Volume 114, Issue 11, 15 March 2001
 COMMUNICATIONS


Photodissociation of oriented HXeI molecules in the gas phase
View Description Hide DescriptionWe report the first production of the molecule HXeI, which is bound by ionic forces, in the gas phase. The molecule is generated by the photodissociation of HI molecules on a large cluster and is identified by detecting the asymmetric distribution of the H atom fragments of the oriented HXeI. The orientation is achieved in a combined pulsed laser and weak electrostatic field making use of the large anisotropy in the polarizability and the large dipole moment of this molecule.

Quantum dynamics on new potential energy surfaces for the reaction
View Description Hide DescriptionWe report dynamics on two interpolated global potential energy surfaces (PES) for the reaction. The first PES is based on both and MRCI/augccpVTZ ab initio calculations. In the second version, the energies at the interpolation data points are improved to the UCCSD(T)/augccpVQZ level. Fully converged integral cross sections were calculated on these two PESs for the title reaction for the ground rovibrational and two rotationally excited initial states. Good agreement between theory and experiment for rate coefficients for temperatures up to 1050 K are only achieved on the second PES.
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 ARTICLES

 Theoretical Methods and Algorithms

Sixdimensional quantum calculations of highly excited vibrational energy levels of hydrogen peroxide and its deuterated isotopomers
View Description Hide DescriptionWe report accurate calculations of vibrational energy levels of HOOH, DOOD, and HOOD up to 10 000 cm^{−1} above the zeropoint energy levels on a highquality ab initiopotential energy surface. These energies were determined by the Lanczos algorithm based on repetitive matrixvector multiplication. The sixdimensional 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 onedimensional pseudospectral 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 lowlying 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. Kramersrestricted twocomponent configuration interaction
View Description Hide DescriptionAs 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 twocomponent 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 twocomponent 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 lowlying electronic states and the wellknown problem case thallium, which often serves as a rigorous test system in relativistic electronic structure calculations.

Rate coefficient calculation for diffusioninfluenced reversible reactions with longerrange reactivities
View Description Hide DescriptionThe chemically relevant socalled phenomenological forward and reverse rate coefficients of reversible bimolecular solutionreactions, for nonlocal reactivities defined via attributed irreversible diffusionkinetic 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 socalled 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 wellknown Noyes formula to reversible reactions result when the corresponding WilemskiFixman closure approximations of the irreversible and reversible rate coefficients are inserted.

Calculations using generalized valence bond based Møller–Plesset perturbation theory
View Description Hide DescriptionThis work presents calculations on small molecules using secondorder Møller–Plesset perturbation theory with a generalized valence bond reference wave function. Møller–Plesset perturbation theory applied to a generalized valence bond reference (GVBMP2) currently provides the best tradeoff between accuracy and computational feasibility among the methodologies of electronic structure. Frienser and coworkers have shown that the computational effort required for the GVBMP2 methodology scales as no more than the third power of the size of the system, while that for the coupledcluster and completeactive 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 GVBMP2 are in better agreement with experiment than those computed at the GVB level, and in close agreement with those obtained from the coupledcluster plus singles and doubles with triples substitutions, which requires significantly more effort.
 Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Charge transfer interaction in the acetic acid–benzene cation complex
View Description Hide DescriptionGeometrical 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 iontrap 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 cisisomer of acetic acid in the hydrogenbonded complex (horizontal cisisomer). The weak one is assigned to the vertical transisomer where the transisomer of acetic acid interacts with the πelectron 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 transisomers 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 transisomer can be observed in this region in addition to a local excitation band of the horizontal cisisomer. We assign the 17 500 cm^{−1} band to the charge transfer transition of the vertical transisomer and the 24 500 cm^{−1} band to the π–π transition of the horizontal cisisomer. 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.

Photoionization study of formed in the reaction system
View Description Hide DescriptionA photoionizationefficiency 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 dischargeflow 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 GAUSSIAN2 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

Infrared and millimeter wave spectra of the complex in the A internal rotation state
View Description Hide DescriptionThe 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.

Rotational spectrum and molecular structure of
View Description Hide DescriptionThe 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 Areduced 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.

The reactions: Statetostate dynamics and models of energy disposal
View Description Hide DescriptionThe 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.

The infrared spectra of the cations trapped in solid neon
View Description Hide DescriptionWhen 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 gasphase studies for the three infraredactive fundamentals of The corresponding fundamentals of and have been observed for the first time, as have been many of the fundamentals of the partially deuteriumsubstituted cations, in experiments on isotopically enriched samples. When the effects of anharmonicity are considered (including the occurrence of “negative anharmonicity” for the outofplane 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 leastsquares 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 parameters
View Description Hide DescriptionAb 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 outofplane 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 transition
View Description Hide DescriptionThe laser excited, jetcooled 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 nonPoissonian collision model of energy relaxation and stochastic motion in condensed phase processes
View Description Hide DescriptionThe effect of energy dissipation on transport and activated rate processes in condensed phase is analyzed in detail within the nonPoissonian 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 nonPoissonian 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 intermediatetostrong 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 simulations
View Description Hide DescriptionWe 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 selfconsistent manner the tails of the simulated pair distribution functions by analytical expressions evaluated by residue analysis of shortranged 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.

Equation of state and structure of binary mixtures of hard ddimensional hyperspheres
View Description Hide DescriptionComputer 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.

Selfdiffusion near the liquid–vapor critical point
View Description Hide DescriptionWhile 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 LennardJones fluid, to evaluate the behavior of the selfdiffusion coefficient in the critical region We find a weakly anomalous behavior at near critical densities which we attribute to both the broadening of this localdensity distribution and the enhancement of mean local density.

Heat capacity of the liquid–liquid mixture perfluoroheptane and 2,2,4trimethylpentane near the critical point
View Description Hide DescriptionThe heat capacity of the liquid–liquid mixture perfluoroheptane and 2,2,4trimethylpentane (also known as isooctane) 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 twoscalefactor 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.

Mechanism of dynamic nuclear polarization in high magnetic fields
View Description Hide DescriptionSolidstate 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 4amino TEMPO as the source of electron polarization. Since the breadth of the 4amino 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 4amino 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 crossrelaxation 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 crossrelaxation in the inhomogeneously broadened 4amino TEMPOEPR line.