True nature of an archetypal self-assembly system: Mobile Au-thiolate species on Au(111)

Alkanethiol self-assembled monolayer (SAM) phases on Au(111) have been assumed to involve direct S head group bonding to the substrate. Using x-ray standing wave experiments, we show the thiolate actually bonds to gold adatoms; self-organization in these archetypal SAM systems must therefore be governed by the movement of these Au-S-R moieties on the surface between two distinct local hollow sites on the surface. The results of recent ab initio total energy calculations provide strong support for this description, and a rationale for the implied significant molecular mobility in these systems

Experimental measurement and prediction of ionic liquid ionisation energies

Ionic liquid (IL) valence electronic structure provides key descriptors for understanding and predicting IL properties. The ionisation energies of 60 ILs are measured and the most readily ionised valence state of each IL (the highest occupied molecular orbital, HOMO) is identified using a combination of X-ray photoelectron spectroscopy (XPS) and synchrotron resonant XPS. A structurally diverse range of cations and anions were studied. The cation gave rise to the HOMO for nine of the 60 ILs presented here, meaning it is energetically more favourable to remove an electron from the cation than the anion. The influence of the cation on the anion electronic structure (and vice versa) were established; the electrostatic effects are well understood and demonstrated to be consistently predictable. We used this knowledge to make predictions of both ionisation energy and HOMO identity for a further 516 ILs, providing a very valuable dataset for benchmarking electronic structure calculations and enabling the development of models linking experimental valence electronic structure descriptors to other IL properties, e.g. electrochemical stability. Furthermore, we provide design rules for the prediction of the electronic structure of ILs

Allylic ionic liquid electrolyte-assisted electrochemical surface passivation of LiCoO2 for advanced, safe lithium-ion batteries

Room-temperature ionic liquid (RTIL) electrolytes have attracted much attention for use in advanced, safe lithium-ion batteries (LIB) owing to their nonvolatility, high conductivity, and great thermal stability. However, LIBs containing RTIL-electrolytes exhibit poor cyclability because electrochemical side reactions cause problematic surface failures of the cathode. Here, we demonstrate that a thin, homogeneous surface film, which is electrochemically generated on LiCoO2 from an RTIL-electrolyte containing an unsaturated substituent on the cation (1-allyl-1-methylpiperidinium bis(trifluoromethanesulfonyl)imide, AMPip-TFSI), can avert undesired side reactions. The derived surface film comprised of a high amount of organic species from the RTIL cations homogenously covered LiCoO2 with a ,25 nm layer and helped suppress unfavorable thermal reactions as well as electrochemical side reactions. The superior performance of the cell containing the AMPip-TFSI electrolyte was further elucidated by surface, electrochemical, and thermal analyses.open1

Monolayer to Bilayer Structural Transition in Confined Pyrrolidinium-Based Ionic Liquids

Ionic liquids can be intricately nanostructured in the bulk and at interfaces resulting from a delicate interplay between interionic and surface forces. Here we report the structuring of a series of dialkylpyrrolidinium-based ionic liquids induced by confinement. The ionic liquids containing cations with shorter alkyl chain substituents form alternating cation-anion monolayer structures on confinement to a thin film, whereas a cation with a longer alkyl chain substituent leads to bilayer formation. The crossover from monolayer to bilayer structure occurs between chain lengths of n = 8 and 10 for these pyrrolidinium-based ionic liquids. The bilayer structure for n = 10 involves full interdigitation of the alkyl chains; this is in contrast with previous observations for imidazolium-based ionic liquids. The results are pertinent to these liquids' application as electrolytes, where the electrolyte is confined inside the pores of a nanoporous electrode, for example, in devices such as supercapacitors or batteries. © 2013 American Chemical Society

Heteroatom Modified Polymer Immobilized Ionic Liquid Stabilized Ruthenium Nanoparticles: Efficient Catalysts for the Hydrolytic Evolution of Hydrogen from Sodium Borohydride

Ruthenium nanoparticles stabilised by polymer immobilized ionic liquids catalyse the hydrolytic release of hydrogen from sodium borohydride. The composition of the polymer influences performance and ruthenium nanoparticles stabilised by an amine-decorated imidazolium-based polymer immobilised ionic liquid (RuNP@NH2-PIILS) was the most efficient with a maximum initial turnover frequency (TOF) of 177 moleH2.molRu−1.min−1, obtained at 30°C with a catalyst loading of 0.08 mol%; markedly higher than that of 69 molH2.molRu−1.min−1 obtained with 5 wt% Ru/C and one of the highest to be reported for a RuNP catalyst. The apparent activation energy (Ea) of 38.9 kJ mol−1 for the hydrolysis of NaBH4 catalysed by RuNP@NH2-PIILS is lower than that for the other polymer immobilized ionic liquid stabilised RuNPs, which is consistent with its efficacy. Comparison of the initial rates of hydrolysis in H2O and D2O catalysed by RuNP@NH2-PIILS gave a primary kinetic isotope effect (kH/kD) of 2.3 which supports a mechanism involving rate limiting oxidative addition of one of the O-H bonds in a strongly hydrogen-bonded surface-coordinated [BH3H−]—-H2O ensemble. The involvement of a surface-coordinated borohydride is further supported by an inverse kinetic isotope effect of 0.65 obtained from a comparison of the initial rates for the hydrolysis of NaBH4 and NaBD4 under the conditions of catalysis i.e., at a high hydride/catalyst mole ratio. Interestingly though, when the comparison of the initial rates of hydrolysis of NaBH4 and NaBD4 was conducted in dilute solution with a hydride/catalyst mole ratio of 1 a kinetic isotope effect (kH/kD) of 2.72 was obtained; this would be more consistent with concerted activation of both an O-H and B-H bond in the rate limiting step, possibly via a concerted oxidative addition-hydride transfer in the surface-coordinated hydrogen-bonded ensemble. Catalyst stability and reuse studies showed that RuNP@NH2-PIILS retained 71% of its activity over five runs; the gradual drop in the initial TOF with run number appears to be due to passivation of the catalyst by the sodium borate by-product as well as an increase in viscosity of the reaction mixture rather than leaching of the catalyst

FDIONIC18 atomic charges of sulfur in ionic liquids: experiments and calculations

Experimental near edge X-ray absorption fine structure (NEXAFS) spectra, X-ray photoelectron (XP) spectra and Auger electron spectra are reported for sulfur in ionic liquids (ILs) with a range of chemical structures. These values provide experimental measures of the atomic charge in each IL and enable evaluation of the suitability of NEXAFS spectroscopy and XPS for probing relative atomic charge of sulfur. In addition, we use Auger electron spectroscopy to show that when XPS binding energies differ by less than 0.5 eV, conclusions on atomic charge should be treated with caution. Our experimental data provides a benchmark for calculations of atomic charge of sulfur obtained using different methods. Atomic charges were computed for lone ions and ion pairs, both in the gas phase (GP) and in a solvation model based on density (SMD), with a wide range of ion pair conformers considered. Three methods were used to compute atomic charges: charges from electrostatic potential using a grid based method (ChelpG), natural bond orbital (NBO) population analysis and Bader’s atoms in molecules (AIM) approach. By comparing experimental and calculated measures of atomic charge of sulfur, we provide an order for the sulfur atoms, ranging from most negative to most positive atomic charge. Furthermore, we show that both ChelpG and NBO are reasonable methods for calculating atomic charge of sulfur in ILs, based on agreement with both XPS and NEXAFS spectroscopy results. However, atomic charges of sulfur derived from ChelpG are found to display significant, non-physical conformational dependence. Only small differences in individual atomic charge of sulfur were observed between lone ion (GP) and ion pair IL(SMD) model systems, indicating that ion–ion interactions do not strongly influence individual atomic charges