David P. Pullman

Quantitative Structure-Property Relationships for Longitudinal, Transverse, and Molecular Static Polarizabilities in Polyynes

The present work reports for the first time quantitative structure-property relationships, derived at the benchmark CCSD(T)/cc-PVTZ level of theory that estimate the static longitudinal, transverse, and molecular polarizability in polyynes (C2nH2), as a function of their length (L). In the case of independent electron models, regardless of the form of the nuclei potential that the electrons experience, the polarizability increases strongly with system size, scaling as L(4). In contrast, the static longitudinal polarizability in polyynes have a considerably weaker length-dependence (L(1.64)). This is shown to predominantly arise from electron-electron repulsion rather than electron correlation by a systematic study of the polarizability length dependence in several simple quantum mechanical systems (e.g., particle-in-box, simple harmonic oscillator) and other molecular systems (e.g., H2, H2(+), polyynes). Decrease of the electron-electron repulsion term is suggested to be the key factor in enhancing nonlinear polarizability characteristics of linear oligomeric and polymeric materials.

On the viability of single atom abstraction in the dissociative chemisorption of O2 on the Al(111) surface

The dissociativechemisorption of O 2 on the Al(111) surface is investigated by means of a Monte Carlo simulation that incorporates two mechanisms that have been proposed for this reaction in the literature: single atom abstraction and two-atom adsorption that generates translationally hot atoms on the surface. A comparison is made to the much-debated STM results of Brune et al. [J. Chem. Phys. 99, 2128 (1993)], in which the oxygen island density (number of islands per binding site) was determined as a function of coverage. Since the two-atom channel has been discussed heavily in the literature, we focus primarily on the abstraction mechanism. We show that atom abstraction in its basic form is incompatible with the STM results; however, we propose two simple modifications that enable atom abstraction to reproduce the STM results. In the first modification, the probability of dissociation is higher at sites next to preexisting O adatoms. In essence, we are proposing that the increased Al–O bond strength at sites next to preexisting O adatoms [Jacobsen et al., Phys. Rev. B 52, 14954 (1995)] stabilizes the transition state for dissociation. If atom abstraction is assumed to be the only operative mechanism, and if its probability increases by a factor of ∼10 next to a site that is occupied versus unoccupied, the STM island density data can be approximately reproduced. In the second modification, the abstracted atom is permitted to make a single hop in the direction of a preexisting, nearby O adatom. The allowance of merely a single, directed hop has a dramatic effect on the coverage dependence of the island density.

The sphere-in-contact model of carbon materials

AbstractA sphere-in-contact model is presented that is used to build physical models of carbon materials such as graphite, graphene, carbon nanotubes and fullerene. Unlike other molecular models, these models have correct scale and proportions because the carbon atoms are represented by their atomic radius, in contrast to the more commonly used space-fill models, where carbon atoms are represented by their van der Waals radii. Based on a survey taken among 65 undergraduate chemistry students and 28 PhD/postdoctoral students with a background in molecular modeling, we found misconceptions arising from incorrect visualization of the size and location of the electron density located in carbon materials. Based on analysis of the survey and on a conceptual basis we show that the sphere-in-contact model provides an improved molecular representation of the electron density of carbon materials compared to other molecular models commonly used in science textbooks (i.e., wire-frame, ball-and-stick, space-fill). We therefore suggest that its use in chemistry textbooks along with the ball-and-stick model would significantly enhance the visualization of molecular structures according to their electron density. Graphical AbstractA sphere-in-contact model of C60-fullerene

Microwave spectra and ab initio studies of Ar-propane and Ne-propane complexes: Structure and dynamics

Microwave spectra in the 7-26 MHz region have been measured for the van der Waals complexes, Ar-CH3CH2CH3, Ar-(13)CH3CH2CH3, 20Ne-CH3CH2CH3, and 22Ne-CH3CH2CH3. Both a- and c-type transitions are observed for the Ar-propane complex. The c-type transitions are much stronger indicating that the small dipole moment of the propane (0.0848 D) is aligned perpendicular to the van der Waals bond axis. While the 42 transition lines observed for the primary argon complex are well fitted to a semirigid rotor Hamiltonian, the neon complexes exhibit splittings in the rotational transitions which we attribute to an internal rotation of the propane around its a inertial axis. Only c-type transitions are observed for both neon complexes, and these are found to occur between the tunneling states, indicating that internal motion involves an inversion of the dipole moment of the propane. The difference in energy between the two tunneling states within the ground vibrational state is 48.52 MHz for 20Ne-CH3CH2CH3 and 42.09 MHz for 22Ne-CH3CH2CH3. The Kraitchman substitution coordinates of the complexes show that the rare gas is oriented above the plane of the propane carbons, but shifted away from the methylene carbon, more so in Ne propane than in Ar propane. The distance between the rare gas atom and the center of mass of the propane, Rcm, is 3.823 A for Ar-propane and 3.696 A for Ne-propane. Ab initio calculations are done to map out segments of the intermolecular potential. The global minimum has the rare gas almost directly above the center of mass of the propane, and there are three local minima with the rare gas in the plane of the carbon atoms. Barriers between the minima are also calculated and support the experimental results which suggest that the tunneling path involves a rotation of the propane subunit. The path with the lowest effective barrier is through a C2v symmetric configuration in which the methyl groups are oriented toward the rare gas. Calculating the potential curve for this one-dimensional model and then calculating the energy levels for this potential roughly reproduces the spectral splittings in Ne-propane and explains the lack of splittings in Ar-propane.