Structural studies of ammonia and metallic lithium-ammonia solutions

The technique of hydrogen/deuterium isotopic substitution has been used to extract detailed information concerning the solvent structure in pure ammonia and metallic lithium-ammonia solutions. In pure ammonia we find evidence for approximately 2.0 hydrogen bonds around each central nitrogen atom, with an average N-H distance of 2.4 A. On addition of alkali metal, we observe directly significant disruption of this hydrogen bonding. At 8 mol % metal there remains only around 0.7 hydrogen bond per nitrogen atom. This value decreases to 0.0 for the saturated solution of 21 mol % metal, as all ammonia molecules have then become incorporated into the tetrahedral first solvation spheres of the lithium cations. In conjunction with a classical three-dimensional computer modeling technique, we are now able to identify a well-defined second cationic solvation shell. In this secondary shell the nitrogen atoms tend to reside above the faces and edges of the primary tetrahedral shell. Furthermore, the computer-generated models reveal that on addition of alkali metal the solvent molecules form voids of approximate radius 2.5-3.0 A. Our data therefore provide new insight into the structure of the polaronic cavities and tunnels, which have been theoretically predicted for lithium-ammonia solutions.     

Ca-intercalated graphite as a hydrogen storage material: Stability against decomposition into CaH2 and graphite

We have used calculations based on density functional theory to investigate the energetics of hydrogen absorption in calcium-intercalated graphites. We focus particularly on the absorption energy and the stability of the hydrogenated material with respect to decomposition into graphite and calcium hydride, which is essential if this material is to be used for practical H₂ storage. The calculations are performed with two commonly used approximations for the exchange-correlation energies. Our calculations confirm earlier predictions that the absorption energy is approximately −0.2 to −0.4 eV, which is favourable for practical use of Ca-intercalated graphite as a hydrogen storage medium. However, we find that the hydrogenated material is strongly unstable against decomposition. Our results therefore explain recent experiments which show that H₂ does not remain stable in CaC₆ but instead forms a hydride plus graphite.     

High temperature hydrogenation of CaC6

The structure and superconducting properties of high temperature hydrogenated calcium-graphite intercalation compound, CaC₆ have been investigated using room temperature X-ray diffraction, and temperature and field dependence of magnetisation. It is found that the hydrogenation can only decompose the CaC₆ phase, and generate a mixture of CaH₂ and graphite as the final compound. The hydrogenation of CaC₆ also reveals a degradation of its superconducting properties. The experimental results are discussed in detail and it is found that the formation of stable CaH₂ and deintercalation are the main source for observed phase separation and suppression in superconductivity.     

The liquid structure of the solvents dimethylformamide (DMF) and dimethylacetamide (DMA)

The liquid structure of the commonly used solvents dimethylformamide (DMF) and dimethylacetamide (DMA)were measured using state-of-the-art state neutron diffraction augmented with isotopic substitution (NDIS) and interpreted with empirical potential structure refinement (EPSR). Both solvents are found to develop rich local ordering with similar local packing densities, though with differences related to their three-dimensional molecular structure. While DMF’s dipole preferentially orientates anti-parallel to maximise hydrogen bonding, DMA favours parallel arrangement maximising non-directional dispersive forces. The highly-developed local orientational structure found in these solvents rationalises their ability to solvate a range of charged and neutral nanomaterials and highlights that the understanding of nanomaterial dispersions is a multi-body problem in which the geometry of the molecule, as well its dipole moment, must be incorporated.     

Proton dynamics in lithium-ammonia solutions and expanded metals

Quasielastic neutron scattering has been used to study proton dynamics in the system lithium-ammonia at concentrations of 0, 4, 12, and 20 mole percent metal (MPM) in both the liquid and solid (expanded metal) phases. At 230 K, in the homogenous liquid state, we find that the proton self-diffusion coefficient first increases with metal concentration, from 5.6x10(-5) cm2 s(-1) in pure ammonia to 7.8x10(-5) cm2 s(-1) at 12 MPM. At higher concentrations we note a small decrease to a value of 7.0x10(-5) cm2 s(-1) at 20 MPM (saturation). These results are consistent with NMR data, and can be explained in terms of the competing influences of the electron and ion solvation. At saturation, the solution freezes to form a series of expanded metal compounds of composition Li(NH3)4. Above the melting point, at 100 K, we are able to fit our data to a jump-diffusion model, with a mean jump length (l) of 2.1 A and residence time (tau) of 3.1 ps. This model gives a diffusion coefficient of 2.3x10(-5) cm2 s(-1). In solid phase I (cubic, stable from 88.8 to 82.2 K) we find that the protons are still undergoing this jump diffusion, with l=2.0 A and tau=3.9 ps giving a diffusion coefficient of 1.8x10(-5) cm2 s(-1). Such motion gives way to purely localized rotation in solid phases IIa (from 82.2 to 69 K) and IIb (stable from 69 to 25 K). We find rotational correlation times (tau(rot)) of the order of 2.0 and 7.3 ps in phases IIa and IIb, respectively. These values can be compared with a rotational mode in solid ammonia with tau(rot) approximately 2.4 ps at 150 K.     

Monte Carlo and molecular dynamics simulations of methane in potassium montmorillonite clay hydrates at elevated pressures and temperatures

The structure and dynamics of methane in hydrated potassium montmorillonite clay have been studied under conditions encountered in sedimentary basin and compared to those of hydrated sodium montmorillonite clay using computer simulation techniques. The simulated systems contain two molecular layers of water and followed gradients of 150 bar km(-1) and 30 K km(-1) up to a maximum burial depth of 6 km. Methane particle is coordinated to about 19 oxygen atoms, with 6 of these coming from the clay surface oxygen. Potassium ions tend to move away from the center towards the clay surface, in contrast to the behavior observed with the hydrated sodium form. The clay surface affinity for methane was found to be higher in the hydrated K-form. Methane diffusion in the two-layer hydrated K-montmorillonite increases from 0.39 x 10(-9) m2 s(-1) at 280 K to 3.27 x 10(-9) m2 s(-1) at 460 K compared to 0.36 x 10(-9) m2 s(-1) at 280 K to 4.26 x 10(-9) m2 s(-1) at 460 K in Na-montmorillonite hydrate. The distributions of the potassium ions were found to vary in the hydrates when compared to those of sodium form. Water molecules were also found to be very mobile in the potassium clay hydrates compared to sodium clay hydrates.     

Synthetic manipulation of the triflone group : Formation from alcohols,constructions, and conversion to ketones and amines

The rearrangement of trifluoromethanesulfinates to trifluoromethanesulfones (“triflones”) was developed as a synthetic method for obtaining these compounds Their utility as reagents for the construction of carbon skeletons is explored with regard to reactions such as alkylation, conjugation addition, and cycloaddition. In addition, the conversion of triflones to more common functionality is described.     

Studies of the radical polymerization of styrene in the presence of polybutadiene

Polymerization of styrene in solutions containing polybutadiene can be used for study of the intermediate stages in the radical polymerization of the monomer.     

A rapid method for isolation of human hemoglobin benzo[a]pyrene diol epoxide derived adducts using high performance liquid chromatography

A method is described to isolate rapidly human hemoglobin-benzo[a]pyrene diol epoxide adducts. A combination of 300 A pore size C4 reversed phase HPLC to effect separation of adducted protein from native protein, and mu-bore C18 reversed phase HPLC to isolate and partially characterize proteolytic peptide adducts (by UV), was used.     

Transition metal hydrazide-based hydrogen-storage materials: the first atoms-in-molecules analysis of the Kubas interaction

Molecular models of the M-H(2) binding sites of experimentally characterised amorphous vanadium hydrazide gels are studied computationally using gradient corrected density functional theory, to probe the coordination number of the vanadium in the material and the nature of the interaction between the metal and the H(2) molecules. The H(2) is found to bind to the vanadium through the Kubas interaction, and the first quantum theory of atoms-in-molecules analysis of this type of interaction is reported. Strong correlation is observed between the electron density at the H-H bond critical point and the M-H(2) interaction energy. Four coordinate models give the best reproduction of the experimental data, suggesting that the experimental sites are four coordinate. The V-H(2) interaction is shown to be greater when the non-hydrazine based ligand, THF, of the experimental system is altered to a poorer π-acceptor ligand. Upon altering the metal to Ti or Cr the M-H(2) interaction energy changes little but the number of H(2) which may be bound decreases from four (Ti) to two (Cr). It is proposed that changing the metal from V to Ti may increase the hydrogen storage capacity of the experimental system. A 9.9 wt% maximum storage capacity at the ideal binding enthalpy for room temperature performance is predicted when the Ti metal is combined with a coordination sphere containing 2 hydride ligands.