Superalkali-Alkalide Interactions and Ion Pairing in Low-Polarity Solvents.

The nature of anionic alkali metals in solution is traditionally thought to be "gaslike" and unperturbed. In contrast to this noninteracting picture, we present experimental and computational data herein that support ion pairing in alkalide solutions. Concentration dependent ionic conductivity, dielectric spectroscopy, and neutron scattering results are consistent with the presence of superalkali-alkalide ion pairs in solution, whose stability and properties have been further investigated by DFT calculations. Our temperature dependent alkali metal NMR measurements reveal that the dynamics of the alkalide species is both reversible and thermally activated suggesting a complicated exchange process for the ion paired species. The results of this study go beyond a picture of alkalides being a "gaslike" anion in solution and highlight the significance of the interaction of the alkalide with its complex countercation (superalkali).

Electronic excitation in bulk and nanocrystalline alkali halides.

The lowest energy excitations in bulk alkali halides are investigated by considering five different excited state descriptions. It is concluded that excitation transfers one outermost halide electron in the fully ionic ground state to the lowest energy vacant s orbital of one closest cation neighbour to produce the excited state termed dipolar. The excitation energies of seven salts were computed using shell model description of the lattice polarization produced by the effective dipole moment of the excited state neutral halogen-neutral metal pair. Ab initio uncorrelated short-range inter-ionic interactions computed from anion wavefunctions adapted to the in-crystal environment were augmented by short-range electron correlation contributions derived from uniform electron-gas density functional theory. Dispersive attractions including wavefunction overlap damping were introduced using reliable semi-empirical dispersion coefficients. The good agreement between the predicted excitation energies and experiment provides strong evidence that the excited state is dipolar. In alkali halide nanocrystals in which each ionic plane contains only four ions, the Madelung energies are significantly reduced compared with the bulk. This predicts that the corresponding intra-crystal excitation energies in the nanocrystals, where there are two excited states depending on whether the halide electron is transferred to a cation in the same or in the neighbouring plane, will be reduced by almost 2 eV. For such an encapsulated KI crystal, it has been shown that the greater polarization in the excited state of the bulk crystal causes these reductions to be lowered to a 1.1 eV-1.5 eV range for the case of charge transfer to a neighbouring plane. For intra-plane charge transfer the magnitude of the polarization energy is further reduced thus causing the excitation in these encapsulated materials to be only 0.2 eV less than in the bulk crystal.

Theoretical study of the structures and electronic properties of all-surface KI and CsI nanocrystals encapsulated in single walled carbon nanotubes.

The structural and electronic properties of all-surface KI and CsI crystals encapsulated in single-walled carbon nanotubes are investigated theoretically with an ionic and atomistic approach using the GULP program. The short-range interactions, derived from Dirac-Fock wavefunctions, were augmented with damped dipole-dipole and dipole-quadrupole dispersive attractions. The uncorrelated interionic interactions computed using the relativistic crystal ion and relativistic integral programs accounted for anion in-crystal modifications while being exact given the ion wavefunctions. All the short-range correlation energies and the uncorrelated interactions between the ions and carbon atoms were computed using the density functional theory of a uniform electron gas of infinite extent. Unphysical self-interactions were removed by scaling the exchange interaction with a Rae factor derived from a study of the adsorption of noble gases on graphite. The predictions for the nonencapsulated crystals agreed well with those previously derived from a global analytic theory based on the Born model. This provided a good description of the contraction of the interplane distance (b) relative to the separation (R(e)) in the rocksalt structured bulk material although failing to account for the observed dilation of the intraplane ionic separations (a). Introduction of the interactions with the nanotube wall, including the ion-nanotube dispersive attractions, increased the predicted a values although these were still significantly smaller than experiment. The predicted b separations were reduced compared with those for the nonencapsulated crystals to values significantly less than observed. It is explained why introducing any ion-nanotube interactions that are sufficiently attractive as to reproduce the experimental a values must significantly underestimate the b separations. The partial transfer of anion electrons to the nanotube carbon atoms, not considered hitherto, was described by decomposing the intra-atomic interactions of both the nanotube pi- and the iodide 5p-electrons into an effective one-electron term plus the repulsion between electrons in the same orbital. These energies were derived from electronic structure computations with the additional interspecies electrostatic repulsions derived from the GULP program. Structural predictions are presented as a function of the number (n) of electrons transferred from each anion. For both KI and CsI, the structure predicted by that computation, which minimized the total energy, in contrast to the other calculations, agreed well with experiment reproducing both the significant dilation of a and the smaller contraction of b. The respective n values (n(t)) predicting the lowest energies are 0.278 and 0.285. These results are supported by comparing the experimental frequencies of Raman modes attributable to vibrations of nanotubes encapsulating KI with the corresponding frequencies for systems where independently known numbers of electrons were transferred to the nanotubes. In both the encapsulated KI and CsI systems, the charge transfer is driven by the reduction of the electron repulsion on delocalizing some anion charge over the significantly greater number of nanotube carbon atoms. A simplified analytic model, which reproduces the charge transfers, explains why n(t) is slighter larger for CsI and also predicts that n(t) will be insensitive to the structure of the nanotube.

A theoretical study of the cohesion of noble gases on graphite.

The interactions of the noble gases with a graphene sheet are investigated theoretically. The short range repulsive interaction between the noble gas and each carbon atom is described using Hartree-Fock atomic densities and a local density functional theory with the exchange functional corrected for the finite range of the interaction by introducing a Rae-type correction depending on the effective number of electrons. The long range interactions are introduced as the sum of the Axilrod-Teller triple-dipole interaction plus the dipole-dipole and dipole-quadrupole dispersive attractions damped according to the theory of Jacobi and Csanak. The energy arising from the interactions between the permanent quadrupoles on the carbon atoms with the dipole they induce on the noble gas is negligible, being nonzero only on account of the atomistic structure of graphene. The mobile and delocalized nature of the graphene pi electrons causes the effective number of electrons to be around 500 rather than that of 12 appropriate for a system of entirely localized interactions with individual carbon atoms. Inclusion of the Axilrod-Teller term is required to obtain reliable predictions for the binding energies and equilibrium geometries. Absorption of a noble gas atom is predicted to occur at the site above the center of a six membered ring although this is preferred over two other sites by only about 5 meV. The methods presented for generating all the potentials can be applied to derive the interactions between any ion and carbon atom in the wall of a single-walled nanotube. Knowledge of these interactions is required to study the alkali halide nanocrystals encapsulated in single-walled carbon nanotubes of current interest.

Fundamental global model for the structures and energetics of nanocrystalline ionic solids.

This paper presents a general theory elucidating the relationships between the structures and cohesive energetics of alkali halide nanocrystals consisting of small sections of bulk rocksalt structures with m(1) and m(2) rows but infinite along the z axis. The theory introduces the electrostatic interactions between the ions treated as point charges and the short-range repulsions between the closest ion neighbors with the latter terms written in the Born form Ar(-)(n). Minimum energy structures are defined by the distances a(e) and b(e) separating the closest ions perpendicular and parallel to the z direction. The ratio a(e)/b(e), defining the crystal shape, is independent of the strength A of the short-range repulsion, greater than the bulk value of unity, and increases with decrease of the crystal cross section or n. This ratio tends toward unity in the hard sphere limit of infinite n. Both b(e)/R(6:6)(e) and a(e)/R(6:6)(e), with the bulk separation R(6:6)(e), are less than one, increase with increase of the crystal cross section or n, and are independent of A if this is independent of structure. The structural dependence of A increases its value with a decreasing crystal cross section rendering closer to unity the ratios a(e)/b(e), b(e)/R(6:6)(e), and a(e)/R(6:6)(e). Energy gains on relaxing the crystal toward equilibrium from its bulk separations decrease with increase of the crystal cross section or n, being about 60 kJ/mol for a one-dimensional chain with n = 6 but 0.5 kJ/mol for m(1) = m(2) = 4 with n = 12. The energy gained on relaxing to a structure with a(e)/b(e) constrained at unity is about 10 times greater than the further energy gains consequent on removing this constraint. The present theory neglecting the interaction between ions and the encapsulating nanotube explains the experimentally measured b(e)/R(6:6)(e) ratios. The observation that the a(e)/R(6:6)(e) values are greater than one shows that ion-wall interactions are important in determining the values of a(e).