUMO orbital energy in Kohn–Sham DFT—in contrast to the Hartree–Fock model—approximates an excited electron, which is correct for excitations in compact molecules. In CT transitions the energy of the LUMO of the acceptor molecule should instead describe an added electron, i.e., approximate the electron affinity. To obtain a contribution that compensates for the difference, a specific divergence of fxc is required in rigorous TDDFT, and a suitable asymptotically correct form of the kernel fxcasymp is proposed. The importance of the asymptotic correction of fxc is demonstrated with the calculation of ωCT(R) for the prototype diatomic system HeBe at various separations R(He–Be). The TDDFT–ALDA curve ωCT(R) roughly resembles the benchmark ab initio curve ωCTCISD(R) of a configuration interaction calculation with single and double excitations in the region R = 1–1.5 Å, where a sizable He–Be interaction exists, but exhibits the wrong behavior ωCT(R)≪ωCTCISD(R) at large R. The TDDFT curve obtained with fxcasymp however approaches ωCTCISD(R) closely in the region R = 3–10 Å. Then, the adequate rigorous TDDFT approach should interpolate between the LDA/GGA ALDA xc kernel for excitations in compact systems and fxcasymp for weakly interacting fragments and suitable interpolation expressions are considered. © 2004 American Institute of Physics.
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31.15.E- Density-functional theory
31.15.A- Ab initio calculations
31.15.vn Electron correlation calculations for diatomic molecules
34.70.+e Charge transfer
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy

Dissipative quantum dynamics with the surrogate Hamiltonian approach. A comparison between spin and harmonic baths

David Gelman, Christiane P. Koch, and Ronnie Kosloff

J. Chem. Phys. 121, 661 (2004); http://dx.doi.org/10.1063/1.1759312 (11 pages) | Cited 16 times

Online Publication Date: 24 June 2004

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The dissipative quantum dynamics of an anharmonic oscillator coupled to a bath is studied with the purpose of elucidating the differences between the relaxation to a spin bath and to a harmonic bath. Converged results are obtained for the spin bath by the surrogate Hamiltonian approach. This method is based on constructing a system–bath Hamiltonian, with a finite but large number of spin bath modes, that mimics exactly a bath with an infinite number of modes for a finite time interval. Convergence with respect to the number of simultaneous excitations of bath modes can be checked. The results are compared to calculations that include a finite number of harmonic modes carried out by using the multiconfiguration time-dependent Hartree method of Nest and Meyer [J. Chem. Phys. 119, 24 (2003)]. In the weak coupling regime, at zero temperature and for small excitations of the primary system, both methods converge to the Markovian limit. When initially the primary system is significantly excited, the spin bath can saturate restricting the energy acceptance. An interaction term between bath modes that spreads the excitation eliminates the saturation. The loss of phase between two cat states has been analyzed and the results for the spin and harmonic baths are almost identical. For stronger couplings, the dynamics induced by the two types of baths deviate. The accumulation and degree of entanglement between the bath modes have been characterized. Only in the spin bath the dynamics generate entanglement between the bath modes. © 2004 American Institute of Physics.
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03.65.Ge Solutions of wave equations: bound states
02.50.Ga Markov processes

Classical spin and quantum-mechanical descriptions of geometric spin frustration

Dadi Dai and Myung-Hwan Whangbo

J. Chem. Phys. 121, 672 (2004); http://dx.doi.org/10.1063/1.1760749 (9 pages) | Cited 17 times

Online Publication Date: 24 June 2004

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Geometric spin frustration (GSF) in isolated plaquettes with local spin s, i.e., an equilateral-triangle spin trimer and a regular-tetrahedron spin tetramer, was examined on the basis of classical spin and quantum-mechanical descriptions to clarify their differences and similarities. An analytical proof was given for how the state degeneracy and the total spin S of their ground states depend on the local spin s. The quantum-mechanical conditions for the occurrence of GSF in isolated plaquettes were clarified, and their implications were explored. Corner sharing between plaquettes and how it affects GSF in the resulting spin systems was examined. © 2004 American Institute of Physics.
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75.10.Dg Crystal-field theory and spin Hamiltonians
75.25.-j Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source x-ray scattering, etc.)
75.10.Jm Quantized spin models, including quantum spin frustration

Efficient and reliable numerical integration of exchange-correlation energies and potentials

Andreas M. Köster, Roberto Flores-Moreno, and J. Ulises Reveles

J. Chem. Phys. 121, 681 (2004); http://dx.doi.org/10.1063/1.1759323 (10 pages) | Cited 51 times

Online Publication Date: 24 June 2004

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An adaptive numerical integrator for the exchange-correlation energy and potential is presented. It uses the diagonal elements of the exchange-correlation potential matrix as a grid generating function. The only input parameter is the requested grid tolerance. In combination with a defined cell function the adaptive grid generation scales almost linear with the number of basis functions in a system. With the adaptive numerical integrator the self-consistent field energy error, which is due to the numerical integration of the exchange-correlation energy, converges with increasing adaptive grid size to a reference value. The performance of the adaptive numerical integration is analyzed using molecules with first, second, and third row elements. Especially for transition metal systems the adaptive numerical integrator shows considerably improved performance and reliability. © 2004 American Institute of Physics.
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31.15.E- Density-functional theory
31.15.xr Self-consistent-field methods
02.30.Cj Measure and integration
02.60.Jh Numerical differentiation and integration

Potential energy surface discontinuities in local correlation methods

Nicholas J. Russ and T. Daniel Crawford

J. Chem. Phys. 121, 691 (2004); http://dx.doi.org/10.1063/1.1759322 (6 pages) | Cited 31 times

Online Publication Date: 24 June 2004

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We have examined the occurence of discontinuities in bond-breaking potential energy surfaces given by local correlation methods based on the Pulay–Saebø orbital domain approach. Our analysis focuses on three prototypical dissociating systems: the C-F bond in fluoromethane, the C-C bond in singlet, ketene, and the central C-C bond in propadienone. We find that such discontinuities do not occur in cases of homolytic bond cleavage due to the inability of the Pipek–Mezey orbital localization method to separate singlet-coupled charges on distant fragments. However, for heterolytic bond cleavage, such as that observed in singlet ketene and propadienone, discontinuities occur both at stretched geometries and near equilibrium. These discontinuities are usually small, but may be of the same order of magnitude as the localization error in some cases. © 2004 American Institute of Physics.
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31.50.-x Potential energy surfaces
33.15.Fm Bond strengths, dissociation energies
31.15.V- Electron correlation calculations for atoms, ions and molecules

Parallel iterative reaction path optimization in ab initio quantum mechanical/molecular mechanical modeling of enzyme reactions

Haiyan Liu, Zhenyu Lu, G. Andrés Cisneros, and Weitao Yang

J. Chem. Phys. 121, 697 (2004); http://dx.doi.org/10.1063/1.1759318 (10 pages) | Cited 19 times

Online Publication Date: 24 June 2004

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The determination of reaction paths for enzyme systems remains a great challenge for current computational methods. In this paper we present an efficient method for the determination of minimum energy reaction paths with the ab initio quantum mechanical/molecular mechanical approach. Our method is based on an adaptation of the path optimization procedure by Ayala and Schlegel for small molecules in gas phase, the iterative quantum mechanical/molecular mechanical (QM/MM) optimization method developed earlier in our laboratory and the introduction of a new metric defining the distance between different structures in the configuration space. In this method we represent the reaction path by a discrete set of structures. For each structure we partition the atoms into a core set that usually includes the QM subsystem and an environment set that usually includes the MM subsystem. These two sets are optimized iteratively: the core set is optimized to approximate the reaction path while the environment set is optimized to the corresponding energy minimum. In the optimization of the core set of atoms for the reaction path, we introduce a new metric to define the distances between the points on the reaction path, which excludes the soft degrees of freedom from the environment set and includes extra weights on coordinates describing chemical changes. Because the reaction path is represented by discrete structures and the optimization for each can be performed individually with very limited coupling, our method can be executed in a natural and efficient parallelization, with each processor handling one of the structures. We demonstrate the applicability and efficiency of our method by testing it on two systems previously studied by our group, triosephosphate isomerase and 4-oxalocrotonate tautomerase. In both cases the minimum energy paths for both enzymes agree with the previously reported paths. © 2004 American Institute of Physics.
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82.39.Fk Enzyme kinetics
87.15.R- Reactions and kinetics
87.14.E- Proteins
87.15.A- Theory, modeling, and computer simulation
82.20.-w Chemical kinetics and dynamics

Quantum-classical dynamics of scattering processes in adiabatic and diabatic representations

Panchanan Puzari, Biplab Sarkar, and Satrajit Adhikari

J. Chem. Phys. 121, 707 (2004); http://dx.doi.org/10.1063/1.1758700 (15 pages) | Cited 28 times

Online Publication Date: 24 June 2004

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We demonstrate the workability of a TDDVR based [J. Chem. Phys. 118, 5302 (2003)], novel quantum-classical approach, for simulating scattering processes on a quasi-Jahn–Teller model [J. Chem. Phys. 105, 9141 (1996)] surface. The formulation introduces a set of DVR grid points defined by the Hermite part of the basis set in each dimension and allows the movement of grid points around the central trajectory. With enough trajectories (grid points), the method converges to the exact quantum formulation whereas with only one grid point, we recover the conventional molecular dynamics approach. The time-dependent Schrödinger equation and classical equations of motion are solved self-consistently and electronic transitions are allowed anywhere in the configuration space among any number of coupled states. Quantum-classical calculations are performed on diabatic surfaces (two and three) to reveal the effects of symmetry on inelastic and reactive state-to-state transition probabilities, along with calculations on an adiabatic surface with ordinary Born–Oppenheimer approximation. Excellent agreement between TDDVR and DVR results is obtained in both the representations. © 2004 American Institute of Physics.
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03.65.Ge Solutions of wave equations: bound states
31.30.Gs Hyperfine interactions and isotope effects
02.10.Ab Logic and set theory
31.15.xr Self-consistent-field methods
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors

Non-Hermitian quantum mechanics: Wave packet propagation on autoionizing potential energy surfaces

N. Moiseyev, S. Scheit, and L. S. Cederbaum

J. Chem. Phys. 121, 722 (2004); http://dx.doi.org/10.1063/1.1709867 (4 pages) | Cited 10 times

Online Publication Date: 24 June 2004

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The correspondence between the time-dependent and time-independent molecular dynamic formalisms is shown for autoionizing processes. We demonstrate that the definition of the inner product in non-Hermitian quantum mechanics plays a key role in the proof. When the final state of the process is dissociative, it is technically favorable to introduce a complex absorbing potential into the calculations. The conditions which this potential should fulfill are briefly discussed. An illustrative numerical example is presented involving three potential energy surfaces. © 2004 American Institute of Physics.
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33.80.Eh Autoionization, photoionization, and photodetachment
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions

A multidimensional discrete variable representation basis obtained by simultaneous diagonalization

Richard Dawes and Tucker Carrington

J. Chem. Phys. 121, 726 (2004); http://dx.doi.org/10.1063/1.1758941 (11 pages) | Cited 15 times

Online Publication Date: 24 June 2004

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Direct product basis functions are frequently used in quantum dynamics calculations, but they are poor in the sense that many such functions are required to converge a spectrum, compute a rate constant, etc. Much better, contracted, basis functions, that account for coupling between coordinates, can be obtained by diagonalizing reduced dimension Hamiltonians. If a direct product basis is used, it is advantageous to use discrete variable representation (DVR) basis functions because matrix representations of functions of coordinates are diagonal in the DVR. By diagonalizing matrices representing coordinates it is straightforward to obtain the DVR that corresponds to any direct product basis. Because contracted basis functions are eigenfunctions of reduced dimension Hamiltonians that include coupling terms they are not direct product functions. The advantages of contracted basis functions and the advantages of the DVR therefore appear to be mutually exclusive. A DVR that corresponds to contracted functions is unknown. In this paper we propose such a DVR. It spans the same space as a contracted basis, but in it matrix representations of coordinates are diagonal. The DVR basis functions are chosen to achieve maximal diagonality of coordinate matrices. We assess the accuracy of this DVR by applying it to model four-dimensional problems. © 2004 American Institute of Physics.
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31.15.-p Calculations and mathematical techniques in atomic and molecular physics

Analytical energy gradients for local second-order Møller–Plesset perturbation theory using density fitting approximations

Martin Schütz, Hans-Joachim Werner, Roland Lindh, and Frederick R. Manby

J. Chem. Phys. 121, 737 (2004); http://dx.doi.org/10.1063/1.1760747 (14 pages) | Cited 72 times

Online Publication Date: 24 June 2004

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An efficient method to compute analytical energy derivatives for local second-order Møller–Plesset perturbation energy is presented. Density fitting approximations are employed for all 4-index integrals and their derivatives. Using local fitting approximations, quadratic scaling with molecular size and cubic scaling with basis set size for a given molecule is achieved. The density fitting approximations have a negligible effect on the accuracy of optimized equilibrium structures or computed energy differences. The method can be applied to much larger molecules and basis sets than any previous second-order Møller–Plesset gradient program. The efficiency and accuracy of the method is demonstrated for a number of organic molecules as well as for molecular clusters. Examples of geometry optimizations for molecules with 100 atoms and over 2000 basis functions without symmetry are presented. © 2004 American Institute of Physics.
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31.15.V- Electron correlation calculations for atoms, ions and molecules
02.60.-x Numerical approximation and analysis
33.15.Bh General molecular conformation and symmetry; stereochemistry
31.15.xp Perturbation theory

Density functional theory study of H and H2 interacting with NiAl(110)

P. Rivière, H. F. Busnengo, and F. Martín

J. Chem. Phys. 121, 751 (2004); http://dx.doi.org/10.1063/1.1747970 (10 pages) | Cited 25 times

Online Publication Date: 24 June 2004

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We present results of extensive density functional theory (DFT) calculations for H and H2 interacting with NiAl(110). Continuous representations of the full dimensional potential energy surface (PES) for the H/NiAl(110) and H2/NiAl(110) systems are obtained by interpolation of the DFT results using the corrugation reducing procedure. We find a minimum activation energy barrier of ∼300 meV for dissociative adsorption of H2, which is consistent with the energy threshold obtained in molecular beam experiments for H2 (ν=0). We explain vibrational enhancement observed in experiments as the consequence of vibrational softening in the entrance channel over the most reactive surface site. The H2/NiAl(110) PES shows a high surface site selectivity: for energies up to 0.1 eV above threshold, H2 adsorption can only take place around top-Ni sites (within a circle of radius ∼0.3 Å). A strong energetic corrugation is observed: energy barriers for dissociation vary by more than 1 eV between the most and the least reactive sites. In contrast, geometric corrugation is much less pronounced and comparable to that of low index single metal surfaces like Cu or Pt. © 2004 American Institute of Physics.
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31.15.E- Density-functional theory
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions
34.50.Lf Chemical reactions
33.20.Tp Vibrational analysis
68.43.Mn Adsorption kinetics
33.15.Fm Bond strengths, dissociation energies

A simple model for NMR relaxation in the presence of internal motions with dynamical coupling

Daniel Abergel and Geoffrey Bodenhausen

J. Chem. Phys. 121, 761 (2004); http://dx.doi.org/10.1063/1.1756867 (8 pages) | Cited 9 times

Online Publication Date: 24 June 2004

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In this paper some effects of dynamical coupling between two interaction-carrying vectors on the internal auto- and cross-correlation functions are investigated in the limit of small amplitude motions. A linearized Langevin approach allows the derivation of explicit expressions for these correlation functions and for the corresponding order parameters. © 2004 American Institute of Physics.
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33.25.+k Nuclear resonance and relaxation
36.20.Ey Conformation (statistics and dynamics)
02.30.Hq Ordinary differential equations
31.15.-p Calculations and mathematical techniques in atomic and molecular physics
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Selective infrared photodissociation of protonated para-fluorophenol isomers: Substitution effects in oxonium and fluoronium ions

Nicola Solcà and Otto Dopfer

J. Chem. Phys. 121, 769 (2004); http://dx.doi.org/10.1063/1.1756132 (4 pages) | Cited 19 times

Online Publication Date: 24 June 2004

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Isomer-selective infrared photodissociation (IRPD) spectra are obtained for the first time for protonated polyfunctional aromatic molecules isolated in the gas phase. IRPD spectra of the oxonium and fluoronium isomers of protonated para-fluorophenol (C6H6FO+) were separately obtained by monitoring resonant photo-induced H2O and HF loss, respectively. Analysis of the F–H, O–H, and C–H stretch wave numbers provides valuable spectroscopic information on the chemical properties of these reactive intermediates, in particular on the substitution effects of functional groups. © 2004 American Institute of Physics.
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33.20.Ea Infrared spectra
33.80.Gj Diffuse spectra; predissociation, photodissociation
33.15.Fm Bond strengths, dissociation energies
82.30.Nr Association, addition, insertion, cluster formation
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)

Free energy perturbation study of water dimer dissociation kinetics

Yi Ming, Geeling Lai, Chinghang Tong, Robert H. Wood, and Douglas J. Doren

J. Chem. Phys. 121, 773 (2004); http://dx.doi.org/10.1063/1.1756574 (5 pages) | Cited 10 times

Online Publication Date: 24 June 2004

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An efficient approach is described for using accurate ab initio calculations to determine the rates of elementary condensation and evaporation processes that lead to nucleation of aqueous aerosols. The feasibility of the method is demonstrated in an application to evaporation rates of water dimer at 230 K. The method, known as ABC-FEP (ab initio/classical free energy perturbation), begins with a calculation of the potential of mean force for the dissociation (evaporation) of small water clusters using a molecular dynamics (MD) simulation with a model potential. The free energy perturbation is used to calculate how changing from the model potential to a potential calculated from ab initio methods would alter the potential of mean force. The difference in free energy is the Boltzmann-weighted average of the difference between the ab initio and classical potential energies, with the average taken over a sample of configurations from the MD simulation. In principle, the method does not require a highly accurate model potential, though more accurate potentials require fewer configurations to achieve a small sampling error in the free energy perturbation step. To test the feasibility of obtaining accurate potentials of mean force from ab initio calculations at a modest number of configurations, the free energy perturbation method has been used to correct the errors when some standard models for bulk water (SPC, TIP4P, and TIP4P⋅FQ) are applied to water dimer. To allow a thorough exploration of sampling issues, a highly accurate fit to results of accurate ab initio calculations, known as SAPT-5s, as been used a proxy for the ab initio calculations. It is shown that accurate values for a point on the potential of mean force can be obtained from any of the water models using ab initio calculations at only 50 configurations. Thus, this method allows accurate simulations of small clusters without the need to develop water models specifically for clusters. © 2004 American Institute of Physics.
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82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.20.Pm Rate constants, reaction cross sections, and activation energies
31.15.xp Perturbation theory
31.15.A- Ab initio calculations
33.15.Bh General molecular conformation and symmetry; stereochemistry
36.40.Mr Spectroscopy and geometrical structure of clusters
31.15.xv Molecular dynamics and other numerical methods
34.20.Gj Intermolecular and atom-molecule potentials and forces

On the interconversion pathway of HBO↔BOH

Qian Peng, Yubin Wang, Bing Suo, Qizhen Shi, and Zhenyi Wen

J. Chem. Phys. 121, 778 (2004); http://dx.doi.org/10.1063/1.1760746 (5 pages) | Cited 9 times

Online Publication Date: 24 June 2004

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The potential energy surfaces have been constructed for the 1A′, 3A′, and 3A″ states of HBO by using the multireference perturbation theory with the basis set cc-pVTZ (6d,10f ). Two stationary points and a transition state have been characterized on all the three surfaces, which are in good agreement with available experiments and previous calculations. The interconversion pathways from metastable boron hydroxide BOH to the considerably more stable HBO are expounded based on the nature of the surfaces. © 2004 American Institute of Physics.
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33.50.Hv Radiationless transitions, quenching
31.50.Df Potential energy surfaces for excited electronic states
82.20.Kh Potential energy surfaces for chemical reactions
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces

Nonadiabatic alignment of asymmetric top molecules: Rotational revivals

Mikael D. Poulsen, Emmanuel Péronne, Henrik Stapelfeldt, Christer Z. Bisgaard, Simon S. Viftrup, Edward Hamilton, and Tamar Seideman

J. Chem. Phys. 121, 783 (2004); http://dx.doi.org/10.1063/1.1760731 (9 pages) | Cited 31 times

Online Publication Date: 24 June 2004

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The rotational revival structure of asymmetric top molecules, following irradiation by an intense picosecond laser pulse, is explored theoretically and experimentally. Numerically we solve nonperturbatively for the rotational dynamics of a general asymmetric top subject to a linearly polarized intense pulse, and analyze the dependence of the dynamical alignment on the field and system parameters. Experimentally we use time-resolved photofragment imaging to measure the alignment of two molecules with different asymmetry, iodobenzene, and iodopentafluorobenzene. Our numerical results explain the experimental observations and generalize them to other molecules. The rotational revival structure of asymmetric tops differs qualitatively from the intensively studied linear top case. Potentially it provides valuable structural information about molecules. © 2004 American Institute of Physics.
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37.10.Mn Slowing and cooling of molecules
37.10.Pq Trapping of molecules
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Sn Rotational analysis
37.10.Vz Mechanical effects of light on atoms, molecules, and ions
33.15.Bh General molecular conformation and symmetry; stereochemistry

Autoionization and neutral dissociation of superexcited HI studied by two-dimensional photoelectron spectroscopy

Yasumasa Hikosaka and Koichiro Mitsuke

J. Chem. Phys. 121, 792 (2004); http://dx.doi.org/10.1063/1.1758212 (8 pages) | Cited 1 time

Online Publication Date: 24 June 2004

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Two-dimensional photoelectron spectroscopy of hydrogen iodide (HI) has been performed in the photon energy region of 11.10–14.85 eV, in order to investigate dynamical properties on autoionization and neutral dissociation of Rydberg states HI∗(RA) converging to HI+( 2Σ1/2+). A two-dimensional photoelectron spectrum exhibits strong vibrational excitation of HI+(math 2Π) over a photon energy region from ∼12 to 13.7 eV, which is attributable to the autoionizing feature of the 5dπ HI∗(RA) state. A noticeable set of stripes in the photon energy region of 13.5–14.5 eV is assigned as resulting from autoionization of the atomic Rydberg states of I∗ converging to I+ (3P0 or 3P1). The formation of I∗ is understood in terms of predissociation of multiple HI∗(RA) states by way of the repulsive Rydberg potential curves converging to HI+(4Π1/2). © 2004 American Institute of Physics.
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33.80.Eh Autoionization, photoionization, and photodetachment
33.80.Gj Diffuse spectra; predissociation, photodissociation
31.50.Df Potential energy surfaces for excited electronic states
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.60.+q Photoelectron spectra
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
33.20.Tp Vibrational analysis

Some symmetry-induced isotope effects in the kinetics of recombination reactions

Russell T Pack and Robert B. Walker

J. Chem. Phys. 121, 800 (2004); http://dx.doi.org/10.1063/1.1758697 (13 pages) | Cited 12 times

Online Publication Date: 24 June 2004

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Symmetry-induced isotope effects in recombination and collision-induced dissociation reactions are discussed. Progress on understanding the anomalous isotope effects in ozone is reviewed. Then, calculations are performed for the simpler reaction xNe+yNe+H⇔xNeyNe+H, where x and y label either identical or different isotopes. The atomic masses in the model are chosen so that symmetry is the only difference between the systems. Starting from a single potential energy surface, the properties of the bound, quasibound, and continuum states of the neon dimer are calculated. Then, the vibration rotation infinite order sudden approximation is used to calculate cross sections for all possible inelastic and dissociative processes. A rate constant matrix that exactly satisfies detailed balance is constructed. It allows recombination to occur both via direct three-body collisions and via tunneling into the quasibound states of the energy transfer mechanism. The eigenvalue rate coefficients are determined. Significant isotope effects are clearly found, and their behavior depends on the pressure, temperature, and mechanism of the reaction. Both spin statistics and symmetry breaking produce isotope effects. Under most conditions the breaking of symmetry enhances the rates, but a wide spectrum of effects is observed; they range from isotope effects with a normal mass dependence to huge, mass-independent isotope effects to cancellation and even to reversal of the isotope effects. This is the first calculation of symmetry-induced isotope effects in recombination rates from first principles. The relevance of the present effects to ozone recombination is discussed. © 2004 American Institute of Physics.
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82.20.Tr Kinetic isotope effects including muonium
82.20.Pm Rate constants, reaction cross sections, and activation energies
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.20.Kh Potential energy surfaces for chemical reactions
01.30.Rr Surveys and tutorial papers; resource letters
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
33.15.Bh General molecular conformation and symmetry; stereochemistry
82.20.Xr Quantum effects in rate constants (tunneling, resonances, etc.)

Observation of a reactive resonance in the integral cross section of a six-atom reaction: F+CHD3

Jingang Zhou, Jim J. Lin, and Kopin Liu

J. Chem. Phys. 121, 813 (2004); http://dx.doi.org/10.1063/1.1761051 (6 pages) | Cited 33 times

Online Publication Date: 24 June 2004

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The title reaction was investigated under crossed-beam conditions at collisional energies ranging from about 0.4 to 7.5 kcal/mol. Product velocity distributions were measured by a time-sliced, velocity-map imaging technique to explicitly account for the density-to-flux transformation factors. Both the state-resolved, pair-correlated excitation functions and vibrational branching ratios are presented for the two isotopic product channels. An intriguing resonance tunneling mechanism occurring near the reaction threshold for the HF+CD3 product channel is surmized, which echoes the reactive resonances found previously for the F+HD→HF+D reaction and more recently for the F+CH4 reaction. © 2004 American Institute of Physics.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Hf Product distribution
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Tp Vibrational analysis
82.20.Xr Quantum effects in rate constants (tunneling, resonances, etc.)

The role of dimers in evaporation of small argon clusters

Ismo Napari and Hanna Vehkamäki

J. Chem. Phys. 121, 819 (2004); http://dx.doi.org/10.1063/1.1763148 (4 pages) | Cited 5 times

Online Publication Date: 24 June 2004

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Evaporation of small Lennard-Jones argon clusters has been studied using molecular dynamic simulations. An extensive library of clusters with 4, 5, 6, 11, and 21 atoms has been obtained from an earlier study. Analysis of the evaporation properties of the clusters indicate, that the fraction of dimer evaporations of all evaporation events increases with the total energy of the cluster. The fraction of evaporated dimers from clusters with a constant lifetime is independent of the cluster size for short-lived clusters and increases with cluster size for long-lived clusters. Only a few percent of the clusters which are long lived enough to participate in vapor–liquid nucleation decay by emitting dimers. The mean cluster lifetime as a function of total energy shows the same exponentially decreasing trend for monomer and dimer evaporation channels. The fraction of trimer evaporations is found to be vanishingly small. © 2004 American Institute of Physics.
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36.40.Ei Phase transitions in clusters
61.20.Ja Computer simulation of liquid structure
64.70.F- Liquid-vapor transitions
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