Repulsive Van Der Waals Interactions Research Articles

Lithium Diffusion in LixCoO2 Electrode Materials

Abstract The minimum energy migration path and the minimum barrier height of lithium diffusion are discussed for the layer compound LixCoO2. The Universal Force Field (UFF) is adapted for use in solid state compounds. Molecular dynamics simulations with additional impulse dynamics method, performed at a microcanonical ensemble, point out that the lithium migration between the Co–O octahedron layers takes place by ion hopping from one octahedral to another octahedral site via an interstitial tetrahedral site. The barrier height of lithium migration is estimated to be 28.7 kJ mol−1 (0.30 eV) for an original layer distance of 2.54 Å; a potential energy profile is developed from molecular mechanics calculations. The observation that the repulsive van der Waals interaction between Li+ and neighboring O2− ions is the most important contribution to the barrier height is consistent with the fact that the activation energy of lithium diffusion is very sensitive to the layer distance, within the experimentally observed values.

Analysis of the Intermolecular Contacts within Sickle Hemoglobin Fibers: Effect of Site-Specific Substitutions, Fiber Pitch, and Double-Strand Disorder

An atomic model of the sickle hemoglobin (HbS) fiber was synthesized by combining the molecular coordinates of the fiber (obtained from electron microscopy) with atomic coordinates of the sickle hemoglobin double strand (obtained from X-ray crystallography). The model is stereochemically acceptable. The majority of polymerization-sensitive HbS mutants are located at fiber contact sites and the majority of the mutants that do not affect polymerization are not located at contact sites. The residues at intermolecular contacts in the fiber model are reported. We have searched the coordinate space in the vicinity of the EM reconstructions to find models with alternative sets of coordinates that satisfy the mutant data, contain 5-Å contacts between double strands, and are stereochemically acceptable. This involved a systematic examination over 297 different models. The alternative fiber models were generated with a range of fiber pitch, double-strand positions, and double-strand polarity. Models which had unacceptably close contacts between atoms, failed to satisfy the mutant data, or did not have 5-Å contacts between double strands were considered unacceptable. None of the acceptable alternative fiber models improved the agreement between the polymerization behavior of HbS mutants and their contact site location. However, several models could account for the polymerization data equally well. Residue locations for single-site HbS mutations that could discriminate between alternative fiber models are proposed. The twist of HbS fibers varies in an apparent random manner with an average rotation of 7.8 ± 2.5° per molecule and a maximum rotation of 16° per molecule. The number of interdouble-strand contacts as a function of fiber twist shows a broad maximum around 9° and may account for the observed range of fiber pitch. This study shows that the upper limit on the fiber twist could result from a loss of axial contacts and repulsive van der Waals interactions between residues involved in interstrand contacts. The loss of axial contacts limits the radial growth of the fiber. In the appendix we analyze the methodology used by I. Cretegny and S. J. Edelstein [(1993) J. Mol. Biol. 230, 733-738] to build a model of the fiber. Our examination reveals shortcomings in the methodology of Cretegny and Edelstein. One result of these shortcomings is that the model synthesized by Cretegny and Edelstein is not stereochemically acceptable because it gives rise to a large number of excessively close (less than 1.4 Å) atom-atom contacts, suggesting interpenetration of the molecular envelopes.

Silyl group internal rotation in S1 phenylsilane and phenylsilane cation: Experiments and ab initio calculations

Resonant two-photon ionization (R2PI) and pulsed field ionization (PFI) were used to measure S1–S0 and cation–S1 spectra of internally cold phenylsilane. We measure the adiabatic ionization potentials IP(phenylsilane)=73 680±5 cm−1, IP(phenylsilane ⋅Ar)=73 517±5 cm−1 and IP(phenylsilane ⋅Ar2)=73 359±5 cm−1. We assign many low lying torsion–vibration levels of the S1 (à 1A1) state and of X̃ 2B1 of phenylsilane+. In both states, the pure torsional transitions are well fit by a simple sixfold hindered rotor Hamiltonian. The results for the rotor inertial constant B and internal rotation potential barrier V6 are, in S1, B=2.7±0.2 cm−1 and V6=−44±4 cm−1; in the cation, B=2.7±0.2 cm−1 and V6=+19±3 cm−1. The sign of V6 and the conformation of minimum energy are inferred from spectral intensities of bands terminating on the 3a″1 and 3a″2 torsional levels. In S1 the staggered conformation is most stable, while in the cation ground state the eclipsed conformation is most stable. For all sixfold potentials whose absolute phase is known experimentally, the most stable conformer is staggered in the neutral states (S0 and S1 p-fluorotoluene, S1 toluene, S1 p-fluorotoluene) and eclipsed in the cationic states (ground state toluene+ and phenylsilane+). In phenylsilane+ we estimate several potential energy coupling matrix elements between torsional and vibrational states. For small V6, the term PαPa in the rigid-frame model Hamiltonian strongly mixes the 6a′1 and 6a′2 torsional states, which mediates further torsion–vibrational coupling. In addition, the cation X̃ 2B1 vibrational structure is badly perturbed, apparently by strong vibronic coupling with the low-lying à 2A2 state. Accordingly, ab initio calculations find a substantial in-plane distortion of the equilibrium geometry of the X̃ 2B1 state, while the à 2A2 state is planar and symmetric. The calculations also correctly predict the lowest energy conformer for S0 states and for cation ground states. Finally, we adapt the natural resonance theory (NRT) of Glendening and Weinhold to suggest why sixfold barriers for methyl and silyl rotors are uniformly small, while some threefold barriers are quite large. The phase of the sixfold potential is apparently determined by a subtle competition between two types of rotor-ring potential terms: attractive donor–acceptor interactions and repulsive van der Waals interactions (steric effects).

Oxygen‐17 NMR of selected N‐Nitrosamines. A preliminary report

AbstractOxygen‐17 NMR spectra of some N‐nitrosamines are discussed in terms of steric and conjugative effects. Relationships have been observed between rotational barriers, conformations and 17O chemical shifts. A tentative linear correlation has been determined between repulsive Van der Waals interactions and some N‐nitrosopiperidines.

17O NMR spectroscopic study of substituted 9‐fluorenones

AbstractThe 17O NMR chemical shift data for 23 9‐fluorenones in acetonitrile at 75°C are reported. 17O NMR data for the 3‐substituted 9‐fluorenones are correlated with 17O NMR data for 4‐substituted acetophenones. Chemical shift data for 1‐halogeno‐9‐fluorenones are deshielded relative to the 3‐isomers, suggesting important repulsive van der Waals interactions. The keto carbonyl signal for 9‐fluorenone‐1‐carboxylic acid is substantially shielded, as expected, from intramolecular hydrogen bonding. The data for several multi‐substituted 9‐fluorenones demonstrate that substituent effects are additive for the 9‐fluorenones.

17O NMR spectroscopy: Origin of deshielding effects in rigid, planar molecules

Abstract 17O NMR data for hindered carbonyls in planar quinones and flavones showed large deshielding effects and correlated with estimated repulsive van der Waals interactions, providing new insights into steric phenomenon at carbonyl oxygens.

Oxygen-17 NMR spectroscopy: torsion angle relationships in aryl carboxylic esters, acids and amides

/sup 1/ /sup 7/O NMR spectroscopic data (natural abundance in acetonitrile at 75/sup 0/C) were obtained for the following series of electronically similar, sterically hindered compounds: aromatic methyl esters, aromatic carboxylic acids, and aromatic amides. Torsional angles were calculated by the molecular mechanics (MM2) method. Linear regression analysis of the estimated torsion angles and the /sup 17/O chemical shift data for each series yielded the following results (series, slope delta/degree, correlation coefficient): esters (C=O), 0.70, 0.997; esters (-0-), 0.43, 0.992; acids (-CO/sub 2/H), 0.56, 0.994; amides (C=O), 0.84, 0.942; N,N-dimethylamides (C=O), 0.6, 0.991. The results are discussed in terms of minimization of repulsive van der Waals interactions by rotation of the functional group out of the plane of the aromatic ring.

Intramolecular van der waals interactions and chemical shifts: A model for β‐ and γ‐effects

AbstractA correlation has been discovered between the local steric van der Waals energy as calculated by the MM2 force field and the chemical shift of a resonant nucleus within classes of saturated aliphatic phosphines, amines, hydrocarbons and monofunctional alcohols. Root‐mean‐square errors found for 37 phosphorus, 35 nitrogen and 164 carbon nuclei are 10.9, 5.9 and 2.8 ppm, respectively. It is found that repulsive van der Waals interactions are associated with deshielding effects in the observed chemical shifts, and attractive interactions with smaller shielding effects. A simple physical picture emerges which describes both β‐ and γ‐effects under one common model. It is found that β‐effects are associated with repulsive interactions while, contrary to earlier approaches γ‐effects are due basically to attractive interactions.

Repulsive van der Waals and Dipole-Quadrupole Interaction in the Excited LiCs Molecule

The authors report strong repulsive van der Waals and dipole-quadrupole interactions in the $\ensuremath{\Sigma}$ and $\ensuremath{\Pi}$ potentials of the LiCs molecule dissociating into Li $2P$ and Cs $6S$ atoms. Theoretical results derived from second-order perturbation theory are verified by investigating experimentally the wavelength dependence of the quasistatic wings of the Li resonance lines broadened by Cs ground-state atoms.

Applications of net repulsive van der Waals forces between different particles, macromolecules, or biological cells in liquids

When the sum of the interactions of two different materials with a liquid medium is larger than the sum of the interactions between the two materials and those between the molecules of the liquid medium themselves, the net van der Waals interaction between the two materials, immersed in that liquid medium, will be repulsive. It can be demonstrated that this situation prevals when the surface tension of the liquid medium has a value intermediate between the values of the interfacial tensions of the two materials. Net repulsive van der Waals interactions are shown to be responsible for the rejection of particles by solidifying melts, and for the incompatibility of polymers in solution. Repulsive van der Waals forces can be employed in the dissociation of antigen—antibody complexes, in the elution steps of hydrophobic chromatography, affinity chromatography and reversed-phase chromatography, and generally in the detachment of liquid and solid materials from solid substrates (e.g., washing, dry-cleaning, petroleum extraction from oil shales or tar sands). Finally, the implications of extremely small van der Waals attractions are discussed in connection with the stability of particle suspensions, and the strategy for devising optimal destabilization procedures.

Repulsive Van Der Waals Interactions: Their Role in Various Separation Methods

Abstract It has long been surmised, as a theoretical curiosity, that conditions could arise under which the sign of the van der Waals interaction between two different uncharged bodies, surrounded by a liquid, might be negative, i.e. that such bodies would repel each other. This possibility is implicit in Hamaker's classical paper on van der Waals-London interactions, although, remarkably enough, Hamaker somehow became persuaded that this view was erroneous, and wrote his paper with the apparent purpose to gainsay it: “If two particles are embedded in a fluid and the London-van der Walls force between particles and fluid is greater than between the particles themselves, it might be thought that the resultant action will be a repulsion rather than an attraction. As has been pointed out to the author by Dr. J.H. de Boer, this conclusion does not hold. Owing to a peculiar property of the London-van der Waals forces, the resultant force is generally attractive even when the particles are surrounded by fluid. ...