Volume 113, Issue 11, 15 September 2000
 COMMUNICATIONS


Quantum calculations of the effect of bend excitation in methane on the HCl rotational distribution in the reaction
View Description Hide DescriptionWe report novel, reduced dimensionality quantum calculations of the effect of bend excitation in methane on the rotational distribution of HCl in the reaction We use a modification of a semiempirical potential developed by Yu and Nyman in these calculations. Reaction probabilities for zero total angular momentum are obtained using an wave packet method for selected HCl rotational states (in the ground vibrational state) and summed over final CH bend/stretch states in The calculations are done in two ranges of the total energy. In the low energy range only the ground bend/stretch state of is open, and in the higher energy range that state and the second excited bend state are energetically open. With just the ground state of open we find a cold, unimodal rotational distribution of HCl, in qualitative agreement with experiment. With the opening of the excited bend state we find a multimodal HCl rotational distribution. We rationalize these results using a simple Franck–Condon argument.

Experimental evidence of polarization effects on exchangeable cations trapped in zeolites
View Description Hide DescriptionThe evolution of the activation energy of the conductivity depends on the nature of the exchanged cations, and differs in faujasites X and Y. This surprising phenomenon, reported in many works, is not yet satisfactorily explained. A qualitative explanation is proposed based on wellknown results obtained in the study of the interactions between chemical species, by means of density functional theory.

Electric dipole polarizability of one excesselectron alkali–halide cluster
View Description Hide DescriptionIn this communication, we present the first measurement of the electric dipole polarizability of alkali–halide clusters with one excess electron. The polarizability is strongly size dependent. Very large values are observed for certain sizes. Ionization potentialmeasurement is also presented for comparison.
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 ARTICLES

 Theoretical Methods and Algorithms

Fast simulation of dynamic twodimensional nuclear magnetic resonance spectra for systems with many spins or exchange sites
View Description Hide DescriptionAn efficient sparsematrixbased numerical method is constructed to simulate twodimensional nuclear magnetic resonance spectra of manyspin systems including the effects of chemical exchange and/or relaxation. The method employs efficient numerical time propagation requiring operations in the case of an spin 1/2 system. Pulses are treated with a fast implementation algorithm achieving scaling (case of spins 1/2). The method is tested in simulations of doublequantumfilter correlation spectroscopy and exchange spectroscopy experiments on five and sevenspin systems with two sites. Observed scaling is consistent with the analytic predictions. © 2000 American Institute of Physics.

On the classical limit for electronic structure and dynamics in the orbital approximation
View Description Hide DescriptionThe classical limit is shown to provide a description exactly equivalent to the quantum mechanical one in the approximation where each electron is assigned to an orbital. Strictly speaking it is therefore not a limit but an alternative way of solving the problem. There are some merits of this reformulation, most notably in that it brings the phase of the orbitals to the forefront, on equal footing as the occupancies. This allows one to discuss, e.g., electron localization, in a clearer manner. But computationally the classical description is not superior. There will be a definite advantage for more realistic electronic Hamiltonians, i.e., for implementing configuration interaction, and/or when the nuclear motion is coupled to the electronic dynamics. In this paper we limit attention to a derivation and discussion of the simple orbital approximation.

A molecular based derivation of the nucleation theorem
View Description Hide DescriptionWe show that, for condensation in an almost ideal vapor, the nucleationtheorem is essentially a consequence of the law of mass action. The usual form of the theorem, with the effects of the translational degrees of freedom of the cluster included, is then derived using statistical mechanics and molecular theory, but only under the assumptions that the cluster excludes a volume to the surrounding vapor and that the vapor is ideal. The form of the result obtained via molecular theory is such that it appears unlikely (but not impossible) that the theorem remains valid for cases when the vapor is nonideal. This suggests that further work is necessary before the theorem can be regarded as established. We also consider the effects of the presence of a carrier gas.

An approximate short time Laplace transform inversion method
View Description Hide DescriptionThe “standard” numerical methods used for inverting the Laplace transform are based on a regularization of an exact inversion formula. They are very sensitive to noise in the Laplace transformed function. In this article we suggest a different strategy. The inversion formula we use is an approximate one, but it is stable with respect to noise. The new approximate expression is obtained from a short time expansion of the Bromwich inversion formula. We show that this approximate result can be significantly improved when iterated, while remaining stable with respect to noise. The iterated method is exact for the class of functions of type The method is applied to a harmonic model of the stilbene molecule, to a truncated exponent series, and to the flux–flux correlation function for the parabolic barrier. These examples demonstrate the utility of the method for application to problems of interest in molecular dynamics.

Brueckner based generalized coupled cluster theory: Implicit inclusion of higher excitation effects
View Description Hide DescriptionA generalization of the single reference Coupled Cluster parameterization for the ground statewave function is proposed that includes substitution operators that annihilate the reference determinant, but which act nontrivially on the correlated part of the wave function. It is shown that an inclusion of such twobody operators can mimic the effect of conventional connected triple and higher excitation operators. Results obtained with Brueckner based Generalized Coupled Cluster Doubles theory (BGCCDversion x) are found to be comparable in accuracy to CCSD(T) and CCSDT for a number of difficult test cases. In the current version of the BGCCD approach we obtain correlated ionization potentials and electron affinities as a byproduct of a ground state calculation. This multistate nature of the BGCCDX approach can give rise to problems with intruder states similar as in Fock Space Coupled Clustertheory.

Multidimensional reactive rate calculations in dissipative chaotic systems
View Description Hide DescriptionUsing concepts from transient chaos and stochastic dynamics, we develop a perturbative solution for multidimensional activated rate processes. The solution is applicable to the underdamped regime where system dynamics prevails over bath fluctuations. The baseline of the method is the partition of the multidimensional reactive flux in a chaotic system to a sum of independent fluxes in onedimensional systems. The partition is based on the underlying dynamics of the multidimensional system. The method is fast and explains the high and low temperature dependence of multidimensional reaction rates.

Rigorous solutions of diatomic molecule oscillator with empirical potential function in phase space
View Description Hide DescriptionWithin the framework of the quantum phasespace representation established by TorresVega and Frederick, the rigorous solutions of the Schrödinger equation of the diatomic molecule oscillator with an empirical potential function are solved and discussed, and the Heisenberg uncertainty principle is interpreted in this physical system.
 Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

The direct production of in the reaction of with the ethyl radical
View Description Hide DescriptionA new product channel that yields vibrationally excited in the reaction of the ethyl radical with is experimentally observed by timeresolvedFourier transform infrared emission spectroscopy. The branching ratios for the different vibrational states are estimated to be 0.21±0.06, 0.27±0.03, 0.14±0.02, 0.08±0.02, 0.07±0.02, 0.07±0.02, 0.06±0.02, 0.05±0.02, and 0.05±0.02 for respectively. Previously, only the and channels were known. Kinetics tests are provided to verify that the CO is produced directly in the reaction and not from secondary chemistry. The two possible new product channels are and The implications of this previously unexplored reaction channel for combustion chemistry and the possible mechanisms for this reaction are discussed.

The statetostate predissociation dynamics of OC–HF upon HF stretch excitation
View Description Hide DescriptionPhotofragment angular and state distributions have been measured following the vibrational predissociation of the OC–HF complex. An Fcenter laser is used to pump the fundamental H–F stretching vibration of the complex and a second Fcenter laser is used to probe the rotational states of the HF fragment as a function of recoil angle. The complex dissociates via two different sets of channels, one that produces (intermolecular transfer) and the other transfer). Analysis of the data gives correlated final state distributions, as well as an accurate value for the dissociation energy of the complex, namely

Renner–Teller induced photodissociation of HCO in the first absorption band: Determination of linewidths for the states by filterdiagonalization
View Description Hide DescriptionWe present new calculations on the Renner–Teller induced decay of the vibrational states of using accurate ab initiopotential energy surfaces. The dynamics calculations are performed by employing filter diagonalization and an absorbing optical potential in the exit channel. The objective of this investigation is twofold: the completion of earlier timedependent wave packet calculations by determining resonance widths for allvibrational states for projection quantum number —up to 2.75 eV above the dissociation threshold—and the determination of the widths for the longlived states. In the latter case, a clearcut dependence, where J is the total angular momentum, is observed indicating that the rate determining step is Kresonance interaction between and 2 states. The experimentally observed Jindependent contribution (0.22–0.5 cm^{−1}), which dominates the linewidth for small values of J, is not accounted for by our calculations. Arguments are put forward, that it is caused by spin–orbit interaction, which is not included in our treatment.

An accurate global ab initio potential energy surface for the electronic state of HOBr
View Description Hide DescriptionA global, analytical potential energy surface for the ground electronic state of HOBr has been determined using highly correlated multireference configuration interactionwave functions and explicit basis set extrapolations of large correlation consistent basis sets. The ab initio data have been fit to an analytical functional form that accurately includes both the HOBr and HBrO minima, as well as all dissociation asymptotes. Small adjustments to this surface are made based on the limited experimental data available and by indirectly taking into account the effects of spin–orbit coupling on the dissociation channel. Vibrational energy levels are calculated variationally for both HOBr and HBrO up to the dissociation limit using a truncation/recoupling method. The HOBr isomer is calculated to contain 708 bound vibrational energy levels, while the HBrO minimum lies above the dissociation limit but is calculated to have 74 “quasibound,” localized eigenstates. Infrared intensities for all of these vibrational transitions are also calculated using MRCI dipole moment functions. The assignment of the HOBr states is complicated by strong stretch–bend resonances even at relatively low energies. In contrast to the HOCl case, these state mixings made it particularly difficult to assign the relatively intense OH overtone bands above The vibrational density of states of HOBr at the dissociation limit is determined to be 0.16 states/cm^{−1}. Comparisons to recent work on HOCl using similar methods are made throughout.

Temperature effect on the deactivation of electronically excited potassium by hydrogen molecule
View Description Hide DescriptionTimeresolved fluorescences from varied K excited states are monitored as a function of pressure. According to a threelevel model, the rate coefficients of collisional deactivation for the and states at 473 K have been determined to be 4.94±0.15, 5.30±0.15, and In addition, the collision transfer of transition may be derived to be 5.03±0.21, 4.68±0.30, and showing dominance of the state deactivation processes owing to the effect of nearresonance energy transfer. As the temperature is varied, the activation energies for the collisions of and atoms with respectively, may be estimated to be 5.38±0.33, 4.39±0.16, and 3.23±0.19 kJ/mol. The first two values are roughly consistent with the theoretical calculations of 3.1 and 0.9 kJ/mol in symmetry predicted by Rossi and Pascale. The obtained energy barriers are small enough to allow for occurrence of the harpoon mechanism, a model applicable to the reactions between and alkali atoms such as K, Rb, and Cs. Among them, collisions appear to be the first case to possess a slight energy barrier. This finding of energy barrier may account for the discrepancy for the state reactivity towards observed between K (or Rb) and Cs atoms.

Theoretical simulations of the excitation spectrum: Spectroscopic manifestation of a linear isomer?
View Description Hide DescriptionPossible manifestations of a linear isomer of a rare gas–halogen molecule van der Waals complex in its excitation spectrum are analyzed using a continuous oneparametric family of Xstate potential energy surfaces (PESs) with variable depths of minima in the Tshaped and linear configurations. For the complex as an example, the propensities in the frequencies and intensities of the representative transitions from Tshaped and linear isomers are analyzed and the variation of the whole spectrum with the topology of the Xstate PES is established. Qualitatively good agreement with the experimental spectrum clearly suggests that the unassigned secondary band of the observed spectrum is likely formed by transitions from the linear isomer, whose energy is very close to that of the Tshaped one. Present results provide strong evidence for the possibility to detect a linear isomer of rare gas–halogen molecule complexes via conventional excitation spectroscopy. © 2000 American Institute of Physics.

Anharmonic treatment of the lowestenergy conformers of glycine: A theoretical study
View Description Hide DescriptionThe structure and energetics of the four lowestenergy conformers of glycine were determined at the MP2/augccpVDZ level of theory. The optimized structural parameters for these conformers agree with previous theoretical results obtained by highly correlated ab initio methods and with available experimental data. The only structure with planar heavy atom arrangement is conformer I (global minimum), the other conformers have nonplanar heavy atom arrangements. In accordance with temperature dependence studies of the vibrational spectra in various rare gas environments, conformers III and IV have small interconversion barriers to conformer I (940 and 740 cm^{−1}). Our calculations have shown that fulldimensional anharmonic treatment is required for an accurate description of the vibrational modes in various glycine conformers. The most pronounced effect has been observed for conformer II with the intramolecular O–H⋅⋅⋅N bond. The theoretical results obtained at the MP2/augccpVDZ level reproduce quantitatively the argon matrix experiments. The calculation uses the quartic force field approximation in the framework of secondorder perturbation theory. An estimate of the higherorder correction is also given.

Finite size effects and rotational relaxation in superfluid helium nanodroplets: Microwaveinfrared doubleresonance spectroscopy of cyanoacetylene
View Description Hide DescriptionMicrowaveinfrared doubleresonance spectroscopy has been used to probe the solvation environment and its influence on the rotational relaxation of a cyanoacetylene molecule embedded in a superfluidnanodroplet. The results support a model in which (within any given rotational state) the guest molecules are distributed over a set of spectroscopically inequivalent states which are most likely “particleinabox” states originating from the confinement of the guest molecule within the droplet. Revisitation of previously collected microwave–microwave doubleresonance data suggests that transitions between these states occur at a rate which is comparable to the rotational relaxation rate, but not fast enough as to produce motionally narrowed, homogeneous absorption lines. The relative intensities of the rotational lines in the microwaveinfrared doubleresonance spectra are observed to depend strongly on the average droplet size. In the large droplet limit we can explain the observed pattern by invoking a “strong collision” regime, i.e., one in which the branching ratios of the rotational relaxation do not depend on the initial rotational state. For small droplets we speculate that, because of finite size effects, the density of (surface) states may become discontinuous, producing deviations from the “thermal” behavior of the larger systems.

Fragmentation of small sodium clusters
View Description Hide DescriptionFragmentation of small sodium clusters was studied by performing both ab initio and classical molecular dynamics simulations. In ab initio calculations at 1200–2400 K, neutral sodium clusters with 10 and 13 atoms ejected both monomers and dimers. The observed behavior is in agreement with previous calculations stating that the electronic shell oscillations diminish strongly as a function of temperature. The fragmentation rates obtained with the ab initio method are consistent with the Kelvin equation for the equilibrium vapor pressure of small clusters. The differences between the results obtained using different models reflect the differences between the dissociation energies calculated correspondingly.

Electronic states of the copper, silver, and gold silicides and their ions
View Description Hide DescriptionThe results of theoretical calculations for the ground state and lowlying excited states of SiCu, SiAg, and SiAu, and their ions and are presented. Calculations were carried out with highlevel correlated methods including relativistic corrections at the level of the Douglas–Kroll approximation. The ground state data are compared with the recent experimental findings and they differ in the assignment of the groundstate symmetry. All neutral silicides are predicted to have the electronic ground state of symmetry, in agreement with earlier theoretical data. The neutral species and both negative and positive ions of silicides are found to be quite stable in the ground electronic state and in several lowlying excited states. The relativistic effects bring significant contribution to the stabilization of the gold silicide and its ions in all electronic states investigated in this paper. © 2000 American Institute of Physics.