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15 Aug 2000

Volume 113, Issue 7, pp. 2517-2935

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Dissociative electron attachment to gas-phase 5-bromouracil

H. Abdoul-Carime, M. A. Huels, F. Brüning, E. Illenberger, and L. Sanche

J. Chem. Phys. 113, 2517 (2000); http://dx.doi.org/10.1063/1.1306654 (5 pages) | Cited 60 times

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We report measurements of dissociative electron attachment (DEA) to gaseous 5-bromouracil (BrU) for incident electron energies between 0 and 16 eV. Low energy electron impact on BrU leads not only to the formation of a long lived parent anion BrU−, but also various anion fragments resulting from endo- and exo-cyclic bond ruptures, such as Br−, uracil-yl anions, i.e., (U-yl)−, OCN−, and a 68 amu anion tentatively attributed to H2C3NO−. The incident electron energy dependent signatures of either the Br− and (U-yl)− yields (at 0, 1.4, and 6 eV), or the OCN− and H2C3NO− yields (at 1.6 and 5.0 eV) suggests competing DEA channels for anion fragment formation. The production cross sections, at 0 eV incident electron energy, for BrU−, Br−, and (U-yl)− are estimated to be about 6×10−15, 6×10−14, and 1.0×10−15 cm2, respectively. © 2000 American Institute of Physics.
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34.80.Ht Dissociation and dissociative attachment
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back to top Theoretical Methods and Algorithms

Self-adaptive quadrature and numerical path integration

Dubravko Sabo, J. D. Doll, and David L. Freeman

J. Chem. Phys. 113, 2522 (2000); http://dx.doi.org/10.1063/1.1305743 (8 pages) | Cited 2 times

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In the present paper we explore the use of generalized Gaussian quadrature methods in the context of equilibrium path integral applications. Using moment techniques, we devise a compact, self-adaptive approach for use in conjunction with selected classes of interaction potentials. We demonstrate that, when applicable, the resulting approach reduces appreciably the number of potential energy evaluations required in equilibrium path integral simulations. © 2000 American Institute of Physics.
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03.65.Ta Foundations of quantum mechanics; measurement theory

The mapping of the local contributions of Fermi and Coulomb correlation into intracule and extracule density distributions

Xavier Fradera, Miquel Duran, and Jordi Mestres

J. Chem. Phys. 113, 2530 (2000); http://dx.doi.org/10.1063/1.1305920 (14 pages) | Cited 2 times

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The contributions of the correlated and uncorrelated components of the electron-pair density to atomic and molecular intracule I(r) and extracule E(R) densities and its Laplacian functions ∇2I(r) and ∇2E(R) are analyzed at the Hartree–Fock (HF) and configuration interaction (CI) levels of theory. The topologies of the uncorrelated components of these functions can be rationalized in terms of the corresponding one-electron densities. In contrast, by analyzing the correlated components of I(r) and E(R), namely, IC(r) and EC(R), the effect of electron Fermi and Coulomb correlation can be assessed at the HF and CI levels of theory. Moreover, the contribution of Coulomb correlation can be isolated by means of difference maps between IC(r) and EC(R) distributions calculated at the two levels of theory. As application examples, the He, Ne, and Ar atomic series, the C2−2, N2, O2+2 molecular series, and the C2H4 molecule have been investigated. For these atoms and molecules, it is found that Fermi correlation accounts for the main characteristics of IC(r) and EC(R), with Coulomb correlation increasing slightly the locality of these functions at the CI level of theory. Furthermore, IC(r), EC(R), and the associated Laplacian functions, reveal the short-ranged nature and high isotropy of Fermi and Coulomb correlation in atoms and molecules. © 2000 American Institute of Physics.
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31.15.xr Self-consistent-field methods
31.15.V- Electron correlation calculations for atoms, ions and molecules

On the condensed Fukui function

P. Fuentealba, P. Pérez, and R. Contreras

J. Chem. Phys. 113, 2544 (2000); http://dx.doi.org/10.1063/1.1305879 (8 pages) | Cited 68 times

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A critical comparison among recently proposed methods for evaluating the condensed Fukui function neglecting relaxation effects is presented. The sign of the condensed Fukui function is discussed and arguments for a positive definite condensed Fukui function are given. Our numerical calculations in two series of molecules show that: (i) the condensed Fukui function can give, in general, valuable information about the site selectivity in chemical reactions and systematization in a family of molecules. In particular, it has been shown that the selectivity towards protonation in anilines and derivatives molecules can be correctly assessed by the electrophilic Fukui function described in this paper. Within this approach non-negative values for the condensed Fukui function are obtained for the relevant protonation sites in these polyfunctional systems; and (ii) the solvent effects on the condensed Fukui function are negligible, confirming a recently presented theoretical prediction. © 2000 American Institute of Physics.
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31.15.E- Density-functional theory
82.20.-w Chemical kinetics and dynamics
82.30.Nr Association, addition, insertion, cluster formation

Microcanonical temperature and its Arrhenius relation to lifetimes in isomerization dynamics of clusters

Kazuo Takatsuka and Tomohiro Yanao

J. Chem. Phys. 113, 2552 (2000); http://dx.doi.org/10.1063/1.1305866 (11 pages) | Cited 8 times

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In a previous study of isomerization dynamics of clusters as a chaotic conservative system, we proposed a temperature, called the microcanonical temperature [C. Seko and K. Takatsuka, J. Chem. Phys. 104, 8613 (1996)], which is expected to characterize a phase space distribution on a constant energy plane. In contrast to the standard view of equal a priori distribution in phase space, we note a fact that this distribution usually becomes sharply localized with a single peak, if projected onto the potential energy coordinate. The microcanonical temperature is defined as a kinetic energy at which this projected distribution takes the maximum value. Then the most probable statistical events should be dominated by those components in vicinity of the peak, provided that the projected distribution is singly and sharply peaked and the associated dynamics is ergodic. The microcanonical temperature can be similarly redefined in the individual potential basins. Here in the present article a numerical fact is highlighted that the inverse of the lifetime of an isomer bears an Arrhenius-type relation with thus defined local microcanonical temperature assigned to the corresponding potential basin. We present an analysis of how the Arrhenius relation can arise. © 2000 American Institute of Physics.
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36.40.Jn Reactivity of clusters
82.30.Qt Isomerization and rearrangement
05.20.-y Classical statistical mechanics

Small-core multiconfiguration-Dirac–Hartree–Fock-adjusted pseudopotentials for post-d main group elements: Application to PbH and PbO

Bernhard Metz, Hermann Stoll, and Michael Dolg

J. Chem. Phys. 113, 2563 (2000); http://dx.doi.org/10.1063/1.1305880 (7 pages) | Cited 197 times

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Relativistic pseudopotentials (PPs) of the energy-consistent variety have been generated for the post-d group 13–15 elements, by adjustment to multiconfiguration Dirac–Hartree–Fock data based on the Dirac–Coulomb–Breit Hamiltonian. The outer-core (n−1)spd shells are explicitly treated together with the nsp valence shell, with these PPs, and the implications of the small-core choice are discussed by comparison to a corresponding large-core PP, in the case of Pb. Results from valence ab initio one- and two-component calculations using both PPs are presented for the fine-structure splitting of the ns2np2 ground-state configuration of the Pb atom, and for spectroscopic constants of PbH (X 2Π1/2, 2Π3/2) and PbO (X 1Σ+). In addition, a combination of small-core and large-core PPs has been explored in spin-free-state shifted calculations for the above molecules. © 2000 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

On the temperature, equipartition, degrees of freedom, and finite size effects: Application to aluminum clusters

J. Jellinek and A. Goldberg

J. Chem. Phys. 113, 2570 (2000); http://dx.doi.org/10.1063/1.1305821 (13 pages) | Cited 41 times

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The relationship between statistical ensembles (especially microcanonical ensemble) and dynamics, the equipartition theorem, and the notion of dynamical temperature are reexamined with an emphasis on finite size effects. A (dynamical) equipartition ansatz (postulate) is formulated and the notion of dynamical degrees of freedom is introduced. The utility of the dynamical degrees of freedom as an analysis tool is discussed and illustrated in applications to model aluminum clusters. © 2000 American Institute of Physics.
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36.40.Sx Diffusion and dynamics of clusters
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
05.20.-y Classical statistical mechanics

Exploring the ab initio/classical free energy perturbation method: The hydration free energy of water

Shinichi Sakane, Eric M. Yezdimer, Wenbin Liu, Jose A. Barriocanal, Douglas J. Doren, and Robert H. Wood

J. Chem. Phys. 113, 2583 (2000); http://dx.doi.org/10.1063/1.1305862 (11 pages) | Cited 16 times

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The ab initio/classical free energy perturbation (ABC-FEP) method proposed previously by Wood et al. [J. Chem. Phys. 110, 1329 (1999)] uses classical simulations to calculate solvation free energies within an empirical potential model, then applies free energy perturbation theory to determine the effect of changing the empirical solute–solvent interactions to corresponding interactions calculated from ab initio methods. This approach allows accurate calculation of solvation free energies using an atomistic description of the solvent and solute, with interactions calculated from first principles. Results can be obtained at a feasible computational cost without making use of approximations such as a continuum solvent or an empirical cavity formation energy. As such, the method can be used far from ambient conditions, where the empirical parameters needed for approximate theories of solvation may not be available. The sources of error in the ABC-FEP method are the approximations in the ab initio method, the finite sample of configurations, and the classical solvent model. This article explores the accuracy of various approximations used in the ABC-FEP method by comparing to the experimentally well-known free energy of hydration of water at two state points (ambient conditions, and 973.15 K and 600 kg/m3). The TIP4P-FQ model [J. Chem. Phys. 101, 6141 (1994)] is found to be a reliable solvent model for use with this method, even at supercritical conditions. Results depend strongly on the ab initio method used: a gradient-corrected density functional theory is not adequate, but a localized MP2 method yields excellent agreement with experiment. Computational costs are reduced by using a cluster approximation, in which ab initio pair interaction energies are calculated between the solute and up to 60 solvent molecules, while multi-body interactions are calculated with only a small cluster (5 to 12 solvent molecules). Sampling errors for the ab initio contribution to solvation free energies are ±2 kJ/mol or less when 50–200 configurations are used. Using the largest clusters and most accurate ab initio methods, ABC-FEP predicts hydration free energies of water at both state points that agree with equations of state, within the sampling error. These results are the first calculation of a free energy of solvation at extreme conditions from a fully atomistic model with ab initio methods. © 2000 American Institute of Physics.
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82.30.-b Specific chemical reactions; reaction mechanisms
61.20.-p Structure of liquids
82.60.-s Chemical thermodynamics

Direct iterative solution of the generalized Bloch equation. II. A general formalism for many-electron systems

Holger Meißner and Josef Paldus

J. Chem. Phys. 113, 2594 (2000); http://dx.doi.org/10.1063/1.1305321 (18 pages) | Cited 16 times

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A general nonperturbative formulation of the recently proposed [H. Meißner and E. O. Steinborn, Int. J. Quantum Chem. 61, 777 (1997); Part I] quadratic iterative scheme for the wave function expansion coefficients (WECs), enabling a direct solution of the generalized Bloch equation, is given for the ab initio electronic Hamiltonians, thus enabling the computation of the molecular electronic structure. The method exploits the concepts of a multidimensional reference or model space, a (non-Hermitian) effective Hamiltonian, and the generalized Bloch equation. The formulation in terms of WECs provides a considerable freedom in the design of various approximation schemes by combining direct iterations on WECs with their approximation by disconnected cluster components based on the exponential cluster ansatz for the wave operator. The resulting formalism is capable of handling a rather large class of both ground and excited states. While the general formulation represents a multiconfigurational, multireference scheme, a special attention is paid to its two-dimensional state selective or state specific version. © 2000 American Institute of Physics.
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31.15.-p Calculations and mathematical techniques in atomic and molecular physics
31.50.Df Potential energy surfaces for excited electronic states
31.15.A- Ab initio calculations
02.60.-x Numerical approximation and analysis

Direct iterative solution of the generalized Bloch equation. III. Application to H2-cluster models

Holger Meißner and Josef Paldus

J. Chem. Phys. 113, 2612 (2000); http://dx.doi.org/10.1063/1.1305322 (10 pages) | Cited 10 times

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A state-selective multiconfigurational single-reference method that was outlined in the preceding paper of this series (H. Meißner and J. Paldus, J. Chem. Phys. 113, 2594 (2000); preceding paper), and is based on a quadratic iterative algorithm enabling the direct solution of the generalized Bloch equation, is applied to several model systems consisting of interacting hydrogen molecules, nowadays referred to as the H4, S4, and H8 models. These exactly solvable models are often used to test the efficacy of post-Hartree–Fock methods in their ability to recover both the dynamic and nondynamic correlation energies, since they enable a continuous variation of the degree of quasidegeneracy from the degenerate to nondegenerate limit by varying a single geometrical parameter, while simulating the dissociation of one or more single bonds. Various approximation schemes that were outlined in Part II, as well as their combinations, are tested and their performance evaluated. The size-extensivity deviations of those approximations that do not rely on the exponential cluster ansatz for the wave operator are also examined using larger hydrogen molecule clusters. It is shown that the so-called BQ4 approximation performs extremely well in all cases and even outperforms the externally corrected, reduced multireference (RMR) CCSD in the quasidegenerate region of geometries. © 2000 American Institute of Physics.
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36.40.Cg Electronic and magnetic properties of clusters
02.60.-x Numerical approximation and analysis
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)

Direct iterative solution of the generalized Bloch equation. IV. Application to H2, LiH, BeH, and CH2

Holger Meißner and Josef Paldus

J. Chem. Phys. 113, 2622 (2000); http://dx.doi.org/10.1063/1.1305323 (16 pages) | Cited 13 times

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The state selective, one- and two-determinantal versions of the recently proposed direct iterative approach to the solution of the generalized Bloch equation [H. Meißner and J. Paldus, J. Chem. Phys. 113, 2612 (2000); Part II] are applied to the ground and excited states of several test molecular ab initio models for which the exact full configuration interaction results are available. Both closed- and open-shell states of different spin multiplicity are considered, as well as the excited states belonging to the same symmetry species as does the ground state. The versatility and flexibility of this approach stems from the possibility of introducing the cluster expansion based approximations of highly excited clusters at various levels of the iterative scheme, leading to a sequence of approximations including both the configuration interaction and coupled cluster methods with singles and doubles as a special case. Both the reliability and the efficiency of these various approximations is examined, and potentially promising approximation schemes are identified. © 2000 American Institute of Physics.
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31.15.-p Calculations and mathematical techniques in atomic and molecular physics
31.50.Df Potential energy surfaces for excited electronic states
02.60.-x Numerical approximation and analysis
31.15.bw Coupled-cluster theory

Hydrogen bonding at the diatomics-in-molecules level: Water clusters

Bella L. Grigorenko, Alexander V. Nemukhin, Igor A. Topol, and Stanley K. Burt

J. Chem. Phys. 113, 2638 (2000); http://dx.doi.org/10.1063/1.1303850 (10 pages) | Cited 8 times

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Further developments of the intermolecular diatomics-in-molecules (DIM) theory towards construction of potential energy surfaces of hydrogen-bonded molecular aggregates are presented. Compared to the previously studied hydrogen fluoride clusters (HF)n [J. Chem. Phys. 111, 4442 (1999)], considerably more complicated and challenging systems, namely, water clusters (H2O)n (n = 2–6) have been analyzed in this work. The present DIM, or more precisely, diatomics-in-ionic-systems, scheme is based on the balanced treatment of neutral and ionic contributions to the electronic properties of polyatomic species, and in this case takes into account the mixing of the OH and O−H+ electronic states within the valence bond description of water molecules. The potential curves of diatomic molecules required for the present application, including ionic species O−H, OH+, O2−, have been computed by ab initio quantum chemistry tools. The results of DIM calculations of equilibrium geometry configurations, binding energies, and relative energies for the low-lying isomers of (H2O)n (n = 2–6) are compared to the reference data showing a good predictive power of this method. © 2000 American Institute of Physics.
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36.40.Mr Spectroscopy and geometrical structure of clusters
33.15.Fm Bond strengths, dissociation energies
31.15.xw Valence bond calculations

Flexible transition state theory for a variable reaction coordinate: Derivation of canonical and microcanonical forms

Struan Robertson, Albert F. Wagner, and David M. Wardlaw

J. Chem. Phys. 113, 2648 (2000); http://dx.doi.org/10.1063/1.1305865 (14 pages) | Cited 9 times

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A completely general canonical and microcanonical (energy-resolved) flexible transition state theory (FTST) expression for the rate constant is derived for an arbitrary choice of reaction coordinate. The derivation is thorough and rigorous within the framework of FTST and replaces our previous treatments [Robertson et al., J. Chem. Phys. 103, 2917 (1995); Robertson et al., Faraday Discuss. Chem. Soc. 102, 65 (1995)] which implicitly involved some significant assumptions. The canonical rate expressions obtained here agree with our earlier results. The corresponding microcanonical results are new. The rate expressions apply to any definition of the separation distance between fragments in a barrierless recombination (or dissociation) that is held fixed during hindered rotations at the transition state, and to any combination of fragment structure (atom, linear top, nonlinear top). The minimization of the rate constant with respect to this definition can be regarded as optimizing the reaction coordinate within a canonical or microcanonical framework. The expression is analytic except for a configuration integral whose evaluation generally requires numerical integration over internal angles (from one to five depending on the fragment structures). The form of the integrand in this integral has important conceptual and computational implications. The primary component of the integrand is the determinant of the inverse G-matrix associated with the external rotations and the relative internal motion of the fragments. © 2000 American Institute of Physics.
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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
02.60.Jh Numerical differentiation and integration
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Direct-dynamics approach to catalytic effects: The tautomerization of 3-hydroxyisoquinoline as a test case

Antonio Fernández-Ramos, Zorka Smedarchina, and Marek Z. Zgierski

J. Chem. Phys. 113, 2662 (2000); http://dx.doi.org/10.1063/1.1305744 (9 pages) | Cited 7 times

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The mechanism of tautomerization of 3-hydroxyisoquinoline (3HIQ) in its first excited singlet state is studied theoretically for the isolated molecule and the 1:1 complexes with water (3HIQ/H2O) and acetic acid (3HIQ/AA). It is found that the proton transfer is a tunneling process which is strongly mediated by the motion of the heavier atoms involved in the hydrogen bond bridges. Therefore it is argued that quantitative assessment of the tremendous catalytic effect of complexation observed experimentally is possible only through the evaluation of multidimensional tunneling rate constants. These are addressed using a direct dynamics approach based on the multidimensional instanton model. The potential energy surface, which governs the tautomerization dynamics, is generated from ab initio calculations at CIS/6-31G* and CASSCF(8,8)/6-31G* levels of theory. It is formulated in terms of the normal modes of the transition state and consists of 33, 57, and 72 degrees of freedom for 3HIQ, 3HIQ/H2O, and 3HIQ/AA, respectively. The catalytic effect of complexation is discussed as an interplay between the static component, reflected in the change of geometries and relative stabilities of the three stationary points, and the dynamic one, resulting from the effects of coupling of the tunneling motion to the skeletal modes. Since the coupling parameters reported in the present study are typical for proton transfer along hydrogen bridges, the relative weight of these effects in the overall acceleration of the reaction will be larger in complexes with smaller reduction of the barrier height upon complexation. © 2000 American Institute of Physics.
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82.30.Qt Isomerization and rearrangement
82.30.Vy Homogeneous catalysis in solution, polymers and zeolites
82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
82.20.Hf Product distribution
82.20.Kh Potential energy surfaces for chemical reactions
82.20.Pm Rate constants, reaction cross sections, and activation energies
82.20.Db Transition state theory and statistical theories of rate constants

The electronic structure and chemical bonding of aluminum acetylide: Al2C2 and Al2C2−: An experimental and theoretical investigation

Nathan A. Cannon, Alexander I. Boldyrev, Xi Li, and Lai-Sheng Wang

J. Chem. Phys. 113, 2671 (2000); http://dx.doi.org/10.1063/1.1305881 (9 pages) | Cited 7 times

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We have investigated the electronic structure and chemical bonding of Al2C2 and Al2C2− both experimentally and theoretically. Photoelectron spectra of Al2C2− were obtained at several photon energies. Two anionic isomers were observed: one with a very sharp ground state feature and a low vertical electron binding energy (0.71 eV) and another with a very broad ground state feature with a much higher vertical electron binding energy (1.58 eV). Theoretical calculations were performed at various levels of theory for both the anion and the neutral. We found two isomers with relatively close energies for the anion: a quasilinear acetylide species and a planar-bridged D2h structure. However, only one stable isomer was found for the neutral, which has the acetylide structure. Adiabatic and vertical detachment energies were also calculated for the two anionic isomers and were used to interpret and assign the experimental spectra. We found that the sharp 0.71 eV feature was from the acetylide isomer, whereas the broad 1.58 eV feature was from the D2h isomer. The excellent agreement between the calculated and experimental electron affinities and excitation energies lends considerable credence for the assignments of the two anionic isomers. The structures and bonding of the acetylide neutral and anion and the D2h anion are discussed. © 2000 American Institute of Physics.
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31.15.-p Calculations and mathematical techniques in atomic and molecular physics
33.15.Fm Bond strengths, dissociation energies
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
33.60.+q Photoelectron spectra
31.15.xr Self-consistent-field methods
31.15.E- Density-functional theory
31.15.bw Coupled-cluster theory

Quantum Monte Carlo determination of the atomization energy and heat of formation of propargyl radical

J. A. W. Harkless and W. A. Lester

J. Chem. Phys. 113, 2680 (2000); http://dx.doi.org/10.1063/1.1305878 (4 pages) | Cited 9 times

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The quantum Monte Carlo (QMC) method is used to compute the atomization energy and the heat of formation of the propargyl radical, C3H3. The effective core potential and fixed-node approximations are used in the diffusion Monte Carlo (DMC) variant of QMC. Two generalized gradient approximation density functionals, B3LYP and B3PW91, are also applied for comparison. The atomization energy determined by these methods is 606.12 kcal/mol (B3LYP), 610.24 kcal/mol (B3PW91), and 607.6(0.6) (DMC). The latter compares favorably with separate measurements of 608.0(3.0) and 608.5(1.2) kcal/mol. The ΔHf298 determined by these methods is 84.03 kcal/mol (B3LYP), 79.91 kcal/mol (B3PW91), 82.5(0.6) (DMC), and two independent measurements yield values of 82.5(3.0) and 81.5(1.2) kcal/mol. © 2000 American Institute of Physics.
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33.15.Fm Bond strengths, dissociation energies
82.60.Cx Enthalpies of combustion, reaction, and formation
31.15.E- Density-functional theory

Multi-isotope study of fractionation effects in the ozone formation process

S. Wolf, M. Bitter, D. Krankowsky, and K. Mauersberger

J. Chem. Phys. 113, 2684 (2000); http://dx.doi.org/10.1063/1.1305890 (3 pages) | Cited 11 times

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The isotope fractionation of all ozone isotopologs of mass 48–54 u has been measured with high accuracy. The data represent a standard for the magnitude of the ozone isotope effect since pressure, temperature, and composition of the gas in which ozone is produced are well known. While 17O17O17O and 18O18O18O are depleted by −1.5% and −3.9%, respectively, all others are enriched with 16O17O18O showing the highest value of 19.8%. Enrichments or depletions are independent of the isotope composition of the oxygen gas. © 2000 American Institute of Physics.
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32.10.Bi Atomic masses, mass spectra, abundances, and isotopes

Interpretation and deperturbation of the Λ-type doubling in the a 3Π state of GaH

Mouna Sbata and Joël Schamps

J. Chem. Phys. 113, 2687 (2000); http://dx.doi.org/10.1063/1.1305882 (10 pages) | Cited 1 time

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The unusual J dependence of the Λ doubling splitting that separates e and f levels in the a 3Π state of GaH has been interpreted using a deperturbation study carried out in an effective Hamiltonian point of view and based on a case (c) partition of the Hamiltonian: rather than limiting the zeroth-order part of the Hamiltonian to electronic and vibrational motions, spin-orbit interaction is also included, leaving outside just those effects linked with nuclear rotational motion. Thus the reasons for the peculiar behavior of the Λ splitting come out in a simple and appealing way. Essentially, with such a procedure, the variation with increasing rotation of the Λ-doubling splitting, i.e., the variations of the energy differences between e and f levels, are directly connected with fairly large differences between the rotational parameters (including mandatorily first and second order centrifugal distorsion) occurring in the off-diagonal Coriolis matrix elements within the 3Π block. To a lesser extent, smaller differences between the e and f rotational constants in the diagonal elements also contribute to the pattern of splittings. The consistency of the resulting values of the main fitted difference parameters (3Π0 e/f splitting at null rotation and first-order diagonal and off-diagonal rotational constant differences) is examined on the basis of experimental data on the one hand and new ab initio calculations on the other. Correct orders of magnitude are obtained although rotational constant differences but one are experimentally larger than expected theoretically from a model restricted to the four electronic states dissociating into ground state products. © 2000 American Institute of Physics.
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33.20.Sn Rotational analysis
33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions
31.15.A- Ab initio calculations

Ab initio studies of anionic clusters of water pentamer

Sudhir A. Kulkarni, Libero J. Bartolotti, and Rajeev K. Pathak

J. Chem. Phys. 113, 2697 (2000); http://dx.doi.org/10.1063/1.1301497 (4 pages) | Cited 12 times

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Conformers of water anionic pentamer cluster (H2O)5− have been reported here in using ab initio as well as density functional methods. Relative stabilities of these conformers obtained from inclusion of electron correlation at various levels indicate that they are very close in energies. The plots of singly occupied molecular orbital (SOMO) reveal that the excess electron exhibits stabilization selectively as either a surface state or an internal state. Vibrational frequencies of some of the conformers have also been reported at the MP2 level. © 2000 American Institute of Physics.
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36.40.Cg Electronic and magnetic properties of clusters
36.40.Mr Spectroscopy and geometrical structure of clusters
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
33.15.Mt Rotation, vibration, and vibration-rotation constants
31.15.xp Perturbation theory
31.15.A- Ab initio calculations
31.15.E- Density-functional theory

Vibrational spectra and intramolecular vibrational redistribution in highly excited deuterobromochlorofluoromethane CDBrClF: Experiment and theory

Andreas Beil, Hans Hollenstein, Oliver L. A. Monti, Martin Quack, and Jürgen Stohner

J. Chem. Phys. 113, 2701 (2000); http://dx.doi.org/10.1063/1.1302083 (18 pages) | Cited 16 times

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The rovibrational spectra of deuterobromochlorofluoromethane (CDBrClF) were measured at intermediate (0.1 cm−1) and high resolution (0.0024 cm−1 full bandwidth, half-maximum) by interferometric Fourier transform infrared spectroscopy in the range from the far infrared at 200 cm−1 to the near infrared (12 000 cm−1) covering all the fundamentals and CD stretching overtones up to polyad N = 5. The spectra are completely analyzed in terms of their vibrational assignments to fundamentals, combinations and overtones. At high excitation the analysis reveals the dominant anharmonic coupling between four high frequency vibrational modes; the CD stretching (ν1), two CD bending (ν2,ν3), and the CF stretching mode (ν4). The analysis is carried out using effective model Hamiltonians including three and four vibrational degrees of freedom. We also present vibrational variational calculations on a grid in a four-dimensional normal coordinate subspace. The potential energy and the dipole moment function are calculated ab initio on this grid using self-consistent field second order Møller–Plesset perturbation theory (MP2). Experimental and theoretical results for band positions and integrated intensities as well as effective spectroscopic parameters are found to be in good agreement. The important anharmonic coupling between the CD chromophore and the CF stretching vibration can be described by an effective cubic Fermi resonance coupling constant ksff′ ≈ (50±10) cm−1, which leads to intramolecular vibrational redistribution between the CD and CF chromophores on the femtosecond time scale. Time dependent intramolecular vibrational redistribution processes in CDBrClF are derived in various representations, including time dependent probability densities (“wave packets”) in coordinate space and finally time dependent entropy. © 2000 American Institute of Physics.
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33.20.Ea Infrared spectra
33.20.Tp Vibrational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Vq Vibration-rotation analysis
31.15.xt Variational techniques
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
31.15.A- Ab initio calculations
31.15.xp Perturbation theory
31.15.xr Self-consistent-field methods
33.70.Fd Absolute and relative line and band intensities
31.90.+s Other topics in the theory of the electronic structure of atoms and molecules (restricted to new topics in section 31)
33.70.Jg Line and band widths, shapes, and shifts

Ab initio calculation and spectroscopic analysis of the intramolecular vibrational redistribution in 1,1,1,2-tetrafluoroiodoethane CF3CHFI

Jörg Pochert, Martin Quack, Jürgen Stohner, and Martin Willeke

J. Chem. Phys. 113, 2719 (2000); http://dx.doi.org/10.1063/1.1302084 (17 pages) | Cited 15 times

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We report a new mechanism for intramolecular vibrational redistribution (IVR) in CF3CHFI which couples the CH chromophore vibrations through a strong Fermi resonance to the formal CF stretching normal mode (a heavy atom frame mode) involving the trans F-atom across the CC bond. The analysis is made possible by comparing spectroscopic results with extensive ab initio calculations of the vibrational fundamental and overtone spectra in the range extending to 12 000 cm−1. Potential energy and electric dipole moment hypersurfaces are calculated ab initio by second order Møller–Plesset perturbation theory (MP2) on a grid involving the CH stretching, two CH bending modes and one high frequency CF stretching normal mode. The potentials are scaled to obtain agreement between the experimental spectrum and the theoretical spectrum calculated by a discrete variable representation technique on this grid. Both spectra are then analyzed in terms of three-dimensional (3D) and four-dimensional (4D) effective vibrational Hamiltonians including Fermi- and Darling–Dennison-type resonances between the CH stretching mode and the CH bending modes and the CF stretching mode. The interaction between the CH modes and the CF mode is clearly visible in the experimental and calculated (4D) spectra. The effective Fermi resonance coupling constants [ksff′ ≃ (40±10) cm−1 and ksaf′ ≃ (55±10) cm−1] coupling the CH and CF mode subspaces are of about the same magnitude as the intra-CH chromophore Fermi resonances (ksaa′ ≃ 56 cm−1 and ksbb′ ≃ 42 cm−1, coupling CH stretching mode “s” with the two CH bending modes “a” and “b”). The chiral, pseudo-Cs symmetry breaking coupling (ksab′ ≃ 11 cm−1) is complemented by an equally strong coupling through the CF mode (ksfb′ ≃ 15 cm−1). It is demonstrated that low order perturbation theoretical analysis using potential constants from a polynomial expansion to represent effective coupling constants gives inadequate results with discrepancies ranging about from factors of 2–5. Time dependent population and wave packet analysis shows essentially complete IVR among the CH chromophore modes within about 100 fs, the 3D and 4D evolutions being similar up to about that time. At longer times of about 250 fs, there is substantial excitation of the CF stretching mode (with initial pure CH stretching excitation). The 4D treatment is then essential for a correct description of the dynamics. © 2000 American Institute of Physics.
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31.15.A- Ab initio calculations
33.20.Tp Vibrational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
31.90.+s Other topics in the theory of the electronic structure of atoms and molecules (restricted to new topics in section 31)
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
31.15.xp Perturbation theory
31.15.xt Variational techniques

The ν1 and ν2 bands of Ar⋯HN2+: A joint theoretical/experimental study

Peter Botschwina, Rainer Oswald, Harold Linnartz, and Dorinel Verdes

J. Chem. Phys. 113, 2736 (2000); http://dx.doi.org/10.1063/1.1305263 (5 pages) | Cited 9 times

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A combined theoretical and experimental study of the ν1 and ν2 stretching vibrations of Ar⋯HN2+ is presented. These correlate asymptotically with the almost local NH and NN stretching vibrations of free HN2+, but undergo mode mixing and are strongly influenced by vibrational anharmonicity in the complex. The first observation of the strong ν2 band by means of diode laser absorption spectroscopy in a supersonic planar plasma is reported. Its band origin is found at 2041.1802(3) cm−1. New results are presented for the ν1 band with origin at 2505.5000(2) cm−1. The ground-state rotational constant is determined from both band systems as 0.080 868(6) cm−1. © 2000 American Institute of Physics.
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31.15.A- Ab initio calculations
31.15.bw Coupled-cluster theory
33.20.Ea Infrared spectra

Properties of random state manifolds with applications to intramolecular vibrational redistribution

W. Dietz and S. F. Fischer

J. Chem. Phys. 113, 2741 (2000); http://dx.doi.org/10.1063/1.1305526 (19 pages) | Cited 4 times

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We investigate spectral properties of random manifolds, which consist of a bright state coupled to mutually uncoupled dark states for an equal but otherwise arbitrary distribution of the couplings. Both Poisson and Wigner distributions of the energy spacings of the dark states are taken into account. The Poisson spacing model is solved exactly. The average spectrum is Lorentzian. The average dilution factor comes out to be a function alone of the mean coupling strength normalized to the mean neighbor spacing of the dark states. A simple expression for the explored fraction of the available phase space is obtained. Numerical studies indicate that the normalized coupling even controls the whole distribution of the dilution factor for the Poisson model. For weak mean coupling strength a secondary peak occurs in this distribution for both the Poisson and the Wigner model. A perturbational analysis shows that this peak leads back to accidental resonances of the bright state with single dark states. A simple tier model is suggested for treating the intermixing of vibrational dark basis states in molecules. Results are compared with experimental data. © 2000 American Institute of Physics.
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33.20.Tp Vibrational analysis

Pseudorotation in tetrahydrofuran⋯HF heterodimer

J. L. Alonso, J. C. López, S. Blanco, A. Lesarri, and F. J. Lorenzo

J. Chem. Phys. 113, 2760 (2000); http://dx.doi.org/10.1063/1.1305863 (8 pages) | Cited 8 times

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Seven isotopomers of the hydrogen-bonded heterodimer tetrahydrofuran⋯HF have been investigated in the frequency range 6–18 GHz using molecular-beam Fourier transform microwave spectroscopy. The rotational spectra of C4H8O⋯HF and C4H8O⋯DF exhibit small tunneling splittings, which were not observed for C4D8O⋯HF and the four single 13C isotopomers. These observations analyzed in terms of symmetry considerations allow us to conclude that these splittings are due to pseudorotation within the tetrahydrofuran subunit of the complex and not to HF inversion. The spectroscopic parameters of the complex have been interpreted in terms of a geometry in which tetrahydrofuran has a conformation close to the twisted ring form, with HF lying on the plane bisector to the COC ring angle. © 2000 American Institute of Physics.
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33.20.Bx Radio-frequency and microwave spectra
33.20.Sn Rotational analysis
33.15.Fm Bond strengths, dissociation energies
back to top Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

Densification effects on the Boson peak in vitreous silica: A molecular-dynamics study

P. Jund and R. Jullien

J. Chem. Phys. 113, 2768 (2000); http://dx.doi.org/10.1063/1.1305861 (4 pages) | Cited 10 times

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We perform classical molecular-dynamics simulation to study the effect of densification on the vibrational spectrum of a model silica glass. We concentrate this study on the so-called Boson peak and compare our results, obtained from a direct diagonalization of the dynamical matrix, with experimental Raman data. We show that, upon densification, the position of the Boson peak shifts towards higher frequencies while its magnitude decreases which is in agreement with a recent experimental study. © 2000 American Institute of Physics.
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63.50.-x Vibrational states in disordered systems
61.43.Fs Glasses
61.43.Bn Structural modeling: serial-addition models, computer simulation
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