Volume 114, Issue 11, 15 March 2001
Index of content:
- Theoretical Methods and Algorithms
Six-dimensional quantum calculations of highly excited vibrational energy levels of hydrogen peroxide and its deuterated isotopomers114(2001); http://dx.doi.org/10.1063/1.1348274View Description Hide Description
We report accurate calculations of vibrational energy levels of HOOH, DOOD, and HOOD up to 10 000 cm−1 above the zero-point energy levels on a high-quality ab initiopotential energy surface. These energies were determined by the Lanczos algorithm based on repetitive matrix-vector multiplication. The six-dimensional vibrational Hamiltonian in the diatom–diatom Jacobi coordinate system was discretized in a mixed basis/grid representation. A direct product potential optimized discrete variable representation was used for the radial coordinates, while nondirect product spherical harmonics were employed for the angular degrees of freedom. The calculation and storage of the potential matrix in the angular finite basis representation were avoided by using a series of one-dimensional pseudo-spectral transformations to a direct product angular coordinate grid. The diatom–diatom exchange symmetry, when applicable, was incorporated into the basis, which significantly enhanced the efficiency for symmetric isotopomers. A few hundred low-lying vibrational levels of each isotopomer were assigned and compared with experimental data.
The generalized active space concept for the relativistic treatment of electron correlation. I. Kramers-restricted two-component configuration interaction114(2001); http://dx.doi.org/10.1063/1.1349076View Description Hide Description
As a prelude to a series of presentations dealing with the treatment of electron correlation and special relativity, we present the theoretical background and the implementation of a new two-component relativistic configuration interaction program. It is based on the method of generalized active spaces which has been extended from a nonrelativistic implementation to make use of two-component Hamiltonians and time reversal and double point group symmetry at both the spinor and Slater determinant level. We demonstrate how the great computational effort arising from such a general approach—the treatment of spin–orbit interaction and electron correlation in a fully variational framework—can be markedly reduced by the use of the aforementioned symmetries. Evidence for the performance of the program is given through a number of calculations on light systems with a significant spin–orbit splitting in low-lying electronic states and the well-known problem case thallium, which often serves as a rigorous test system in relativistic electronic structure calculations.
Rate coefficient calculation for diffusion-influenced reversible reactions with longer-range reactivities114(2001); http://dx.doi.org/10.1063/1.1350577View Description Hide Description
The chemically relevant so-called phenomenological forward and reverse rate coefficients of reversible bimolecular solutionreactions, for nonlocal reactivities defined via attributed irreversible diffusion-kinetic schemes [W. Naumann and A. Molski, J. Chem. Phys. 103, 3474 (1995)], are exactly expressed by formal operator expressions. It is shown that this rate coefficient definition corresponds to the so-called integral encounter theory by Gopich, Kipriyanov, and Doktorov [J. Chem. Phys. 110, 10888 (1999)], an isolated reactive pair approximation. Assuming detailed balance, for the bimolecular isomerization the operator expressions lead to exact relations with the rate coefficients of the irreversible partial reactions and Generalizations of the well-known Noyes formula to reversible reactions result when the corresponding Wilemski-Fixman closure approximations of the irreversible and reversible rate coefficients are inserted.
114(2001); http://dx.doi.org/10.1063/1.1345513View Description Hide Description
This work presents calculations on small molecules using second-order Møller–Plesset perturbation theory with a generalized valence bond reference wave function. Møller–Plesset perturbation theory applied to a generalized valence bond reference (GVB-MP2) currently provides the best tradeoff between accuracy and computational feasibility among the methodologies of electronic structure. Frienser and co-workers have shown that the computational effort required for the GVB-MP2 methodology scales as no more than the third power of the size of the system, while that for the coupled-cluster and complete-active space methods scales as the seventh or worse power of system size. The GVB wave function is a qualitative wave function. Spectroscopic parameters and energetics at the GVB level are in qualitative agreement with experimentally determined values. The calculations presented in this work demonstrate that spectroscopic parameters computed using GVB-MP2 are in better agreement with experiment than those computed at the GVB level, and in close agreement with those obtained from the coupled-cluster plus singles and doubles with triples substitutions, which requires significantly more effort.
- Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry
114(2001); http://dx.doi.org/10.1063/1.1349082View Description Hide Description
Geometrical and electronic structures of the acetic acid–benzene cation complex, are studied experimentally and theoretically. Experimentally, a vibrational spectrum of in the supersonic jet is measured in the 3000–3680 cm−1 region using an ion-trap photodissociationspectrometer. An electronic spectrum is also observed with this spectrometer in the 12 000–29 600 cm−1 region. Theoretically, ab initio molecular orbital calculations are performed for geometry optimization and evaluation of vibrational frequencies and electronic transition energies. The vibrational spectrum shows two distinct bands in the O–H stretching vibrational region. The frequency of the strong band (3577 cm−1) is close to that of the O–H stretching vibration of acetic acid and the weak one is located at 3617 cm−1. On the basis of geometry optimizations and frequency calculations, the strong band is assigned to the O–H stretching vibration of the cis-isomer of acetic acid in the hydrogen-bonded complex (horizontal cis-isomer). The weak one is assigned to the vertical trans-isomer where the trans-isomer of acetic acid interacts with the π-electron system of the benzene cation. The weakness of the high frequency band in the photodissociationspectrum is attributed to the binding energy larger than the photon energy injected. Only hot vertical trans-isomers can be dissociated by the IR excitation. The electronic spectrum exhibits two bands with intensity maxima at 17 500 cm−1 and 24 500 cm−1. The calculations of electronic excitation energies and oscillator strengths suggest that charge transfer bands of the vertical trans-isomer can be observed in this region in addition to a local excitation band of the horizontal cis-isomer. We assign the 17 500 cm−1 band to the charge transfer transition of the vertical trans-isomer and the 24 500 cm−1 band to the π–π transition of the horizontal cis-isomer. The calculations also suggest that the charge transfer is induced through the intermolecular bond formed between a carbon atom of benzene and the carbonyl oxygen atom of acetic acid.
114(2001); http://dx.doi.org/10.1063/1.1349078View Description Hide Description
A photoionization-efficiency spectrum of was measured over the wavelength range 108–142 nm by means of a photoionizationmass spectrometer coupled to a synchrotron as the source of radiation. Gaseous was generated in a discharge-flow reactor involving Cl, and at room temperature via these sequential reactions: According to the PIE spectrum of thus obtained, the ionization energy is eV. Based on GAUSSIAN-2 calculations, the observed ionization of near the threshold region is likely to form from singlet ionizing to doublet the calculated ionization energy 9.064 eV agrees with the experimental value. The adiabatic ionization energy of and appearance energy of from were determined to be eV and eV, respectively; the dissociation energy of the bond is thus derived to be kcal
114(2001); http://dx.doi.org/10.1063/1.1349425View Description Hide Description
The weakly bound van der Waals complex has been observed spectroscopically for the first time in the infrared (C–O stretching, ≈2143 cm−1) and millimeter wave (80–107 GHz) regions. The spectraanalyzed here resemble quite closely those of the rare gas–carbon monoxide complexes, like Ne–CO and Ar–CO, and they almost certainly arise from complexes composed of in the lowest rotational state of A symmetry. The effective ground state intermolecular separation is 3.994 Å. Predictions are given here for the and 1 pure rotational microwave transitions of in the A state. The infrared spectrum shows numerous additional transitions which must be due to composed of methane in the F and E symmetry states, but these have not yet been assigned. Future microwave measurements on these F and E states will aid further progress on the infrared spectrum.
114(2001); http://dx.doi.org/10.1063/1.1346637View Description Hide Description
The rotational spectra of four isotopomers of including nuclear quadrupolehyperfine structure in the -containing isotopomers, have been observed in the 6.5–19 GHz region with a Fourier transform microwave spectrometer and analyzed using the Watson A-reduced Hamiltonian with the inclusion of nuclear quadrupole coupling interactions where applicable. The effective structure of the complex, obtained by fitting the structural parameters to the moments of inertia of each isotopomer, is approximately slipped parallel, with oxygen in and sulfur in OCS occupying the obtuse vertices of the quadrilateral formed by the two subunits. The intermolecular distance is 3.5166(2) Å, with and OCS forming angles of 68.5(3)° and 99.6(2)° with the intermolecular axis, respectively. This structure is also supported by a Kraitchman analysis. Comparisons of the structure of with those of and show that the isoelectronic and behave similarly in their intermolecular interactions with OCS while the difference between the isovalent OCS and in their interactions with mainly arises from steric effects. The nuclear quadrupole coupling constants of the two nitrogen nuclei in do not definitively indicate a perturbation of the electronic distribution of in the complex. However, an electrostatic calculation of the electric fields at the atomic positions due to OCS shows that the perturbation is small and is therefore rendered unobservable due to the large uncertainties in the nuclear quadrupole coupling constants of the central nitrogen.
114(2001); http://dx.doi.org/10.1063/1.1349089View Description Hide Description
The rovibrational state distributions for the product of the reactions at 1.6 eV collision energy are reported. The results are compared to measurements made on the kinematically and energetically similar and reactions as well as the atom–diatom reactions For the title reactions, as for all the comparison reactions, the product appears in few of the energetically accessible states. This is interpreted as the result of a kinematic constraint on the product translational energy. Characteristic of the reactions we have previously studied, the title reactions show increasing rotational excitation of the product with increasing vibrational excitation of it, a correlation that gets stronger as the size of the alkane increases. Trends and variations in the product energy disposal are analyzed and explained by a localized reactionmodel. This model predicates a truncation of the opacity function due to competing reactive sites in the polyatomic alkane reactant, and a relaxation of the otherwise tight coupling of energy and angular momentum conservation, because the polyatomic alkyl radical product is a sink for angular momentum.
114(2001); http://dx.doi.org/10.1063/1.1349087View Description Hide Description
When a dilute mixture of in neon is subjected to Penning ionization and/or photoionization by neon atoms in their first excited states, between 16.6 eV and 16.85 eV, and the products are rapidly frozen at approximately 5 K, the infrared spectrum of the resulting deposit includes three relatively prominent product absorptions which agree well with the positions determined in earlier gas-phase studies for the three infrared-active fundamentals of The corresponding fundamentals of and have been observed for the first time, as have been many of the fundamentals of the partially deuterium-substituted cations, in experiments on isotopically enriched samples. When the effects of anharmonicity are considered (including the occurrence of “negative anharmonicity” for the out-of-plane deformation fundamental), the positions of these absorptions agree satisfactorily with those predicted in an earlier study from the fit of an ab initiopotential surface to the experimental data for as well as with those predicted in the present study from a least-squares force constant adjustment to the frequencies observed for the various isotopomers.
The Jahn–Teller and related effects in the cyclopentadienyl radical. I. The ab initio calculation of spectroscopically observable parameters114(2001); http://dx.doi.org/10.1063/1.1348275View Description Hide Description
Ab initio calculations are performed for the and states of the cyclopentadienyl radical. An important goal of these calculations is to guide the analysis of the experimentally observed – electronic spectrum. Vibrational frequencies for both the and state are reported. Large changes in frequency between the states for out-of-plane vibrations are found, leading to the expectation that overtones of these modes will appear strongly in the spectrum. Additionally, spectroscopically obtainable parameters describing the Jahn–Teller effect are calculated for the state. Using all this information the electronic spectrum is predicted for both and
The Jahn–Teller and related effects in the cyclopentadienyl radical. II. Vibrational analysis of the electronic transition114(2001); http://dx.doi.org/10.1063/1.1348276View Description Hide Description
The laser excited, jet-cooled electronic spectrum of the cyclopentadienyl radical yields detailed information about the vibronic structure of both its and states. A straightforward assignment of the vibronic structure is presented. The state vibronic structure reveals a comprehensive picture of the Jahn–Teller distortion of its potential energy surface. The molecular parameters characterizing the Jahn–Teller interaction provide the stabilization energy and distorted geometry, which are compared to previous experimental and ab initio results.
- Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation
The stationary non-Poissonian collision model of energy relaxation and stochastic motion in condensed phase processes114(2001); http://dx.doi.org/10.1063/1.1349708View Description Hide Description
The effect of energy dissipation on transport and activated rate processes in condensed phase is analyzed in detail within the non-Poissonian collision model (NPCM). The NPCM is a generalized variant of the collision model (CM) describing the instantaneous change of the velocity of probe particles induced by random collisions with particles of a medium. Unlike the conventional CM, the NPCM assumes the non-Poissonian collision statistics. In this work we concentrate on the stationary variant of the NPCM (SNPCM), which differs from the nonstationary NPCM (NNPCM) discussed in previous studies by the proper treatment of the collision statistics ensuring the time homogeneity of the process. The SNPCM is shown to be free of inconsistencies inherent in the NNPCM. In particular, the SNPCM reproduces the physically natural relations between the average parameters (the average displacement and velocity,correlation functions, etc.) well known in the transport theory. The SNPCM describes properly the specific features of the processes under study, for example, the kinetic cage effect predicted earlier. Within the SNPCM the analytical expressions for the rate of passage over a parabolic barrier, valid in the intermediate-to-strong friction limit, are derived for some particular values of the parameters of the model. The expressions obtained are analyzed in detail.
The decay of pair correlation functions in ionic fluids: A dressed ion theory analysis of Monte Carlo simulations114(2001); http://dx.doi.org/10.1063/1.1350449View Description Hide Description
We analyze the decay of structural correlation functions for 1:1, 1:2, and 2:2 electrolytesolutions obtained from Monte Carlo simulations. It is found that by the use of dressed ion theory and a simple Picard iteration scheme one can extract the leading decay parameters with high accuracy, even from simulations with a rather limited number of ions in the simulation cell. The extraction scheme consists of replacing in a self-consistent manner the tails of the simulated pair distribution functions by analytical expressions evaluated by residue analysis of short-ranged parts of the correlation functions. Numerical results in this work are restricted to primitive modelelectrolytes where the solvent only enters as a dielectric continuum. The leading decay parameters of the simulated correlation functions are compared to results obtained from the hypernetted chain (HNC) approximation. For 1:1 and 1:2 electrolytes in aqueous solution the simulation results confirm predictions from the HNC approximation. For 2:2 electrolytes the HNC results agree qualitatively with the simulations but deviate quantitatively. To investigate artifacts induced by boundary conditions used in the simulations we analyze correlation functions obtained from simulations in a spherical cell as well as with cubic periodic boundary conditions. The results and method of analysis presented are restricted to electrolyte concentrations at which the leading decay terms of the pair distribution functions exhibit monotonic exponential decay.
114(2001); http://dx.doi.org/10.1063/1.1349094View Description Hide Description
Computer simulations have been performed on binary fluid mixtures of hard hyperspheres in four and five dimensions. The equation of state and the radial distribution function have been obtained for a variety of compositions and size ratios. The simulation results for the excess compressibility factor and the contact values of the cross radial distribution functions in both dimensions are described rather accurately by a recent theoretical proposal for these quantities up to a reduced density where some features arise which are reminiscent of a fluid–solid phase transition.
114(2001); http://dx.doi.org/10.1063/1.1349095View Description Hide Description
While detailed descriptions of critical anomalies are well known for thermodynamic variables, the effect of critical fluctuations on microscopic properties is much less well understood. Herein we use the results of molecular dynamics simulation, for the distribution of local densities around a tagged particle in a neat Lennard-Jones fluid, to evaluate the behavior of the self-diffusion coefficient in the critical region We find a weakly anomalous behavior at near critical densities which we attribute to both the broadening of this local-density distribution and the enhancement of mean local density.
Heat capacity of the liquid–liquid mixture perfluoroheptane and 2,2,4-trimethylpentane near the critical point114(2001); http://dx.doi.org/10.1063/1.1350661View Description Hide Description
The heat capacity of the liquid–liquid mixture perfluoroheptane and 2,2,4-trimethylpentane (also known as iso-octane) has been measured for the first time near its upper critical consolute point using an adiabatic calorimeter. The theoretical expression for the heat capacity near the critical point was applied to our combined data runs. The critical exponent α was determined to be which agreed with theoretical predictions. When α was fixed at its theoretical value of 0.11, our value for the amplitude ratio was consistent with experimental determinations and theoretical predictions. However, the two-scale-factor universality ratio X, now consistent among experiments and theories with a value between 0.019 and 0.020, was violated in this system when using the published value for the correlation length.
114(2001); http://dx.doi.org/10.1063/1.1346640View Description Hide Description
Solid-state NMR signal enhancements of about two orders of magnitude (100–400) have been observed in dynamic nuclear polarization (DNP) experiments performed at high magnetic field (5 T) and low temperature (10 K) using the nitroxide radical 4-amino TEMPO as the source of electron polarization. Since the breadth of the 4-amino TEMPOEPR spectrum is large compared to the nuclear Larmor frequency, it has been assumed that thermal mixing (TM) is the dominate mechanism by which polarization is transferred from electron to nuclear spins. However, theoretical explanations of TM generally assume a homogeneously broadened EPR line and, since the 4-amino TEMPO line at 5 T is inhomogeneously broadened, they do not explain the observed DNP enhancements. Accordingly, we have developed a treatment of DNP that explicitly uses electron–electron cross-relaxation to mediate electron–nuclear polarization transfer. The process proceeds via spin flip–flops between pairs of electronic spin packets whose Zeeman temperatures differ from one another. To confirm the essential features of the model we have studied the field dependence of electron–electron double resonance(ELDOR) data and DNP enhancement data. Both are well simulated using a simple model of electron cross-relaxation in the inhomogeneously broadened 4-amino TEMPOEPR line.
- Surfaces, Interfaces, and Materials
114(2001); http://dx.doi.org/10.1063/1.1347964View Description Hide Description
The formation of stable anions induced by the impact of 0–1.6 eV electrons on and condensed at the surface, and embedded within the bulk of solid Kr is investigated by measuring charging of the doped Kr films. Effects of the Pt substrate and under- and over-layer coverage of these molecules on the dissociative electron attachment and stable anion formation (SAF) are studied in detail by varying the film thickness and the position of the molecules within the film. Due to recent advances, we provide new and more reliable values of the absolute cross sections, for SAF. reaches a maximum of at 0.6 eV for embedded in solid Kr; for this maximum is at 0.2 eV. These values correspond to an enhancement of factors of 5 and 10, respectively, relative to measured at the surface. The electron energy dependence of is also evaluated theoretically from calculations performed with the R-matrix formalism extended to include the influence of condensed-matter environments. By incorporating into the theory the band structure of the solid and different values of the polarization induced into the medium by electron capture, we study the effect of these parameters on Comparison between experimental and theoretical allows us to analyze the gas-phase parameters that must be modified to generate condensed-phase cross sections from gas-phase data and to discuss the effect of phenomena which are absent in the gas-phase (e.g., caging).
Influence of noncontact dissipation in the tapping mode: Attempt to extract quantitative information on the surface properties with the local force probe method114(2001); http://dx.doi.org/10.1063/1.1349179View Description Hide Description
In the Tapping mode, a variation of the oscillation amplitude and phase as a function of the tip sample distance is the necessary measurement to access quantitatively to the properties of the surface. In the present work, we give a systematic comparison between experimental data recorded on two surfaces, phase and amplitude, and theoretical curves. With an interaction between the tip and the surface taking into account an attractive and a repulsive term, the analytical approach is unable to properly describe the relationship between the phase variation and the oscillation amplitude variation. When an additional dissipation term is involved, due to the attractive interaction between the tip and the surface, the model gives a good agreement with the recorded data. Particularly, the trends in the phase variations related to the noncontact situations have been found to be amenable to an analysis based upon a simple viscoelastic behavior of the surface.