Featured Researches

Chemical Physics

Bond angle distribution in amorphous germania and silica

The distribution of Ge-O-Ge and Si-O-Si bond angles alpha in amorphous germania and silica is re-determined on the basis of diffraction experiments. The bond angle alpha joining adjacent tetrahedra is the central parameter of any continuous random network description (CRN) of these glasses. New high energy photon diffraction experiments on amorphous germania (at photon energies of 97 and 149 keV) are presented, covering the momentum transfer 0.6-33.5 AA^{-1}. In photon diffraction experiments on GeO2 the contribution of the OO pairs is very small. To obtain a similar information for amorphous SiO2, high energy photon diffraction experiments have been combined with neutron diffraction data on amorphous silica in order to eliminate the OO- partial structure factor. With this technique it is shown that the Si-O-Si angle distribution is fairly narrow (sigma=7.5 degree) and in fact comparable in width to the Ge-O-Ge angle distribution (sigma=8.3 degree), a result which differs from current opinion. The narrower distribution found in this study are in much better agreement to the determinations based on 29Si-MAS-NMR. Among the various models relating the chemical shift to the bond angle, best agreement is found with those models based on the secant model. Sharp components in the bond angle distribution can be excluded within the reached real space resolution of 0.09 AA.

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Chemical Physics

Boson-realization model for the vibrational spectra of tetrahedral molecules

An algebraic model of Boson-realization is proposed to study the vibrational spectra of a tetrahedral molecule, where ten sets of boson creation and annihilation operators are used to construct the Hamiltonian with T d symmetry. There are two schemes in our model. The first scheme provides an eight-parameter fit to the published experimental vibrational eigenvalues of methane with a root-mean-square deviation 11.61 c m −1 . The second scheme, where the bending oscillators are assumed to be harmonic and the interactions between the bending vibrations are neglected, provided a five-parameter fit with a root-mean-square deviation 12.42 c m −1

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Chemical Physics

Boundary Integral Methods for the Poisson Equation of Continuum Dielectric Solvation Models

This paper tests a dielectric model for variation of hydration free energy with geometry of complex solutes in water. It works out some basic aspects of the theory of boundary integral methods for these problems. One aspect of the algorithmic discussion lays the basis for multigrid methods of solution, methods that are likely to be necessary for similarly accurate numerical solution of these models for much larger solutes. Other aspects of the algorithmic work show how macroscopic surfaces such as solution interfaces and membranes may be incorporated and also show how these methods can be transferred directly to periodic boundary conditions. This dielectric model is found to give interesting and helpful results for the variation in solvation free energy with solute geometry. However, it typically significantly over-stabilizes classic attractive ion-pairing configurations. On the basis of the examples and algorithmic considerations, we make some observations about extension of this continuum model incrementally to reintroduce molecular detail of the solvation structure.

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Chemical Physics

C20: Fulleren, Bowl or Ring? New Results from Coupled-Cluster Calculations

Contrary to recent experimental evidence suggesting that the monocyclic ring is the most stable 20-atom carbon species, highly accurate calculations convincingly predict that the smallest fullerene, the dodecahedron C 20 , has the lowest energy. A related corannulene-like bowl is nearly degenerate in energy to the fullerene. Thermodynamic considerations suggest that at formation temperatures of around 700 K the bowl should be the dominant species. The recent application of gradient corrections to LDA which supported the ring structure is qualitatively in error. (RK-94-02)

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Chemical Physics

Charge transfer excitons in C60-dimers and polymers

Charge-transfer (CT) exciton effects are investigated for the optical absorption spectra of crosslinked C60 systems by using the intermediate exciton theory. We consider the C60-dimers, and the two (and three) molecule systems of the C60-polymers. We use a tight-binding model with long-range Coulomb interactions among electrons, and the model is treated by the Hartree-Fock approximation followed by the single-excitation configuration interaction method. We discuss the variations in the optical spectra by changing the conjugation parameter between molecules. We find that the total CT-component increases in smaller conjugations, and saturates at the intermediate conjugations. It decreases in the large conjugations. We also find that the CT-components of the doped systems are smaller than those of the neutral systems, indicating that the electron-hole distance becomes shorter in the doped C60-polymers.

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Chemical Physics

Chemical Reactions of Silicon Clusters

Smalley and co-workers discovered that chemisorption reactivities of silicon clusters vary over three orders of magnitude as a function of cluster size. In particular, they found that \Si{33}, \Si{39}, and \Si{45} clusters are least reactive towards various reagents compared to their immediate neighbors in size. We explain these observations based on our stuffed fullerene model. This structural model consists of bulk-like core of five atoms surrounded by fullerene-like surface. Reconstruction of the ideal fullerene geometry gives rise to four-fold coordinated crown atoms and π -bonded dimer pairs. This model yields unique structures for \Si{33}, \Si{39}, and \Si{45} clusters without any dangling bonds and thus explains their lowest reactivity towards chemisorption of closed shell reagents. This model is also consistent with the experimental finding of Jarrold and Constant that silicon clusters undergo a transition from prolate to spherical shapes at \Si{27}. We justify our model based on an in depth analysis of the differences between carbon and silicon chemistry and bonding characteristics. Using our model, we further explain why dissociative chemisorption occurs on bulk surfaces while molecular chemisorption occurs on cluster surfaces. We also explain reagent specific chemisorption reactivities observed experimentally based on the electronic structures of the reagents. Finally, experiments on \Si{x}X y (X = B, Al, Ga, P, As, AlP, GaAs) are suggested as a means of verifying the proposed model. We predict that \Si{x}(AlP) y and \Si{x}(GaAs) y (x=25,31,37;y=4) clusters will be highly inert and it may be possible to prepare macroscopic samples of these alloy clusters through high temperature reactions.

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Chemical Physics

Chemistry of Nanoscale Semiconductor Clusters

The ground state structures of small silicon clusters are determined through exhaustive tight-binding molecular dynamics simulation studies. These simulations revealed that \Si{11} is an icosahedron with one missing cap, \Si{12} is a complete icosahedron, \Si{13} is a surface capped icosahedron, \Si{14} is a 4-4-4 layer structure with two caps, \Si{15} is a 1-5-3-5-1 layer structure, and \Si{16} is a partially closed cage consisting of five-membered rings. The characteristic feature of these clusters is that they are all surface. Smalley and co-workers discovered that chemisorption reactivities of silicon clusters vary over three orders of magnitude as a function of cluster size. In particular, they found that \Si{33}, \Si{39}, and \Si{45} clusters are least reactive towards various reagents compared to their immediate neighbors in size. We provide insights into this observed reactivity pattern through our stuffed fullerene model. This structural model consists of bulk-like core of five atoms surrounded by fullerene-like surface. Reconstruction of the ideal fullerene geometry gives rise to four-fold coordinated crown atoms and π -bonded dimer pairs. This model yields unique structures for \Si{33}, \Si{39}, and \Si{45} clusters without any dangling bonds and thus explains their lowest reactivity towards chemisorption of closed shell reagents. We also explain why a) these clusters are substantially unreactive compared to bulk surfaces and b) dissociative chemisorption occurs on bulk surfaces while molecular chemisorption occurs on cluster surfaces. Finally, experiments on Si x X y (X = B, Al, Ga, P, As, AlP, GaAs) are suggested as a means of verifying the proposed model.

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Chemical Physics

Coherent Control of Isotope Separation in HD+ Photodissociation by Strong Fields

The photodissociation of the HD+ molecular ion in intense short- pulsed linearly polarized laser fields is studied using a time- dependent wave-packet approach where molecular rotation is fully included. We show that applying a coherent superposition of the fundamental radiation with its second harmonic can lead to asymmetries in the fragment angular distributions, with significant differences between the hydrogen and deuterium distributions in the long wavelength domain where the permanent dipole is most efficient. This effect is used to induce an appreciable isotope separation.

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Chemical Physics

Comparative Study of Multicanonical and Simulated Annealing Algorithms in the Protein Folding Problem

We compare a few variants of the recently proposed multicanonical method with the well known simulated annealing for the effectiveness in search of the energy global minimum of a biomolecular system. For this we study in detail Met-enkephalin, one of the simplest peptides. We show that the new method not only outperforms simulated annealing in the search of the energy groundstate but also provides more statistical-mechanical information about the system.

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