PdCu nanoalloy decorated photocatalysts for efficient and selective oxidative coupling of methane in flow reactors

Methane activation by photocatalysis is one of the promising sustainable technologies for chemical synthesis. However, the current efficiency and stability of the process are moderate. Herein, a PdCu nanoalloy (~2.3 nm) was decorated on TiO2, which works for the efficient, stable, and selective photocatalytic oxidative coupling of methane at room temperature. A high methane conversion rate of 2480 μmol g-1 h-1 to C2 with an apparent quantum efficiency of ~8.4% has been achieved. More importantly, the photocatalyst exhibits the turnover frequency and turnover number of 116 h-1 and 12,642 with respect to PdCu, representing a record among all the photocatalytic processes (λ > 300 nm) operated at room temperature, together with a long stability of over 112 hours. The nanoalloy works as a hole acceptor, in which Pd softens and weakens C-H bond in methane and Cu decreases the adsorption energy of C2 products, leading to the high efficiency and long-time stability.

Highly selective oxidation of benzene to phenol with air at room temperature promoted by water

Phenol is one of the most important fine chemical intermediates in the synthesis of plastics and drugs with a market size of ca. $30b¹ and the commercial production is via a two-step selective oxidation of benzene, requiring high energy input (high temperature and high pressure) in the presence of a corrosive acidic medium, and causing serious environmental issues^2-5. Here we present a four-phase interface strategy with well-designed Pd@Cu nanoarchitecture decorated TiO₂ as a catalyst in a suspension system. The optimised catalyst leads to a turnover number of 16,000-100,000 for phenol generation with respect to the active sites and an excellent selectivity of ca. 93%. Such unprecedented results are attributed to the efficient activation of benzene by the atomically Cu coated Pd nanoarchitecture, enhanced charge separation, and an oxidant-lean environment. The rational design of catalyst and reaction system provides a green pathway for the selective conversion of symmetric organic molecules.

Atomic-scale modelling of organic matter in soil: adsorption of organic molecules and biopolymers on the hydroxylated α-Al2O3 (0001) surface

Binding of organic molecules on oxide mineral surfaces is a key process which impacts the fertility and stability of soils. Aluminium oxide and hydroxide minerals are known to strongly bind organic matter. To understand the nature and strength of sorption of organic carbon in soil, we investigated the binding of small organic molecules and larger polysaccharide biomolecules on α-Al₂O₃ (corundum). We modelled the hydroxylated α-Al₂O₃ (0001) surface, since these minerals' surfaces are hydroxylated in the natural soil environment. Adsorption was modelled using density functional theory (DFT) with empirical dispersion correction. Small organic molecules (alcohol, amine, amide, ester and carboxylic acid) were found to adsorb on the hydroxylated surface by forming multiple hydrogen bonds with the surface, with carboxylic acid as the most favourable adsorbate. A possible route from hydrogen-bonded to covalently bonded adsorbates was demonstrated, through co-adsorption of the acid adsorbate and a hydroxyl group to a surface aluminium atom. Then we modelled the adsorption of biopolymers, fragments of polysaccharides which naturally occur in soil: cellulose, chitin, chitosan and pectin. These biopolymers were able to adopt a large variety of hydrogen-bonded adsorption configurations. Cellulose, pectin and chitosan could adsorb particularly strongly, and therefore are likely to be stable in soil. This article is part of a discussion meeting issue 'Supercomputing simulations of advanced materials'.

Modelling the strength of mineral–organic binding: organic molecules on the α-Al2O3(0001) surface

Organic carbon (OC) is an essential component of soil. Sorption of OC to oxide mineral surfaces is a key process in soil preservation due to its ability to protect OC from microbial degradation. To understand the sorption of OC in soils and obtain a quantitative description of the binding of organic molecules to soil minerals, we investigated the binding of water and small organic molecules, typical building blocks of OC, on α-Al₂O₃, a common soil mineral. α-Al₂O₃ was modelled using (0001)-oriented periodic slabs, using density functional theory calculations with empirical dispersion correction. For water, dissociative adsorption was energetically preferred to molecular adsorption. Amine, amide and carboxylic acid functional groups were found to bind more strongly to this surface compared to water. Alcohol, ether, thiol and ester functional groups had adsorption energies very similar to that of water, while hydrocarbons were found to bind less strongly. Carboxylic acids were the strongest bound surface adsorbates in this study. Dissociated adsorption configurations (where allowed by the molecules' chemical nature) were usually more favourable than molecular adsorption. Hydrogen bonding was found to be a major contributor to the stability of adsorption configurations. This work shows that a number of organic functional groups, in particular amine, amide and carboxylic acids, bind to the α-Al₂O₃(0001) surface more strongly than water; thus they are likely to be adsorbed on this mineral surface under ambient conditions and to provide stability of adsorbed OC.

We evaluated the stability of organic molecules in soil by calculating these molecules' strength of adsorption on the α-Al₂O₃ mineral.

Computational design of graphitic carbon nitride photocatalysts for water splitting

A series of structures based on graphitic carbon nitride (g-C₃N₄), a layered material composed of linked carbon-nitrogen heterocycles arranged in a plane, were investigated by density functional theory calculations. g-C₃N₄ is a semiconductor that absorbs UV light and visible light at the blue end of the visible spectrum, and is widely studied as a photocatalyst for water splitting; however, its photocatalytic efficiency is limited by its poor light-harvesting ability and low charge mobilities. Modifications to the g-C₃N₄ structure could greatly improve its optical and electronic properties and its photocatalytic efficiency. In this work, the g-C₃N₄ structure was modified by replacing the nitrogen linker with heteroatoms (phosphorus, boron) or aromatic groups (benzene, s-triazine and substituted benzenes). Two-dimensional (2D) sheets and three-dimensional (3D) multilayer structures with different stacking types were modelled. Several new structures were predicted to have electronic properties superior to g-C₃N₄ for use as water splitting photocatalysts. In particular, introduction of phosphorus, benzene and s-triazine groups led to band gaps smaller than in the standard g-C₃N₄ (down to 2.4 eV, corresponding to green light). Doping with boron in the linker positions dramatically reduced the band gap (to 1.6 eV, corresponding to red light); the doped material has the valence band position suitable for water oxidation. Our computational study shows that chemical modification of g-C₃N₄ is a powerful method to tune this material's electronic properties and improve its photocatalytic activity.

Quantifying the Ultraslow Desorption Kinetics of 2,6-Naphthalenedicarboxylic Acid Monolayers at Liquid-Solid Interfaces

Kinetic effects in monolayer self-assembly at liquid-solid interfaces are not well explored but can provide unique insights. We use variable-temperature scanning tunneling microscopy (STM) to quantify the desorption kinetics of 2,6-naphthalenedicarboxylic acid (NDA) monolayers at nonanoic acid-graphite interfaces. Quantitative tracking of the decline of molecular coverages by STM between 57.5 and 65.0 °C unveiled single-exponential decays over the course of days. An Arrhenius plot of rate constants derived from fits results in a surprisingly high energy barrier of 208 kJ mol^-1 that strongly contrasts with the desorption energy of 16.4 kJ mol^-1 with respect to solution as determined from a Born-Haber cycle. This vast discrepancy indicates a high-energy transition state. Expanding these studies to further systems is the key to pinpointing the molecular origin of the remarkably large NDA desorption barrier.

Origin of Charge Trapping in TiO2/Reduced Graphene Oxide Photocatalytic Composites: Insights from Theory

Composites of titanium dioxide (TiO₂) and reduced graphene oxide (RGO) have proven to be much more effective photocatalysts than TiO₂ alone. However, little attention has been paid so far to the chemical structure of TiO₂/RGO interfaces and to the role that the unavoidable residual oxygen functional groups of RGO play in the photocatalytic mechanism. In this work, we develop models of TiO₂ rutile (110)/RGO interfaces by including a variety of oxygen functional groups known to be present in RGO. Using hybrid density functional theory calculations, we demonstrate that the presence of oxygen functional groups and the formation of interfacial cross-links (Ti-O-C covalent bonds and strong hydrogen bonds between TiO₂ and RGO) have a major effect on the electronic properties of RGO and RGO-based composites. The electronic structure changes from semimetallic to semiconducting with an indirect band gap, with the lowest unoccupied band positioned below the TiO₂ conduction band and largely localized on RGO oxygen and carbon orbitals, with some contributions of RGO-bonded Ti atoms. We suggest that this RGO-based lowest unoccupied band acts as a photoelectron trap and the indirect nature of the band gap hinders electron-hole recombination. These results can explain the experimentally observed extended lifetimes of photoexcited charge carriers in TiO₂/RGO composites and the enhancement of photocatalytic efficiency of these composites.

Remote functionalization in surface-assisted dehalogenation by conformational mechanics: organometallic self-assembly of 3,3',5,5'-tetrabromo-2,2',4,4',6,6'-hexafluorobiphenyl on Ag(111)

Even though the surface-assisted dehalogenative coupling constitutes the most abundant protocol in on-surface synthesis, its full potential will only become visible if selectivity issues with polybrominated precursors are comprehensively understood, opening new venues for both organometallic self-assembly and on-surface polymerization. Using the 3,3',5,5'-tetrabromo-2,2',4,4',6,6'-hexafluorobiphenyl (Br4F6BP) at Ag(111), we demonstrate a remote site-selective functionalization at room temperature and a marked temperature difference in double- vs. quadruple activation, both phenomena caused by conformational mechanical effects of the precursor-surface ensemble. The submolecularly resolved structural characterization was achieved by Scanning Tunneling Microscopy, the chemical state was quantitatively assessed by X-ray Photoelectron Spectroscopy, and the analysis of the experimental signatures was supported through first-principles Density-Functional Theory calculations. The non-planarity of the various structures at the surface was specifically probed by additional Near Edge X-ray Absorption Fine Structure experiments. Upon progressive heating, Br4F6BP on Ag(111) shows the following unprecedented phenomena: (1) formation of regular organometallic 1D chains via remote site-selective 3,5'-didebromination; (2) a marked temperature difference in double- vs. quadruple activation; (3) an organometallic self-assembly based on reversibility of C-Ag-C linkages with a thus far unknown polymorphism affording both hexagonal and rectangular 2D networks; (4) extraordinary thermal stability of the organometallic networks. Controlled covalent coupling at the previously Br-functionalized sites was not achieved for the Br4F6BP precursor, in contrast to the comparatively studied non-fluorinated analogue.

What can be inferred from moiré patterns? A case study of trimesic acid monolayers on graphite

Self-assembly of benzene-1,3,5-tricarboxylic acid (trimesic acid – TMA) monolayers at the alkanoic acid–graphite interface is revisited. Even though this archetypal model system for hydrogen bonded porous networks is particularly well studied, the analysis of routinely observed superperiodic contrast modulations known as moiré patterns lags significantly behind. Fundamental questions remain unanswered such as, are moiré periodicity and orientation always the same, i.e. is exclusively only one specific moiré pattern observed? What are the geometric relationships (superstructure matrices) between moiré, TMA, and graphite lattices? What affects the moiré pattern formation? Is there any influence from solvent, concentration, or thermal treatment? These basic questions are addressed via scanning tunneling microscopy experiments at the liquid–solid interface, revealing a variety of different moiré patterns. Interestingly, TMA and graphite lattices were always found to be ∼5° rotated with respect to each other. Consequently, the observed variation in the moiré patterns is attributed to minute deviations (<2°) from this preferred orientation. Quantitative analysis of moiré periods and orientations facilitates the determination of the TMA lattice parameter with picometer precision.

Molecular self-assembly of substituted terephthalic acids at the liquid/solid interface: investigating the effect of solvent

Self-assembly of three related molecules – terephthalic acid and its hydroxylated analogues – at liquid/solid interfaces (graphite/heptanoic acid and graphite/1-phenyloctane) has been studied using a combination of scanning tunnelling microscopy and molecular mechanics and molecular dynamics calculations. Brickwork-like patterns typical for terephthalic acid self-assembly have been observed for all three molecules. However, several differences became apparent: (i) formation or lack of adsorbed monolayers (self-assembled monolayers formed in all systems, with one notable exception of terephthalic acid at the graphite/1-phenyloctane interface where no adsorption was observed), (ii) the size of adsorbate islands (large islands at the interface with heptanoic acid and smaller ones at the interface with 1-phenyloctane), and (iii) polymorphism of the hydroxylated terephthalic acids’ monolayers, dependent on the molecular structure and/or solvent. To rationalise this behaviour, molecular mechanics and molecular dynamics calculations have been performed, to analyse the three key aspects of the energetics of self-assembly: intermolecular, substrate–adsorbate and solvent–solute interactions. These energetic characteristics of self-assembly were brought together in a Born–Haber cycle, to obtain the overall energy effects of formation of self-assembled monolayers at these liquid/solid interfaces.

From Au-Thiolate Chains to Thioether Sierpiński Triangles: The Versatile Surface Chemistry of 1,3,5-Tris(4-mercaptophenyl)benzene on Au(111)

Self-assembly of 1,3,5-tris(4-mercaptophenyl)benzene (TMB), a 3-fold symmetric, thiol-functionalized aromatic molecule, was studied on Au(111) with the aim of realizing extended Au-thiolate-linked molecular architectures. The focus lay on resolving thermally activated structural and chemical changes by a combination of microscopy and spectroscopy. Thus, scanning tunneling microscopy (STM) provided submolecularly resolved structural information, while the chemical state of sulfur was assessed by X-ray photoelectron spectroscopy (XPS). Directly after room-temperature deposition, only less well ordered structures were observed. Mild annealing promoted the first structural transition into ordered molecular chains, partly organized in homochiral molecular braids. Further annealing led to self-similar Sierpiński triangles, while annealing at even higher temperatures again resulted in mostly disordered structures. Both the irregular aggregates observed at room temperature and the chains were identified as metal-organic assemblies, whereby two out of the three intermolecular binding motifs are energetically equivalent according to density functional theory (DFT) simulations. The emergence of Sierpiński triangles is driven by a chemical transformation, i.e., the conversion of coordinative Au-thiolate to covalent thioether linkages, and can be further understood by Monte Carlo simulations. The great structural variance of TMB on Au(111) can on one hand be explained by the energetic equivalence of two binding motifs. On the other hand, the unexpected chemical transition even enhances the structural variance and results in thiol-derived covalent molecular architectures.

Nanopatterning of a covalent organic framework host-guest system

We have used a boroxine-based COF as a template for C60-fullerene self-assembly on graphite. Local removal of the COF by STM based nanomanipulation creates nanocorrals that may host other species.

Continuum and atomistic description of excess electrons in TiO2

The modelling of an excess electron in a semiconductor in a prototypical dye sensitised solar cell is carried out using two complementary approaches: atomistic simulation of the TiO2 nanoparticle surface is complemented by a dielectric continuum model of the solvent-semiconductor interface. The two methods are employed to characterise the bound (excitonic) states formed by the interaction of the electron in the semiconductor with a positive charge opposite the interface. Density-functional theory (DFT) calculations show that the excess electron in TiO2 in the presence of a counterion is not fully localised but extends laterally over a large region, larger than system sizes accessible to DFT calculations. The numerical description of the excess electron at the semiconductor-electrolyte interface based on the continuum model shows that the exciton is also delocalised over a large area: the exciton radius can have values from tens to hundreds of Ångströms, depending on the nature of the semiconductor (characterised by the dielectric constant and the electron effective mass in our model).

Thermodynamics of 4,4'-stilbenedicarboxylic acid monolayer self-assembly at the nonanoic acid-graphite interface

A direct calorimetric measurement of the overall enthalpy change associated with self-assembly of organic monolayers at the liquid-solid interface is for most systems of interest practically impossible. In previous work we proposed an adapted Born-Haber cycle for an indirect assessment of the overall enthalpy change by using terephthalic acid monolayers at the nonanoic acid-graphite interface as a model system. To this end, the sublimation enthalpy, dissolution enthalpy, the monolayer binding enthalpy in vacuum, and a dewetting enthalpy are combined to yield the total enthalpy change. In the present study the Born-Haber cycle is applied to 4,4'-stilbenedicarboxylic acid monolayers. A detailed comparison of these two aromatic dicarboxylic acids is used to evaluate and quantify the contribution of the organic backbone for stabilization of the monolayer at the nonanoic acid-graphite interface.

Adsorption studies of p-aminobenzoic acid on the anatase TiO₂(101) surface

The adsorption of p-aminobenzoic acid (pABA) on the anatase TiO2(101) surface has been investigated using synchrotron radiation photoelectron spectroscopy, near edge X-ray absorption fine structure (NEXAFS) spectroscopy, and density functional theory (DFT). Photoelectron spectroscopy indicates that the molecule is adsorbed in a bidentate mode through the carboxyl group following deprotonation. NEXAFS spectroscopy and DFT calculations of the adsorption structures indicate the ordering of a monolayer of the amino acid on the surface with the plane of the ring in an almost upright orientation. The adsorption of pABA on nanoparticulate TiO2 leads to a red shift of the optical absorption relative to bare TiO2 nanoparticles. DFT and valence band photoelectron spectroscopy suggest that the shift is attributed to the presence of the highest occupied molecular orbitals in the TiO2 band gap region and the presence of new molecularly derived states near the foot of the TiO2 conduction band.

Nanoscale electrical investigation of layer-by-layer grown molecular wires

Nanoscopic metal-molecule-metal junctions consisting of Fe-bis(terpyridine)-based ordered nanostructures are grown in layer-by-layer fashion on a solid support. Hopping is demonstrated as the main charge-transport mechanism both experimentally and theoretically.

Thermodynamics of halogen bonded monolayer self-assembly at the liquid–solid interface

The overall enthalpy change associated with hexabromotriphenylene monolayer self-assembly at the heptanoic acid–graphite interface was assessed by an adapted Born–Haber cycle.

Born-Haber cycle for monolayer self-assembly at the liquid-solid interface: assessing the enthalpic driving force

The driving force for self-assembly is the associated gain in free energy with decisive contributions from both enthalpy and entropy differences between final and initial state. For monolayer self-assembly at the liquid-solid interface, solute molecules are initially dissolved in the liquid phase and then become incorporated into an adsorbed monolayer. In this work, we present an adapted Born-Haber cycle for obtaining precise enthalpy values for self-assembly at the liquid-solid interface, a key ingredient for a profound thermodynamic understanding of this process. By choosing terephthalic acid as a model system, it is demonstrated that all required enthalpy differences between well-defined reference states can be independently and consistently assessed by both experimental and theoretical methods, giving in the end a reliable value of the overall enthalpy gain for self-assembly of interfacial monolayers. A quantitative comparison of enthalpy gain and entropy cost reveals essential contributions from solvation and dewetting, which lower the entropic cost and render monolayer self-assembly a thermodynamically favored process.

What Is the Best Anchoring Group for a Dye in a Dye-Sensitized Solar Cell?

We developed a computational procedure to screen many different anchoring groups used or usable to connect a dye to the semiconducting surface in a dye-sensitized solar cell. The procedure leads to a clear identification of the anchoring groups that bind strongly to the surface and facilitate the electron injection at the same time, providing clear-cut indications for the design of new dyes. The complicated interplay of factors that determine the final results (preferred adsorption mode, the anchor's effect on the dye's electronic structure, and dye-semiconductor coupling) is illustrated through a few examples showing how chemical intuition can often be misleading in this problem.

Incorporation dynamics of molecular guests into two-dimensional supramolecular host networks at the liquid-solid interface

The objective of this work is to study both the dynamics and mechanisms of guest incorporation into the pores of 2D supramolecular host networks at the liquid-solid interface. This was accomplished by adding molecular guests to prefabricated self-assembled porous monolayers and the simultaneous acquisition of scanning tunneling microscopy (STM) topographs. The incorporation of the same guest molecule (coronene) into two different host networks was compared, where the pores of the networks either featured a perfect geometric match with the guest (for trimesic acid host networks) or were substantially larger than the guest species (for benzenetribenzoic acid host networks). Even the moderate temporal resolution of standard STM experiments in combination with a novel injection system was sufficient to reveal clear differences in the incorporation dynamics in the two different host networks. Further experiments were aimed at identifying a possible solvent influence. The interpretation of the results is aided by molecular mechanics (MM) and molecular dynamics (MD) simulations.

Temperature control in molecular dynamic simulations of non-equilibrium processes

Thermostats are often used in various condensed matter problems, e.g. when a biological molecule undergoes a transformation in a solution, a crystal surface is irradiated with energetic particles, a crack propagates in a solid upon applied stress, two surfaces slide with respect to each other, an excited local phonon dissipates its energy into a crystal bulk, and so on. In all of these problems, as well as in many others, there is an energy transfer between different local parts of the entire system kept at a constant temperature. Very often, when modelling such processes using molecular dynamics simulations, thermostatting is done using strictly equilibrium approaches serving to describe the NVT ensemble. In this paper we critically discuss the applicability of such approaches to non-equilibrium problems, including those mentioned above, and stress that the correct temperature control can only be achieved if the method is based on the generalized Langevin equation (GLE). Specifically, we emphasize that a meaningful compromise between computational efficiency and a physically appropriate implementation of the NVT thermostat can be achieved, at least for solid state and surface problems, if the so-called stochastic boundary conditions (SBC), recently derived from the GLE (Kantorovich and Rompotis 2008 Phys. Rev. B 78 094305), are used. For SBC, the Langevin thermostat is only applied to the outer part of the simulated fragment of the entire system which borders the surrounding environment (not considered explicitly) serving as a heat bath. This point is illustrated by comparing the performance of the SBC and some of the equilibrium thermostats in two problems: (i) irradiation of the Si(001) surface with an energetic CaF(2) molecule using an ab initio density functional theory based method, and (ii) the tribology of two amorphous SiO(2) surfaces coated with self-assembled monolayers of methyl-terminated hydrocarbon alkoxylsilane molecules using a classical atomistic force field. We discuss the differences in behaviour of these systems due to applied thermostatting, and show that in some cases a qualitatively different physical behaviour of the simulated system can be obtained if an equilibrium thermostat is used.

Modelling the manipulation of C60 on the Si001 surface performed with NC-AFM

We present a theoretical model of manipulation of the C(60) molecule on the Si(001) surface with a non-contact atomic force microscope (NC-AFM). The model relies on the lowering of the energy barrier for the C(60) manipulation due to the interaction of the C(60) with an AFM tip and the subsequent thermal movement of the molecule over this barrier. We performed numerical simulations of these energy barriers for a series of tip positions relative to the molecule to show how the barriers change with the tip position. The values of these barriers are then used in kinetic Monte Carlo simulations to estimate the probability of the C(60) movement for different tip positions and temperatures. Virtual atomic force microscope simulations, which include the kinetic Monte Carlo treatment of the C(60) movement, are then performed to describe in real time the process of movement of the C(60) molecule during an NC-AFM scan. Our results demonstrate that manipulation of the C(60) molecule, which is covalently bound to the surface, is possible with NC-AFM, even though there is no continuous tip-molecule contact, which is known to be a necessary requirement for the C(60) manipulation with scanning tunnelling microscopy. We show that the manipulation event can be identified in real NC-AFM experiments as an abrupt change in the distance of the tip closest approach (topography), and as spikes in the frequency shift and dissipation signals.

Theoretical study of melamine superstructures and their interaction with the Au(111) surface

Using a systematic method, based on considering all possible hydrogen bond connections between two melamine molecules, and ab initio density functional theory (DFT) calculations, we consider the possible planar superstructures that the molecules can form in two dimensions. This is relevant to the assembly of melamine on flat metal surfaces with a small lateral corrugation of the molecule-surface interaction energy. The structures considered include small clusters as well as periodic structures, such as one-dimensional filaments and two-dimensional monolayers. Then, the interaction of melamine structures with the Au(111) surface is considered in detail to elucidate the possible effect of the surface on the formed structures, including the influence of the van der Waals interaction, which is not taken into account in DFT-based methods. The problem of commensurability between the lattices of the gas-phase monolayer and of the substrate is also discussed.

Theoretical modelling of tip effects in the pushing manipulation of C(60) on the Si(001) surface

We present the results of our theoretical studies on the repulsive (pushing) manipulation of a C(60) molecule on the Si(001) surface with several scanning tunnelling microscopy tips. We show that, for silicon tips, tip-C(60) bonds are formed even with tips that do not initially have dangling bonds, and this tip-C(60) interaction drives the manipulation of the molecule. The details of the atomic structure of the tip and its position relative to the molecule do not have a significant effect on the mechanism and the sequence of adsorption configurations during the pushing manipulation of C(60) along the trough, where the trough itself provides a guiding effect. The pushing manipulation is thus a very robust process that occurs largely independently of the tip structure. On the other hand, the pushing manipulation across an Si-Si dimer row into the neighbouring trough proceeds in a more complex way, with tip deformation and detachment more likely to occur. We demonstrate the role of tip deformation and tip-molecule bond rearrangement in the continuous manipulation of the molecule. Finally, we calculate and analyse the forces acting on the tip during manipulation and identify characteristic patterns.

C2 Isomers of C84F40 and C84F44 Are Cuboid and Contain Benzenoid and Naphthalenoid Aromatic Patches

A 1:1 mixture of C84F40 and C84F44, both derived from the D2(IV) isomer, has been isolated from the fluorination of [84]fullerene with either MnF3 or CoF3 at 500 degrees C. The 1D and 2D COSY 19F NMR spectra showed that each derivative is cuboid, having benzenoid rings at four of the six octahedral sites; the two remaining sites have naphthalenoid rings for C84F40, and two slightly offset benzenoid rings for C84F44. The benzenoid rings each have six adjacent sp3-hybridised carbon atoms whilst the naphthalenoid moieties have eight, thus facilitating full delocalisation. In terms of the number and size of aromatic patches, C84F44 is the most aromatic fullerene derivative yet isolated.

Isolation of two seven-membered ring C58 fullerene derivatives: C58F17CF3 and C58F18

Fluorination of C60 at 550 degrees C leads to milligram quantities of two stable fullerene derivatives with 58-carbon cage structures: C58F18 and C58F17CF3. The compounds were characterized by mass spectrometry and fluorine nuclear magnetic resonance spectroscopy, and the data support a heptagonal ring in the framework. The resulting strain, which has hindered past attempts to prepare these smaller quasi-fullerenes, is mitigated here by hybridization change of some of the carbons in the pentagons from sp2 to sp3 because of fluorine addition. The loss of carbon from C60 is believed to occur via sequential fluorine addition to a CC single bond and an adjacent CC bond, followed by loss of a:CF2 carbene.

C1 C70F38 Contains Four Planar Aromatic Hexagons; The Parallel between Fluorination of [60]- and [70]Fullerenes

The main C(1) isomer of C(70)F(38) is shown by single-crystal X-ray analysis to contain four planar aromatic hexagons and four isolated C=C bonds, has two fluorines on the equator, and is related to C(2) C(70)F(38) by means of three 1,3-fluorine shifts. The C(1) and C(2) isomers thus parallel the T and C(3)/C(1) isomers of C(60)F(36) in containing three and four aromatic rings, respectively, and in the fluorine shift relationship.

C2 C70F38 is aromatic, contains three planar hexagons, and has equatorial addends

Single crystal X-ray analysis shows the main (C(2)) isomer of C(70)F(38) to contain three planar delocalised aromatic hexagons (two equivalent and one centred on the C(2) axis), together with seven C[double bond, length as m-dash]C bonds (three pairs and one straddling the C(2) axis); C(70)F(38) is the first high addition level [70]fullerene derivative to be fully characterised, is the first to have equatorial addends, and is calculated to have high stability.

Methylation of [76]fullerene and [84]fullerenes; the first oxahomo derivatives of a higher fullereneElectronic supplementary information (ESI) available: HPLC separation data (Table S1) and calculated heats of formation (Table S2). See http://www.rsc.org/suppdata/ob/b3/b316163c/

Methylation of [76]fullerene by reaction with Al-Ni alloy/NaOH followed by quenching of the intermediate anions with methyl iodide gives a mixture of methylated and methylenated products together with oxide derivatives. The major derivatives are five isomers of C(76)Me(2)(one of C(s) symmetry due to 1,6-C(76)Me(2)) and C(76)(CH(2))(n)(n= 2-4), together with corresponding mono-oxides. The single line (1)H NMR spectrum of mono-oxide C(76)Me(2)O shows it is an oxahomofullerene (ether) the first example derived from [76]fullerene, oxygen being inserted between the CMe groups in 1,6-C(76)Me(2)giving a product of C(2) symmetry. The probable structures of the unsymmetrical dimethyl derivatives have been deduced from heats of formation calculated by AM1 and density functional methods. Bis-oxide C(76)Me(4)O(2) is the first bis oxahomo[76]fullerene to be isolated and gives two equal-intensity lines in the (1)H NMR spectrum showing that it must also have C(2) symmetry; probable structures are considered. Methylation of [84]fullerene takes place less readily and only four C(84)Me(2) derivatives were isolated, two of them in quantities sufficient to show the symmetries as C(1), and either C(2) or C(s).