15N chemical shifts in concentrated aqueous solutions of ammonium salts

¹⁵N n.m.r. spectra have been recorded from salts of ¹⁵N enriched ammonium ions in aqueous solution. Shifts of the resonance frequency have been attributed to direct interaction between the ammonium and its counter ion in solution.     

First emission studies of Tc2X8(2-) systems (X = Cl, Br)

The emission spectra of the solids [n-Bu(4)N](2)Tc(2)X(8) (X = Cl, Br) have been investigated at room temperature and 77 K. In each case, the emission originates in the (1)δ-δ* excited state, as with the rhenium homologues, but has a shorter lifetime.     

The absorption bandshape of a molecular dimer

The thermodynamic properties of a crystal lattice in the harmonic approximation are expressed in terms of a dispersion function conjugate to the spectral distribution of normal-mode vibrations. Frequency moments of the normal-mode distribution provide the necessary and sufficient information for construction of accurate bounds on the dispersion function and on the associated thermodynamic properties. This clarifies the successful employment of moment expansions and approximate frequency spectra in determinations of thermodynamic properties, and establishes the equivalence of recently devised continued-fraction and moment-theory techniques for investigating crystal lattice vibrations. When sufficient numbers of frequency moments are available, accurate histogram approximations to the normal-mode distribution, which correctly image band gaps and van Hove singularities, are obtained from Stieltjes inversion of the dispersion function, thereby providing an alternative to the more customary root-sampling techniques.     

Functionalized fullerenes in self-assembled monolayers

Anisotropy of intermolecular and molecule-substrate interactions holds the key to controlling the arrangement of fullerenes into 2D self-assembled monolayers (SAMs). The chemical reactivity of fullerenes allows functionalization of the carbon cages with sulfur-containing groups, thiols and thioethers, which facilitates the reliable adsorption of these molecules on gold substrates. A series of structurally related molecules, eight of which are new fullerene compounds, allows systematic investigation of the structural and functional parameters defining the geometry of fullerene SAMs. Scanning tunnelling microscopy (STM) measurements reveal that the chemical nature of the anchoring group appears to be crucial for the long-range order in fullerenes: the assembly of thiol-functionalized fullerenes is governed by strong molecule-surface interactions, which prohibit formation of ordered molecular arrays, while thioether-functionalized fullerenes, which have a weaker interaction with the surface than the thiols, form a variety of ordered 2D molecular arrays owing to noncovalent intermolecular interactions. A linear row of fullerene molecules is a recurring structural feature of the ordered SAMs, but the relative alignment and the spacing between the fullerene rows is strongly dependent on the size and shape of the spacer group linking the fullerene cage and the anchoring group. Careful control of the chemical functionality on the carbon cages enables positioning of fullerenes into at least four different packing arrangements, none of which have been observed before. Our new strategy for the controlled arrangement of fullerenes on surfaces at the molecular level will advance the development of practical applications for these nanomaterials.     

Crystallographic recognition controls peptide binding for bio-based nanomaterials

The ability to control the size, shape, composition, and activity of nanomaterials presents a formidable challenge. Peptide approaches represent new avenues to achieve such control at the synthetic level; however, the critical interactions at the bio/nano interface that direct such precision remain poorly understood. Here we present evidence to suggest that materials-directing peptides bind at specific time points during Pd nanoparticle (NP) growth, dictated by material crystallinity. As such surfaces are presented, rapid peptide binding occurs, resulting in the stabilization and size control of single-crystal NPs. Such specificity suggests that peptides could be engineered to direct the structure of nanomaterials at the atomic level, thus enhancing their activity.     

De haas-van alphen effect in stage 2 graphite-AsF5

We have observed dHvA frequencies in several samples of stage 2 graphite-AsF₅ intercalation compound. A consistent interpretation is given in terms of two nested hole surfaces located at each of the 6 corners of the 2D graphite zone. A fractional ionization f = 0.42 is obtained, consistent with spin susceptibility, with the Blinowski-Rigaux analysis of reflectivity, and with recent chemical results.     

Low temperature magnetization of graphite in magnetic fields up to 150 kG

The c-axis magnetisation of pyrolytic graphite at 4.2 K has been measured in fields up to 150 kG. Contrary to the predictions of a recent calculation, graphite remains diamagnetic in the ultra-quantum region.     

205Tl NMR studies: Ion pairing of Tl+ and (CH3)2Tl+ with NO 3 − and ClO 4 − in various solvents

The concentration dependence of the²⁰⁵Tl chemical shifts of Tl⁺ and of (CH₃)₂Tl⁺ ions was determined in several solvents with NO ₃ ^– and ClO ₄ ^– counterions. In general, increased ion pairing caused a low-frequency shift of the²⁰⁵Tl resonance, with the exceptions of (CH₃)₂TlNO₃ inn-butylamine and TlNO₃ in N,N-dimethylformamide (DMF) and in hexamethylphosphorotriamide (HMPA). In HMPA,²⁰⁵Tl linewidths of both Tl⁺ and (CH₃)₂Tl⁺ increased dramatically with dilution below 0.1M. Analysis of the data allowed ion-pair formation constants and²⁰⁵Tl chemical shifts for the ion-paired cation and for the free (solvated) cation to be estimated for some of the solvents.