Volume 113, Issue 11, 15 September 2000
Index of content:
- Theoretical Methods and Algorithms
Fast simulation of dynamic two-dimensional nuclear magnetic resonance spectra for systems with many spins or exchange sites113(2000); http://dx.doi.org/10.1063/1.1288607View Description Hide Description
An efficient sparse-matrix-based numerical method is constructed to simulate two-dimensional nuclear magnetic resonance spectra of many-spin 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 double-quantum-filter correlation spectroscopy and exchange spectroscopy experiments on five- and seven-spin systems with two sites. Observed scaling is consistent with the analytic predictions. © 2000 American Institute of Physics.
113(2000); http://dx.doi.org/10.1063/1.1288915View Description Hide Description
The 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.
113(2000); http://dx.doi.org/10.1063/1.1288802View Description Hide Description
We 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.
113(2000); http://dx.doi.org/10.1063/1.1288790View Description Hide Description
The “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.
113(2000); http://dx.doi.org/10.1063/1.1288912View Description Hide Description
A 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 two-body operators can mimic the effect of conventional connected triple and higher excitation operators. Results obtained with Brueckner based Generalized Coupled Cluster Doubles theory (BGCCD-version 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 by-product of a ground state calculation. This multistate nature of the BGCCD-X approach can give rise to problems with intruder states similar as in Fock Space Coupled Clustertheory.
113(2000); http://dx.doi.org/10.1063/1.1288686View Description Hide Description
Using 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 one-dimensional 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.
113(2000); http://dx.doi.org/10.1063/1.1287282View Description Hide Description
Within the framework of the quantum phase-space representation established by Torres-Vega 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
113(2000); http://dx.doi.org/10.1063/1.1288791View Description Hide Description
A new product channel that yields vibrationally excited in the reaction of the ethyl radical with is experimentally observed by time-resolvedFourier 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.
113(2000); http://dx.doi.org/10.1063/1.1288605View Description Hide Description
Photofragment angular and state distributions have been measured following the vibrational predissociation of the OC–HF complex. An F-center laser is used to pump the fundamental H–F stretching vibration of the complex and a second F-center 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 filter-diagonalization113(2000); http://dx.doi.org/10.1063/1.1288606View Description Hide Description
We 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 time-dependent 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 long-lived states. In the latter case, a clear-cut dependence, where J is the total angular momentum, is observed indicating that the rate determining step is K-resonance interaction between and 2 states. The experimentally observed J-independent 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.
113(2000); http://dx.doi.org/10.1063/1.1288913View Description Hide Description
A 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.
113(2000); http://dx.doi.org/10.1063/1.1288787View Description Hide Description
Time-resolved fluorescences from varied K excited states are monitored as a function of pressure. According to a three-level 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 near-resonance 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.
113(2000); http://dx.doi.org/10.1063/1.1288793View Description Hide Description
Possible manifestations of a linear isomer of a rare gas–halogen molecule van der Waals complex in its excitation spectrum are analyzed using a continuous one-parametric family of X-state potential energy surfaces (PESs) with variable depths of minima in the T-shaped and linear configurations. For the complex as an example, the propensities in the frequencies and intensities of the representative transitions from T-shaped and linear isomers are analyzed and the variation of the whole spectrum with the topology of the X-state 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 T-shaped 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.
113(2000); http://dx.doi.org/10.1063/1.1288914View Description Hide Description
The structure and energetics of the four lowest-energy conformers of glycine were determined at the MP2/aug-cc-pVDZ 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 full-dimensional 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/aug-cc-pVDZ level reproduce quantitatively the argon matrix experiments. The calculation uses the quartic force field approximation in the framework of second-order perturbation theory. An estimate of the higher-order correction is also given.
Finite size effects and rotational relaxation in superfluid helium nanodroplets: Microwave-infrared double-resonance spectroscopy of cyanoacetylene113(2000); http://dx.doi.org/10.1063/1.1288604View Description Hide Description
Microwave-infrared double-resonance 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 “particle-in-a-box” states originating from the confinement of the guest molecule within the droplet. Revisitation of previously collected microwave–microwave double-resonance 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 microwave-infrared double-resonance 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.
113(2000); http://dx.doi.org/10.1063/1.1288380View Description Hide Description
Fragmentation 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.
113(2000); http://dx.doi.org/10.1063/1.1288919View Description Hide Description
The results of theoretical calculations for the ground state and low-lying excited states of SiCu, SiAg, and SiAu, and their ions and are presented. Calculations were carried out with high-level 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 ground-state 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 low-lying 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.
113(2000); http://dx.doi.org/10.1063/1.1286978View Description Hide Description
Time-dependent photodetachmentspectra for small electronically and vibrationally excited negatively chargedcarbon clusters are measured using an electrostaticion trap. The time dependence demonstrates the presence of metastable electronic states with lifetimes in the range of 10 to 200 ms. Comparison is made with available data and theoretical calculations.
- Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation
113(2000); http://dx.doi.org/10.1063/1.1288688View Description Hide Description
First-principles molecular dynamics simulations have been performed on the solvation of in water. Consistent with the available experimental data, we find that the first solvation shell of contains on average 5.2 water molecules. A significant number of water exchanges between the first and second solvation shells are observed. However, the simulations are not long enough to reliably measure the rate of water exchange. Contrary to several previous studies, we do not find any effect of on the orientation of water molecules outside of the first solvation shell. Furthermore, the complete set of structural properties determined by first-principles molecular dynamics is not predicted by any of the known classical simulations.
113(2000); http://dx.doi.org/10.1063/1.1288690View Description Hide Description
The ultrafast optical Kerr-response of water and heavy water has been measured at 1 bar in the temperature range between 273 and 373 K. The nuclear Kerr response of the liquid exhibits a pronounced double exponential decay on longer time scales after dephasing of impulsively perturbed acoustic modes is completed. The time constant, characterizing the slowly decaying exponential component of the Kerr-response function is in quantitative agreement with rotational diffusion time constants of the water molecules obtained form nuclear magnetic resonance(NMR) spin-lattice relaxation rates. A detailed comparison with THz time domain spectroscopy demonstrates that the reorientational dynamics responsible for the long time tail of the Kerr response are due to single molecule as opposed to collective effects. Furthermore, a good agreement between the single molecule rotational diffusion and the Stokes–Einstein–Debye equation is found in the temperature range of thermodynamic stability of the liquid. The time constant, characterizing the fast exponential component of the Kerr-response of water is found to be in qualitative agreement with central Lorentzian linewidths obtained from frequency-domain, depolarized Raman scattering experiments. The temperature dependence of does not follow an Arrhenius-type behavior, which was previously taken as evidence for thermally activated crossing of a librational barrier with concomitant hydrogen-bond breakage. Instead, the temperature dependence of the fast relaxation time constant can be represented adequately by the Speedy–Angell relation which has been shown to accurately describe a number of transport parameters and thermodynamic properties of water.