Volume 113, Issue 7, 15 August 2000
 COMMUNICATIONS


Dissociative electron attachment to gasphase 5bromouracil
View Description Hide DescriptionWe report measurements of dissociative electron attachment (DEA) to gaseous 5bromouracil (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 but also various anion fragments resulting from endo and exocyclic bond ruptures, such as uracilyl anions, i.e., and a 68 amu anion tentatively attributed to The incident electron energy dependent signatures of either the and yields (at 0, 1.4, and 6 eV), or the and 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 and are estimated to be about and respectively.
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 ARTICLES

 Theoretical Methods and Algorithms

Selfadaptive quadrature and numerical path integration
View Description Hide DescriptionIn 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, selfadaptive 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.

The mapping of the local contributions of Fermi and Coulomb correlation into intracule and extracule density distributions
View Description Hide DescriptionThe contributions of the correlated and uncorrelated components of the electronpair density to atomic and molecular intracule and extracule densities and its Laplacian functions and 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 oneelectron densities. In contrast, by analyzing the correlated components of and namely, and 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 and distributions calculated at the two levels of theory. As application examples, the He, Ne, and Ar atomic series, the molecular series, and the molecule have been investigated. For these atoms and molecules, it is found that Fermi correlation accounts for the main characteristics of and with Coulomb correlation increasing slightly the locality of these functions at the CI level of theory. Furthermore, and the associated Laplacian functions, reveal the shortranged nature and high isotropy of Fermi and Coulomb correlation in atoms and molecules.

On the condensed Fukui function
View Description Hide DescriptionA 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 nonnegative 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.

Microcanonical temperature and its Arrhenius relation to lifetimes in isomerization dynamics of clusters
View Description Hide DescriptionIn a previous study of isomerizationdynamics 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 Arrheniustype relation with thus defined local microcanonical temperature assigned to the corresponding potential basin. We present an analysis of how the Arrhenius relation can arise.

Smallcore multiconfigurationDirac–Hartree–Fockadjusted pseudopotentials for post d main group elements: Application to PbH and PbO
View Description Hide DescriptionRelativistic pseudopotentials (PPs) of the energyconsistent variety have been generated for the post group 13–15 elements, by adjustment to multiconfiguration Dirac–Hartree–Fock data based on the Dirac–Coulomb–Breit Hamiltonian. The outercore shells are explicitly treated together with the valence shell, with these PPs, and the implications of the smallcore choice are discussed by comparison to a corresponding largecore PP, in the case of Pb. Results from valence ab initio one and twocomponent calculations using both PPs are presented for the finestructure splitting of the groundstate configuration of the Pb atom, and for spectroscopic constants of PbH and PbO In addition, a combination of smallcore and largecore PPs has been explored in spinfreestate shifted calculations for the above molecules.

On the temperature, equipartition, degrees of freedom, and finite size effects: Application to aluminum clusters
View Description Hide DescriptionThe 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.

Exploring the ab initio/classical free energy perturbation method: The hydration free energy of water
View Description Hide DescriptionThe ab initio/classical free energyperturbation (ABCFEP) 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 energyperturbation 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 ABCFEP 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 ABCFEP method by comparing to the experimentally wellknown free energy of hydration of water at two state points (ambient conditions, and 973.15 K and 600 kg/m^{3}). The TIP4PFQ 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 gradientcorrected 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 multibody 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, ABCFEP 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.

Direct iterative solution of the generalized Bloch equation. II. A general formalism for manyelectron systems
View Description Hide DescriptionA 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 (nonHermitian) 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 twodimensional state selective or state specific version.

Direct iterative solution of the generalized Bloch equation. III. Application to cluster models
View Description Hide DescriptionA stateselective multiconfigurational singlereference 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 postHartree–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 sizeextensivity 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 socalled approximation performs extremely well in all cases and even outperforms the externally corrected, reduced multireference (RMR) CCSD in the quasidegenerate region of geometries.

Direct iterative solution of the generalized Bloch equation. IV. Application to LiH, BeH, and
View Description Hide DescriptionThe state selective, one and twodeterminantal 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 openshell 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.

Hydrogen bonding at the diatomicsinmolecules level: Water clusters
View Description Hide DescriptionFurther developments of the intermolecular diatomicsinmolecules (DIM) theory towards construction of potential energy surfaces of hydrogenbonded molecular aggregates are presented. Compared to the previously studied hydrogen fluoride clusters [J. Chem. Phys. 111, 4442 (1999)], considerably more complicated and challenging systems, namely, water clusters have been analyzed in this work. The present DIM, or more precisely, diatomicsinionicsystems, 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 electronic states within the valence bond description of water molecules. The potential curves of diatomic molecules required for the present application, including ionic species 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 lowlying isomers of are compared to the reference data showing a good predictive power of this method.

Flexible transition state theory for a variable reaction coordinate: Derivation of canonical and microcanonical forms
View Description Hide DescriptionA completely general canonical and microcanonical (energyresolved) 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 Gmatrix associated with the external rotations and the relative internal motion of the fragments.
 Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Directdynamics approach to catalytic effects: The tautomerization of 3hydroxyisoquinoline as a test case
View Description Hide DescriptionThe mechanism of tautomerization of 3hydroxyisoquinoline (3HIQ) in its first excited singlet state is studied theoretically for the isolated molecule and the 1:1 complexes with water 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 tunnelingrate 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/631G* and CASSCF(8,8)/631G* 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, 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.

The electronic structure and chemical bonding of aluminum acetylide: and An experimental and theoretical investigation
View Description Hide DescriptionWe have investigated the electronic structure and chemical bonding of and both experimentally and theoretically. Photoelectron spectra of 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 planarbridged 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 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 anion are discussed. © 2000 American Institute of Physics.

Quantum Monte Carlo determination of the atomization energy and heat of formation of propargyl radical
View Description Hide DescriptionThe quantum Monte Carlo (QMC) method is used to compute the atomization energy and the heat of formation of the propargyl radical, The effective core potential and fixednode approximations are used in the diffusion Monte Carlo (DMC) variant of QMC. Two generalized gradient approximationdensity 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 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.

Multiisotope study of fractionation effects in the ozone formation process
View Description Hide DescriptionThe 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 ozoneisotope effect since pressure, temperature, and composition of the gas in which ozone is produced are well known. While and are depleted by −1.5% and −3.9%, respectively, all others are enriched with showing the highest value of 19.8%. Enrichments or depletions are independent of the isotope composition of the oxygen gas.

Interpretation and deperturbation of the Λtype doubling in the state of GaH
View Description Hide DescriptionThe unusual J dependence of the Λ doubling splitting that separates e and f levels in the 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 zerothorder part of the Hamiltonian to electronic and vibrational motions, spinorbit 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 Λdoubling splitting, i.e., the variations of the energydifferences 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 offdiagonal Coriolis matrix elements within the block. To a lesser extent, smaller differences between the e and frotational constants in the diagonal elements also contribute to the pattern of splittings. The consistency of the resulting values of the main fitted difference parameters splitting at null rotation and firstorder diagonal and offdiagonal 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.

Ab initio studies of anionic clusters of water pentamer
View Description Hide DescriptionConformers of water anionic pentamer cluster 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.

Vibrational spectra and intramolecular vibrational redistribution in highly excited deuterobromochlorofluoromethane CDBrClF: Experiment and theory
View Description Hide DescriptionThe rovibrational spectra of deuterobromochlorofluoromethane (CDBrClF) were measured at intermediate and high resolution full bandwidth, halfmaximum) by interferometric Fourier transform infrared spectroscopy in the range from the far infrared at to the near infrared covering all the fundamentals and CD stretching overtones up to polyad 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 two CD bending and the CF stretching mode 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 fourdimensional normal coordinate subspace. The potential energy and the dipole moment function are calculated ab initio on this grid using selfconsistent 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 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.