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28 May 2006

Volume 124, Issue 20, Articles (20xxxx)

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Addition of NH3 to Al3O3−

Richard B. Wyrwas, Caroline Chick Jarrold, Ujjal Das, and Krishnan Raghavachari

J. Chem. Phys. 124, 201101 (2006); http://dx.doi.org/10.1063/1.2206583 (4 pages) | Cited 4 times

Online Publication Date: 22 May 2006

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Recent computational studies on the addition of ammonia (NH3) to the Al3O3− cluster anion [ A. Guevara-Garcia, A. Martinez, and J. V. Ortiz, J. Chem. Phys. 122, 214309 (2005) ] have motivated experimental and additional computational studies, reported here. Al3O3− is observed to react with a single NH3 molecule to form the Al3O3NH3− ion in mass spectrometric studies. This is in contrast to similarly performed studies with water, in which the Al3O5H4− product was highly favored. However, the anion PE spectrum of the ammoniated species is very similar to that of Al3O4H2−. The adiabatic electron affinity of Al3O3NH3 is determined to be 2.35(5) eV. Based on comparison between the spectra and calculated electron affinities, it appears that NH3 adds dissociatively to Al3O3−, suggesting that the time for the Al3O3−∙NH3 complex to either overcome or tunnel through the barrier to proton transfer (which is higher for NH3 than for water) is short relative to the time for collisional cooling in the experiment.
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36.40.Jn Reactivity of clusters
36.40.Wa Charged clusters
82.80.Ms Mass spectrometry (including SIMS, multiphoton ionization and resonance ionization mass spectrometry, MALDI)
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.20.Xr Quantum effects in rate constants (tunneling, resonances, etc.)
82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)

Observation of a simple vibrational wavepacket in a polyatomic molecule via time-resolved photoelectron velocity-map imaging: A prototype for time-resolved IVR studies

Chris J. Hammond, Katharine L. Reid, and Kate L. Ronayne

J. Chem. Phys. 124, 201102 (2006); http://dx.doi.org/10.1063/1.2204596 (4 pages) | Cited 6 times

Online Publication Date: 22 May 2006

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We have prepared a coherent superposition of the two components of a Fermi resonance in the S1 state of toluene at ∼ 460 cm−1 with a ∼ 1 ps laser pulse and monitored time-resolved photoelectron velocity-map images. The photoelectron intensities oscillate with time in a manner that depends on their kinetic energy, even though full vibrational resolution in the cation is not achieved. Analysis of the time-dependent photoelectron spectra enables information on the composition of the S1 wavepacket to be deduced. Such an experiment, in which a whole set of partially dispersed cation vibrational states are detected simultaneously, suggests an efficient method of studying intramolecular vibrational energy redistribution processes in excited states.
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33.60.+q Photoelectron spectra
34.50.Ez Rotational and vibrational energy transfer
33.70.Fd Absolute and relative line and band intensities

Infrared multiphoton spectra from metal dication complexes in the gas phase

Guohua Wu, Jingang Guan, Georgina D. C. Aitken, Hazel Cox, and Anthony J. Stace

J. Chem. Phys. 124, 201103 (2006); http://dx.doi.org/10.1063/1.2194547 (4 pages) | Cited 2 times

Online Publication Date: 22 May 2006

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Infrared multiphoton spectra have been recorded for the first time from metal dication complexes held in an ion trap. The photofragmentation of [M(pyridine)4]2+ complexes has been observed in the range 920–1090 cm−1 and for M = Cu2+, Mg2+ and Zn2+. The narrow absorption features are identified as vibrational modes of the pyridine molecule and comparisons with calculations suggest that the Mg2+ and Zn2+ ion complexes have D2d (compressed tetrahedron) structures, and that the Cu2+ complex is probably square-planar (D4h).
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33.20.Ea Infrared spectra
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.80.Gj Diffuse spectra; predissociation, photodissociation
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.15.Bh General molecular conformation and symmetry; stereochemistry

A quantum-classical bracket that satisfies the Jacobi identity

Oleg V. Prezhdo

J. Chem. Phys. 124, 201104 (2006); http://dx.doi.org/10.1063/1.2200342 (4 pages) | Cited 12 times

Online Publication Date: 22 May 2006

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A quantum-classical bracket is proposed and is shown to satisfy the Jacobi identity, in contrast to previous definitions that obey this property only up to higher order terms in the Planck constant ℏ. The Jacobi identity is required of a true Lie bracket and ensures that the Lie bracket of constants of motion is also a constant of motion. An explicit calculation of the Jacobi identity highlights the difference between the proposed and traditional definitions. A further example illustrates that the proposed bracket generates a more consistent quantum-classical dynamics than the traditional bracket. The traditional quantum-classical dynamics in the Henon-Heiles system diverges due to higher order ℏ terms. The divergence is eliminated with the proposed bracket.
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03.65.Ta Foundations of quantum mechanics; measurement theory
03.65.Fd Algebraic methods
02.10.Ud Linear algebra

A full dimensional, nine-degree-of-freedom, time-dependent quantum dynamics study for the H2+C2H reaction

Dunyou Wang

J. Chem. Phys. 124, 201105 (2006); http://dx.doi.org/10.1063/1.2206180 (4 pages) | Cited 15 times

Online Publication Date: 22 May 2006

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A full dimensional, nine-degree-of-freedom (9DOF), time-dependent quantum dynamics wave packet approach is presented for the study of the H2+C2H→H+C2H2 reaction system. This is the first full dimensional quantum dynamics study for a diatom-triatom reaction system. The effects of the initial vibrational and rotational excitations of the reactants on the reactivity of this reaction are investigated. This study shows that vibrational excitations of H2 enhance the reactivity; whereas, the vibrational excitations of C2H only have a small effect on the reaction probability. In addition, the bending excitations of C2H, compared to the ground state reaction probability, hinder the reactivity. Comparison of the ground state reaction probabilities of the 9DOF and 8DOF shows the reaction probability from the full dimensional calculation is larger, with more prominent resonance features.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Ej Quantum theory of reaction cross section

Reactive quenching of OH(A math) by D2 studied using crossed molecular beams

Mariví Ortiz-Suárez, Mark F. Witinski, and H. Floyd Davis

J. Chem. Phys. 124, 201106 (2006); http://dx.doi.org/10.1063/1.2206779 (4 pages) | Cited 12 times

Online Publication Date: 22 May 2006

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Reactive quenching of OH(A math,v = 0) by D2 forming HOD+D was studied in crossed molecular beams. The D atom products are primarily forward scattered relative to the incident D2. The dominant mechanism involves a direct reaction from relatively large impact parameters with ∼ 88% of the available energy appearing in HOD internal excitation.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Hf Product distribution

Spin-dependent transport through a magnetic carbon nanotube-molecule junction

C. Zhang, L.-L. Wang, H.-P. Cheng, X.-G. Zhang, and Y. Xue

J. Chem. Phys. 124, 201107 (2006); http://dx.doi.org/10.1063/1.2202739 (4 pages) | Cited 5 times

Online Publication Date: 23 May 2006

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The electronic structure and spin-dependent conductance of a magnetic junction consisting of two Fe-doped carbon nanotubes and a C60 molecule are investigated using a first-principles approach that combines the density functional theory with the nonequilibrium Greens function technique. The tunneling magnetoresistance ratio is found to be 11%. The density of states and transmission coefficient through the molecular junction are analyzed and compared to layered magnetic tunneling junctions. Our findings suggest new possibilities for experiments and for future technology.
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72.25.Mk Spin transport through interfaces
71.20.Tx Fullerenes and related materials; intercalation compounds
72.15.Gd Galvanomagnetic and other magnetotransport effects
73.20.At Surface states, band structure, electron density of states
73.22.-f Electronic structure of nanoscale materials and related systems
75.47.Pq Other materials

New stable multiply charged negative atomic ions in linearly polarized superintense laser fields

Qi Wei, Sabre Kais, and Nimrod Moiseyev

J. Chem. Phys. 124, 201108 (2006); http://dx.doi.org/10.1063/1.2207619 (4 pages) | Cited 7 times

Online Publication Date: 24 May 2006

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Singly charged negative atomic ions exist in the gas phase and are of fundamental importance in atomic and molecular physics. However, theoretical calculations and experimental results clearly exclude the existence of any stable doubly-negatively-charged atomic ion in the gas phase, only one electron can be added to a free atom in the gas phase. In this report, using the high-frequency Floquet theory, we predict that in a linear superintense laser field one can stabilize multiply charged negative atomic ions in the gas phase. We present self-consistent field calculations for the linear superintense laser fields needed to bind extra one and two electrons to form He−, He2−, and Li2−, with detachment energies dependent on the laser intensity and maximal values of 1.2, 0.12, and 0.13 eV, respectively. The fields and frequencies needed for binding extra electrons are within experimental reach. This method of stabilization is general and can be used to predict stability of larger multiply charged negative atomic ions.
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34.80.Lx Recombination, attachment, and positronium formation
34.80.Qb Laser-modified scattering
32.80.-t Photoionization and excitation
31.15.xr Self-consistent-field methods

Generating long-lasting math and math hyperpolarization in small molecules with parahydrogen-induced polarization

Thorsten Jonischkeit, Ute Bommerich, Jörg Stadler, Klaus Woelk, Heiko G. Niessen, and Joachim Bargon

J. Chem. Phys. 124, 201109 (2006); http://dx.doi.org/10.1063/1.2209235 (5 pages) | Cited 20 times

Online Publication Date: 31 May 2006

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Recently, Levitt and co-workers demonstrated that conserving the population of long-lasting nuclear singlet states in weak magnetic fields can lead to a preservation of nuclear spin information over times substantially longer than governed by the (high-field) spin-lattice relaxation time T1. Potential benefits of the prolonged spin information for magnetic resonance imaging and spectroscopy were pointed out, particularly when combined with the parahydrogen induced polarization (PHIP) methodology. In this contribution, we demonstrate that an increase of the effective relaxation time by a factor up to three is achieved experimentally, when molecules hyperpolarized by PHIP are kept in a weak magnetic field instead of the strong field of a typical NMR magnet. This increased lifetime of spin information makes the known PHIP phenomena more compatible with the time scales of biological processes and, thus, more attractive for future investigations.
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33.25.+k Nuclear resonance and relaxation
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back to top Theoretical Methods and Algorithms

Dynamical pruning of static localized basis sets in time-dependent quantum dynamics

Drew A. McCormack

J. Chem. Phys. 124, 204101 (2006); http://dx.doi.org/10.1063/1.2196889 (10 pages) | Cited 5 times

Online Publication Date: 22 May 2006

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We investigate the viability of dynamical pruning of localized basis sets in time-dependent quantum wave packet methods. Basis functions that have a very small population at any given time are removed from the active set. The basis functions themselves are time independent, but the set of active functions changes in time. Two different types of localized basis functions are tested: discrete variable representation (DVR) functions, which are localized in position space, and phase-space localized (PSL) functions, which are localized in both position and momentum. The number of functions active at each point in time can be as much as an order of magnitude less for dynamical pruning than for static pruning, in reactive scattering calculations of H2 on the Pt(211) stepped surface. Scaling of the dynamically pruned PSL (DP-PSL) bases with dimension is considerably more favorable than for either the primitive (direct product) or DVR bases, and the DP-PSL basis set is predicted to be three orders of magnitude smaller than the primitive basis set in the current state-of-the-art six-dimensional reactive scattering calculations.
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82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
68.43.Mn Adsorption kinetics
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.20.Ej Quantum theory of reaction cross section

From local hybrid functionals to “localized local hybrid” potentials: Formalism and thermochemical tests

Alexei V. Arbuznikov, Martin Kaupp, and Hilke Bahmann

J. Chem. Phys. 124, 204102 (2006); http://dx.doi.org/10.1063/1.2196883 (15 pages) | Cited 24 times

Online Publication Date: 22 May 2006

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Hybrid exchange-correlation functionals with position-dependent exact-exchange admixture (local hybrid functionals) have been implemented self-consistently for the first time. Functional derivatives with respect to the occupied orbitals have been derived and were subsequently transformed into local and multiplicative potentials within the framework of the optimized effective potential. The resulting local and multiplicative Kohn-Sham potentials are termed “localized local hybrid” (LLH) potentials. They have been evaluated in calculations of atomization energies for a series of main-group molecules. It is shown that LLH potentials yield somewhat better thermochemical results than non-self-consistent post-GGA calculations with the same local hybrid energy functionals for orbitals obtained with a different potential. The choice of the “local mixing function” (LMF) is discussed. This is the key quantity for the performance of local hybrid functionals that determines the amount of exact-exchange admixture at a given point in space. Careful analyses of average exact-exchange admixtures and of the spatial variation of two different LMFs for various molecules provide insight into the shortcomings of the currently used local hybrid functionals. Beyond a too large average exact-exchange admixture, both LMFs used appear to provide an unbalanced description of exact-exchange admixture across bonds to hydrogen. LLH potentials open the way for property calculations with local hybrid functionals.
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82.60.-s Chemical thermodynamics
31.15.E- Density-functional theory
33.15.Fm Bond strengths, dissociation energies

Advantages of a Lowe-Andersen thermostat in molecular dynamics simulations

E. A. Koopman and C. P. Lowe

J. Chem. Phys. 124, 204103 (2006); http://dx.doi.org/10.1063/1.2198824 (5 pages) | Cited 9 times

Online Publication Date: 22 May 2006

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The Lowe-Andersen thermostat is a momentum conserving and Galilean invariant analog of the Andersen thermostat. Like the Andersen thermostat it has the advantage of being local. We show that by using a minimal thermostat interaction radius in a molecular dynamics simulation, it perturbs the system dynamics to a far lesser extent than the Andersen method. This alleviates a well known drawback of the Andersen thermostat by allowing high thermostatting rates without the penalty of significantly suppressed diffusion in the system.
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61.43.Bn Structural modeling: serial-addition models, computer simulation
68.35.Fx Diffusion; interface formation

Two-photon absorption strength: A new tool for the quantification of two-photon absorption

Rémy Fortrie and Henry Chermette

J. Chem. Phys. 124, 204104 (2006); http://dx.doi.org/10.1063/1.2198530 (6 pages) | Cited 1 time

Online Publication Date: 22 May 2006

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In this paper, we define the two-photon absorption strength, a new characterization tool, similar to the oscillator strength, but for two-photon absorption. It allows the quantification of the two-photon absorption properties of molecular systems which are one-photon transparent. Its definition is such that the corresponding numerical values are around 100 for small molecules. We also show that this new theoretical tool allows the direct comparison of experimental and theoretical data without requiring the introduction of any arbitrary band width. As an example, the experimental and theoretical (AM1+CNDO/S and HF+CIS/3-21G) two-photon absorption properties of the 2,2′-bi(9,9-dihexylfluorene) molecule are compared.
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33.80.-b Photon interactions with molecules
31.15.bu Semi-empirical and empirical calculations (differential overlap, Hückel, PPP methods, etc.)
31.15.xr Self-consistent-field methods
31.15.V- Electron correlation calculations for atoms, ions and molecules

An efficient self-consistent field method for large systems of weakly interacting components

Rustam Z. Khaliullin, Martin Head-Gordon, and Alexis T. Bell

J. Chem. Phys. 124, 204105 (2006); http://dx.doi.org/10.1063/1.2191500 (11 pages) | Cited 19 times

Online Publication Date: 23 May 2006

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An efficient method for removing the self-consistent field (SCF) diagonalization bottleneck is proposed for systems of weakly interacting components. The method is based on the equations of the locally projected SCF for molecular interactions (SCF MI) which utilize absolutely localized nonorthogonal molecular orbitals expanded in local subsets of the atomic basis set. A generalization of direct inversion in the iterative subspace for nonorthogonal molecular orbitals is formulated to increase the rate of convergence of the SCF MI equations. Single Roothaan step perturbative corrections are developed to improve the accuracy of the SCF MI energies. The resulting energies closely reproduce the conventional SCF energy. Extensive test calculations are performed on water clusters up to several hundred molecules. Compared to conventional SCF, speedups of the order of (N/O)2 have been achieved for the diagonalization step, where N is the size of the atomic orbital basis, and O is the number of occupied molecular orbitals.
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36.40.Cg Electronic and magnetic properties of clusters
31.15.xr Self-consistent-field methods

Quantum optimal control: Hessian analysis of the control landscape

Zhenwen Shen, Michael Hsieh, and Herschel Rabitz

J. Chem. Phys. 124, 204106 (2006); http://dx.doi.org/10.1063/1.2198836 (7 pages) | Cited 13 times

Online Publication Date: 25 May 2006

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Seeking an effective quantum control entails searching over a landscape defined as the objective as a functional of the control field. This paper considers the problem of driving a state-to-state transition in a finite level quantum system, and analyzes the local topology of the landscape of the final transition probability in terms of the variables specifying the control field. Numerical calculation of the eigenvalues of the Hessian of the transition probability with respect to the control field variables reveals systematic structure in the spectra reflecting the existence of a generic and simple control landscape topology. An illustration shows that the number of nonzero Hessian eigenvalues is determined by the number of quantum states in the system. The Hessian eigenvectors associated with its nonzero eigenvalues are shown to give insight into the cooperative roles of the control variables. The practical consequences of these findings for quantum control are discussed.
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03.65.Ta Foundations of quantum mechanics; measurement theory
03.65.Fd Algebraic methods
02.10.Yn Matrix theory
02.40.Pc General topology
02.50.Cw Probability theory
02.10.Ud Linear algebra

Optimal control landscapes for quantum observables

Herschel Rabitz, Michael Hsieh, and Carey Rosenthal

J. Chem. Phys. 124, 204107 (2006); http://dx.doi.org/10.1063/1.2198837 (6 pages) | Cited 18 times

Online Publication Date: 25 May 2006

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The optimal control of quantum systems provides the means to achieve the best outcome from redirecting dynamical behavior. Quantum systems for optimal control are characterized by an evolving density matrix and a Hermitian operator associated with the observable of interest. The optimal control landscape is the observable as a functional of the control field. The features of interest over this control landscape consist of the extremum values and their topological character. For controllable finite dimensional quantum systems with no constraints placed on the controls, it is shown that there is only a finite number of distinct values for the extrema, dependent on the spectral degeneracy of the initial and target density matrices. The consequences of these findings for the practical discovery of effective quantum controls in the laboratory is discussed.
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02.10.Yn Matrix theory
03.65.-w Quantum mechanics

Free volume hypothetical scanning molecular dynamics method for the absolute free energy of liquids

Ronald P. White and Hagai Meirovitch

J. Chem. Phys. 124, 204108 (2006); http://dx.doi.org/10.1063/1.2199529 (13 pages) | Cited 4 times

Online Publication Date: 25 May 2006

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The hypothetical scanning (HS) method is a general approach for calculating the absolute entropy, S, and free energy, F, by analyzing Boltzmann samples obtained by Monte Carlo (MC) or molecular dynamics (MD) techniques. With HS applied to a fluid, each configuration i of the sample is reconstructed by gradually placing the molecules in their positions at i using transition probabilities (TPs). With our recent version of HS, called HSMC-EV, each TP is calculated from MC simulations, where the simulated particles are excluded from the volume reconstructed in previous steps. In this paper we remove the excluded volume (EV) restriction, replacing it by a “free volume” (FV) approach. For liquid argon, HSMC-FV leads to an improvement in efficiency over HSMC-EV by a factor of 2–3. Importantly, the FV treatment greatly simplifies the HS implementation for liquids, allowing a much more natural application of the method for MD simulations. Given the success and popularity of MD, the present development of the HSMD method for liquids is an important advancement for HS methodology. Results for the HSMD-FV approach presented here agree well with our HSMC and thermodynamic integration results. The efficiency of HSMD-FV is equivalent to HSMC-EV. The potential use of HSMC(MD)-FV in protein systems with explicit water is discussed.
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61.20.Ja Computer simulation of liquid structure
65.20.-w Thermal properties of liquids

Bounds on the overlap of the Hartree-Fock, optimized effective potential, and density functional approximations with the exact energy eigenstates

S. Thanos and A. K. Theophilou

J. Chem. Phys. 124, 204109 (2006); http://dx.doi.org/10.1063/1.2204601 (6 pages) | Cited 11 times

Online Publication Date: 25 May 2006

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In this paper, we examine the limits of accuracy of the single determinant approximations (Hartree-Fock, optimized effective potential, and density functional theory) to the exact energy eigenstates of many electron systems. We show that an approximate Slater determinant of Sz = M gives maximum accuracy for states with S = M, provided that perturbation theory for the spin up minus spin down potential is applicable. The overlap with the exact energy eigenstates with S ≠ M is much smaller. Therefore, for the case that the emphasis is on wave functions, one must use symmetry preserving theories, although this is at the expense of accuracy in energy.
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31.15.xr Self-consistent-field methods
31.15.E- Density-functional theory
31.15.xp Perturbation theory

Approaches for the calculation of vibrational frequencies in liquids: Comparison to benchmarks for azide/water clusters

Shuzhou Li, J. R. Schmidt, S. A. Corcelli, C. P. Lawrence, and J. L. Skinner

J. Chem. Phys. 124, 204110 (2006); http://dx.doi.org/10.1063/1.2200690 (11 pages) | Cited 21 times

Online Publication Date: 26 May 2006

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Ultrafast vibrational spectroscopy experiments, together with molecular-level theoretical interpretation, can provide important information about the structure and dynamics of complex condensed phase systems, including liquids. The theoretical challenge is to calculate the instantaneous vibrational frequencies of a molecule in contact with a molecular environment, accurately and quickly, and to this end a number of different methods have been developed. In this paper we critically analyze these different methods by comparing their results to accurate benchmark calculations on azide/water clusters. We also propose an optimized quantum mechanics/molecular mechanics method, which for this problem is superior to the other methods.
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78.47.-p Spectroscopy of solid state dynamics
78.30.C- Liquids
61.25.Em Molecular liquids
61.20.Gy Theory and models of liquid structure

Rate processes with dynamical disorder: A direct variational approach

Ananya Debnath, Rajarshi Chakrabarti, and K. L. Sebastian

J. Chem. Phys. 124, 204111 (2006); http://dx.doi.org/10.1063/1.2200695 (6 pages) | Cited 4 times

Online Publication Date: 30 May 2006

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Using path integral approach, we develop variational approximations to the calculation of survival probability for rate processes with dynamical disorder. We derive both upper and lower bounds to the survival probability using Jensen’s inequality. The inequalities involve the use of a trial action for which the path integrals can be evaluated exactly. Any parameter in the trial action can be varied to optimize the bounds. We have also derived a lower bound to the rate of the process. As a simple illustration, we apply the method to the problem of a particle undergoing Brownian motion in a harmonic potential well, in the presence of a delta function sink, for which one can calculate the exact survival probability numerically. The calculation confirms the two inequalities. The method should be very useful in similar but more complex problems where even numerical solution is not possible.
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82.20.Db Transition state theory and statistical theories of rate constants
82.20.Pm Rate constants, reaction cross sections, and activation energies

Herman-Kluk semiclassical dynamics in action-angle representation: New approaches to mapping quantum degrees of freedom

Rajdeep Saha and M. Ovchinnikov

J. Chem. Phys. 124, 204112 (2006); http://dx.doi.org/10.1063/1.2200700 (9 pages) | Cited 6 times

Online Publication Date: 31 May 2006

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A general approach to mapping a discrete quantum mechanical problem by a continuous Hamiltonian is presented. The method is based on the representation of the quantum number by a continuous action variable that extends from −∞ to ∞. The projection of this Hilbert space onto the set of integer quantum numbers reduces the Hamiltonian to a discrete matrix of interest. The theory allows the application of the semiclassical methods to discrete quantum mechanical problems and, in particular, to problems where quantum Hamiltonians are coupled to continuous degrees of freedom. The Herman Kluk semiclassical propagation is used to calculate the nonadiabatic dynamics for a model avoided crossing system. The results demonstrate several advantages of the new theory compared to the existing mapping approaches.
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03.65.Sq Semiclassical theories and applications
02.10.Yn Matrix theory

Atoms-in-molecules in momentum space: A Hirshfeld partitioning of electron momentum densities

P. Balanarayan and Shridhar R. Gadre

J. Chem. Phys. 124, 204113 (2006); http://dx.doi.org/10.1063/1.2198531 (7 pages)

Online Publication Date: 31 May 2006

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A direct application of the Hirshfeld atomic partitioning (HAP) scheme is implemented for molecular electron momentum densities (EMDs). The momentum density contributions of individual atoms in diverse molecular systems are analyzed along with their topographical features and the kinetic energies of the atomic partitions. The proposed p-space HAP-based charge scheme does seem to possess the desirable attributes expected of any atoms in molecules partitioning. In addition to this, the main strength of the p-space HAP is the exact knowledge of the kinetic energy functional and the inherent ease in computing the kinetic energy. The charges derived from HAP in momentum space are found to match chemical intuition and the generally known chemical characteristics such as electronegativity, etc.
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31.15.-p Calculations and mathematical techniques in atomic and molecular physics
31.10.+z Theory of electronic structure, electronic transitions, and chemical binding
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Ab initio calculations of dispersion coefficients for nucleic acid base pairs

Terence P. Haley, Eric R. Graybill, and Slawomir M. Cybulski

J. Chem. Phys. 124, 204301 (2006); http://dx.doi.org/10.1063/1.2197832 (7 pages) | Cited 5 times

Online Publication Date: 22 May 2006

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The results of ab initio calculations of two- and three-body dispersion coefficients for the four most important nucleic acid bases are reported. The isotropic as well as anisotropic coefficients were found by using the time-dependent Hartree-Fock approach and the aug-cc-pVDZ basis set. Single and double excitation coupled-cluster theory with noniterative treatment of triple excitations [CCSD(T)] was used to find the values of static polarizabilities which were subsequently used to estimate the values of the CCSD(T) dispersion coefficients. A comparison of these estimated CCSD(T) dispersion coefficients with coefficients found by using empirical approaches based on atomic contributions revealed that the latter are not reliable.
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31.15.A- Ab initio calculations
31.15.xr Self-consistent-field methods
31.15.bw Coupled-cluster theory
36.20.-r Macromolecules and polymer molecules
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility

The ultraviolet spectrum of the CoCl2 radical, studied at vibrational and rotational resolution

Philip J. Hodges, John M. Brown, and Thomas D. Varberg

J. Chem. Phys. 124, 204302 (2006); http://dx.doi.org/10.1063/1.2188937 (10 pages) | Cited 1 time

Online Publication Date: 22 May 2006

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The laser excitation spectrum of the 327 nm band system of CoCl2, formed in a free-jet expansion, has been recorded at a rotational temperature of approximately 10 K. The spectrum is congested and suffers extensive perturbations. A progression in the excited state symmetric stretching vibration has been identified. The decrease in the symmetric stretching vibrational wave number on excitation is considerable [ν1′ = 195.7(12), ν1″ = 358.1(17) cm−1]. Despite widespread perturbations in the rotational structure of these vibronic bands, they can be confidently assigned to a parallel Ω = 7/2−7/2 transition, consistent with an inverted math ground electronic state. The rotational constant for Comath in the ground state is determined to be 0.056 65(11) cm−1, which corresponds to a value for the zero-point averaged Co–Cl bond length r0 of 2.062 8(40) Å. The perturbations are found to be strongly isotopomer dependent.
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33.20.Lg Ultraviolet spectra
33.20.Tp Vibrational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Sn Rotational analysis
33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions
31.30.Gs Hyperfine interactions and isotope effects

Photodissociation dynamics of the HCNN radical

Ann Elise Faulhaber, Jason R. Gascooke, Alexandra A. Hoops, and Daniel M. Neumark

J. Chem. Phys. 124, 204303 (2006); http://dx.doi.org/10.1063/1.2196890 (8 pages) | Cited 3 times

Online Publication Date: 22 May 2006

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The photodissociation dynamics of the diazomethyl (HCNN) radical have been studied using fast radical beam photofragment translational spectroscopy. A photofragment yield spectrum was obtained for the range of 25 510–40 820 cm−1, and photodissociation was shown to occur for energies above 25 600 cm−1. The only product channel observed was the formation of CH and N2. Fragment translational energy and angular distributions were obtained at several energies in the range covered by the photofragment yield spectrum. The fragment translational energy distributions showed at least two distinct features at energies up to 4.59 eV, and were not well fit by phase space theory at any of the excitation energies studied. A revised C–N bond dissociation energy and heat of formation for HCNN, D0(HC–NN) = 1.139±0.019 eV and ΔfH0(HCNN) = 5.010±0.023 eV, were determined.
Show PACS
33.80.Gj Diffuse spectra; predissociation, photodissociation
82.50.-m Photochemistry
82.20.Hf Product distribution
82.60.Cx Enthalpies of combustion, reaction, and formation
33.15.Fm Bond strengths, dissociation energies
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