Karl Sohlberg

Continuum methods of mechanics as a simplified approach to structural engineering of nanostructures

Recent studies of potential components for nanomachines reveal that for a wide variety of structures, the rigidity of the structure is a key element in its proper performance. Vibrational analysis is an ideal way to study structural rigidity, but standard methods of molecular vibrational analysis are computationally prohibitive for nanostructures with large numbers of atoms. Herein, the vibration of nanotubes is used to demonstrate that continuum methods of vibrational analysis have potential utility in the engineering of nanostructures.

Adsorption of alcohols on γ-alumina (1 1 0 C)

Adsorption of methanol, ethanol, propanol and isopropanol on the -alumina ( 110C )surface is investigated with semiempirical (PM3) cluster calculations. It is found that all four alcohols chemisorb to the alumina surface when they come close to the surface with suitable orientation. The chemisorption is an exothermic process when the OH hydrogen interacts with a surface oxygen atom that is in turn close to a cation vacancy. In this case only O–H interaction is required for successful dehydrogenation. If the surface oxygen has no adjacent vacancies, both the oxygen and hydrogen from the alcohol OH must interact with the surface for successful dehydrogenation. Alkoxide formation by abstraction of the alcohol OH proton is found to be favored over alkoxide production by nucleophilic attack of the alcohol C by a surface Lewis base site followed by C–OH bond scission.

THE BULK AND SURFACE STRUCTURE OF γ-ALUMINA

Despite its widespread use, the composition and structure of γ-alumina have been the subject of controversy for decades. In this article we review the historical attempts to resolve two principal questions that have each been extensively debated, but in independent communities: (1) What is the distribution of vacancies over the two cation sublattices in the nominal spinel structure, and (2) what is the hydrogen content of γ-alumina, if any? We then review how recent investigations have shown that these two questions are, in fact, related, and have a single resolution. The experimental study of γ-alumina surfaces has long been dominated by IR spectroscopy of the hydroxylated surfaces, and 27AI NMR. Both techniques have produced perplexing results. Here we review the development of these surface studies and parallel computational studies. Finally, we review recent computational results that explain the puzzling experimental findings.

Doping of TiO 2 Polymorphs for Altered Optical and Photocatalytic Properties

This paper reviews recent investigations of the influence of dopants on the optical properties of TiO2 polymorphs. The common undoped polymorphs of TiO2 are discussed and compared. The results of recent doping efforts are tabulated, and discussed in the context of doping by elements of the same chemical group. Dopant effects on the band gap and photocatalytic activity are interpreted with reference to a simple qualitative picture of the TiO2 electronic structure, which is supported with first-principles calculations.

A hybrid tandem supersonic beam mass spectrometer for the study of collision-induced dissociation of ions in the energy range <1 to 3000 eV

Abstract A hybrid tandem mass spectrometer has been constructed to study the dynamics of collision-induced dissociation processes in the energy range of less than 1 eV to several keV. A mass- and energy-analyzed high-energy ion beam is decelerated to low energies by a series of cylindrical and rectangular tube lenses. The decelerated ion beam collides with a supersonic neutral beam at right angles. Energy and mass analysis of the fragment ions is performed by a novel hemispherical energy analyzer followed by a quadrupole mass filter. The detector system can be rotated about the collision center to provide angular analysis of scattered fragment ions. Ion beams of moderate intensity have been obtained in the entire energy range, it is especially significant that good intensity is obtained in the lower threshold energy range of 0.2–5 eV. The performance of the instrument has been evaluated by an extensive series of ion transmission, focus, and energy measurements and by comparison of CID results with known mstastable and collision-induced dissociation processes.

Collision-induced dissociation reaction dynamics of the acetone molecular ion

Abstract The collision-induced dissociation (CID) reaction CH3COCH+3 + Ar → CH3CO+ + CH3 + Ar has been studied with a tandem hybrid mass spectrometer (a high-resolution mass spectrometer coupled with a supersonic neutral beam, which uses post-collision energy and mass analysis) at ion laboratory energies in the range 4.5–300 eV. Kinetic energy and angular distributions of the fragment CH3CO+ ions were measured and scattering contour diagrams for this process were constructed. The results show that the dissociation proceeds via low-impact parameter collisions with extensive momentum transfer. At 4.5 eV, the product is primarily backward scattered with some intensity at the center-of-mass (completely inelastic collision). At higher energies the product ion is progressively more forward scattered but over the energy range investigated the maximum intensity never shifts to 0°. Collectively, these results suggest that impulsive mechanisms dominate CID reactions at all energy ranges for this ion and that the dominant mechanism is kinematically different at low and high energy.

On the origin of the heavy atom effect in the fine-structure splitting of the 1 2Π state of alkali metal 2P-rare gas van der Waals molecules

The fine-structure splitting of the (1 2ΠΩ,ν) levels in Li(1s 22p,2P)Ar and Li(1s 22p,2P)Ne is determined. Analysis of the electronic wave functions demonstrates that the heavy atom contribution to the fine-structure splitting results from the antibonding mixture of valence pπ orbitals on the rare gas and metal atoms. Rydberg orbitals do not contribute significantly to the heavy atom effect.

Docking envelopes for the assembly of molecular bearings

To design a process for the assembly or self-assembly of a nanomachine requires knowing the degree of spatial control needed to put the components together. A useful starting point in this area of study is the concept of a docking envelope, which is a continuous region of initial condition parameter space for which two structures will dock. In this paper docking envelopes, determined from molecular dynamics simulation, are presented for the assembly of a molecular bearing consisting of two concentric carbon nanotubes. In the beginning of each simulation the outer nanotube (sleeve) is held in place and the inner nanotube (shaft) starts far away from, but is given an initial velocity toward, the sleeve. The docking envelope in this case is delineated by the initial offset from a coaxial geometry. In order to address recent concerns about the effects of zero-point energy leakage and chaos in classical simulations of nanomachine components, docking envelopes from two types of simulations are presented: fully atomistic (all degrees of freedom included) and rigid body (each nanotube rigid but shaft allowed to rotate and translate).

Semi-empirical study of a prototype rotaxane-based molecular shuttle

Abstract We report the results of AM1 semi-empirical electronic structure calculations on a prototype photochemically driven molecular shuttle based on a rotaxane. As inferred experimentally, the ring component of the rotaxane is shown to favor a different position about the shaft component depending on whether the shaft is in the trans or cis conformation. The calculations show that the shuttling action of the cyclodextrin ring is accompanied by the (de)formation of a kink in the shaft.

The role of symmetry in collisions of N2 with N+2

Classical trajectory methods have been used to explore the excitation of vibrations in gas‐phase collisions of the nitrogen molecular ion with its parent molecule. The near symmetry of the reactants is shown to result in a high probability that the two molecules are excited by an equal amount of energy. This provides a possible explanation of the molecular beam measurements that show that the total number of vibrational energy quanta excited in the collision is, with a high probability, even.