Julia E. Parker

Crystalline Structures of Alkylamide Monolayers Adsorbed on the Surface of Graphite

Synchrotron X-ray and neutron diffraction have been used to determine the two-dimensional crystalline structures of alkylamides adsorbed on graphite at submonolayer coverage. The calculated structures show that the plane of the carbon backbone of the amide molecules is parallel to the graphite substrate. The molecules form hydrogen-bonded dimers, and adjacent dimers form additional hydrogen bonds yielding extended chains. By presenting data from a number of members of the homologous series, we have identified that these chains pack in different arrangements depending on the number of carbons in the amide molecule. The amide monolayers are found to be very stable relative to other closely related alkyl species, a feature which is attributed to the extensive hydrogen bonding present in these systems. The characteristics of the hydrogen bonds have been determined and are found to be in close agreement with those present in the bulk materials.

Behavior of binary alcohol mixtures adsorbed on graphite using calorimetry and scanning tunneling microscopy

The mixing behavior of binary combinations of linear alcohols adsorbed from their liquids is studied by calorimetry and scanning tunneling microscopy (STM). In particular, we consider combinations of primary alcohols that differ by a single methylene group. Where the shorter alcohol has an odd number of carbon atoms, the combination is found to mix, essentially, ideally on the surface. However, for combinations where the shorter alcohol has an even number of carbon atoms, we find that there is molecular complex formation for shorter members but ideal mixing for longer (n>12) homologues. This extends previous work in this area by the determination of the limits of surface molecular complex formation. We also exploit STM to address this unexpected complex formation.

Mixing behaviour of carboxylic acids adsorbed on graphite

Solid monolayer formation by all the linear carboxylic acids from C6 to C20 adsorbed from their liquids to a graphite surface is demonstrated. In addition, we present the two-dimensional phase behaviour of linear monocarboxylic acid mixtures adsorbed on graphite from their liquid mixtures, determined using differential scanning calorimetry. All acid mixtures with alkyl chains that differ by two or three methylene groups (Δn = 2 or 3) exhibit a significant degree of phase separation in the monolayer. Generally, the mixing tendency increases with increasing alkyl chain length for a given Δn, and with more similar chain lengths (e.g. Δn = 1). We report neutron diffraction data that confirms the formation of solid acid monolayers. This structural data also allows us to compare the mixing results with a recent quantitative model for 2-D mixing. The general form of the observed behaviour agrees well with the model, although the characteristic parameters that separate complete mixing, partial mixing and phase separation are different from those found with alkane and alcohol monolayer mixtures.

The monolayer structure of 1,2-bis(4-pyridyl)ethylene physisorbed on a graphite surface

The crystalline monolayer of 1,2-bis(4-pyridyl)ethylene physisorbed on a graphite surface at 0.44 monolayers coverage has been observed and characterized by synchrotron X-ray diffraction and differential scanning calorimetry. The experimentally determined monolayer structure has p2 symmetry with lattice parameters a = 17.77 Å, b = 13.69 Å and ν = 39.7°. The unit cell contains two molecules, which are oriented in a plane parallel to the surface. It is proposed that the molecules are arranged such that they are able to form a weak C–H ··· N hydrogen bond between pyridine groups. The monolayer melts at 414 K, which is unusually close to the bulk melting point for a sub-monolayer coverage system. This molecule is chiral when adsorbed on the surface, but both isomers appear in the unit cell leading to no overall chirality in the monolayer.

Monolayer structures of 4,4′ bipyridine on graphite at sub-monolayer coverage

The formation of crystalline monolayers of 4,4′ bipyridine at submonolayer coverage have been characterized by synchrotron X-ray diffraction. The experimentally determined monolayer unit cell exhibits a small temperature dependence, such that the monolayer structure at higher temperatures has a square unit cell, while at lower temperatures the unit cell becomes very slightly rectangular. Both structures have the same plane group, Pgg. The experimentally determined lattice parameters are a = 11.42 Å, i = 11.42 Å and v (=90° and a = 11.26 Å, b = 11.45 Å, v = 90° for the high and low temperature structures, respectively. There are two molecules per unit cell with the pyridine units of each molecule in a plane parallel to the graphite substrate, although we are not particularly sensitive to this orientation. Interestingly, the monolayer does not melt even at the highest temperatures accessible on the device, 445 K, well above the bulk melting point. This is a very unusual behaviour particularly at these submonolayer coverages.

Tracking the structural changes in pure and heteroatom substituted aluminophosphate, AIPO-18, using synchrotron based X-ray diffraction techniques

We report the structural changes that occur during the thermal removal of organic template molecules that occlude the pores of small pore nanoporous zeolitic solids, AlPO-18, SAPO-18, CoAlPO-18, ZnAlPO-18 and CoSAPO-18. The calcination process is a necessary step in the formation of active catalysts. The studies performed using time-resolved High Resolution Powder Diffraction (HRPD) and High Energy X-ray Diffraction (HEXRD) techniques at various temperatures reveal that changes that take place are dependent on the type of heteroatom present in the nanoporous solids. While time-resolved HRPD shows clear changes in lattice parameters during the removal of physisorbed water molecules and subsequent removal of the organic template, HEXRD data show changes in various near neighbour distances in AlPO-18, SAPO-18, CoAlPO-18, CoSAPO-18 and ZnAlPO-18 during the calcination process. In particular HEXRD reveals the presence of water molecules coordinated to Al(III) ions in the as-synthesised materials. Upon removal of the template and water, these solids show contraction in the cell volume at elevated temperatures while first and second neighbour distances remained almost unchanged.

Combined diffraction and density functional theory calculations of halogen-bonded cocrystal monolayers.

This work describes the combined use of synchrotron X-ray diffraction and density functional theory (DFT) calculations to understand the cocrystal formation or phase separation in 2D monolayers capable of halogen bonding. The solid monolayer structure of 1,4-diiodobenzene (DIB) has been determined by X-ray synchrotron diffraction. The mixing behavior of DIB with 4,4′-bipyridyl (BPY) has also been studied and interestingly is found to phase-separate rather than form a cocrystal, as observed in the bulk. DFT calculations are used to establish the underlying origin of this interesting behavior. The DFT calculations are demonstrated to agree well with the recently proposed monolayer structure for the cocrystal of BPY and 1,4-diiodotetrafluorobenzene (DITFB) (the perfluorinated analogue of DIB), where halogen bonding has also been identified by diffraction. Here we have calculated an estimate of the halogen bond strength by DFT calculations for the DITFB/BPY cocrystal monolayer, which is found to be ∼20 kJ/mol. Computationally, we find that the nonfluorinated DIB and BPY are not expected to form a halogen-bonded cocrystal in a 2D layer; for this pair of species, phase separation of the components is calculated to be lower energy, in good agreement with the diffraction results.

Phase Behavior of Heptanamide Adsorbed on a Graphite Substrate

In this letter, the phase behavior of a saturated alkylamide, heptanamide (C(7)), adsorbed on the surface of graphite using synchrotron X-ray diffraction is presented. The diffraction patterns indicate that heptanamide undergoes a solid-solid phase transition in the monolayer at 330 K from pgg symmetry at lower temperatures to p2 symmetry at high temperatures. Other alkylamides with similar carbon chain lengths do not show this phase change, making the C(7) homologue unusual.

The crystalline structure of the phenazine overlayer physisorbed on a graphite surface

The monolayer crystal structure of phenazine adsorbed on graphite is determined by a combination of synchrotron X-ray diffraction and DFT calculations. The molecules adopt a rectangular unit cell with lattice parameters a = 13.55 Å and b = 10.55 Å, which contains 2 molecules. The plane group of the unit cell is p2gg, and each molecule is essentially flat to the plane of the surface, with only a small amount of out-of-plane tilt. Density functional theory (DFT) calculations find a minimum energy structure with a unit cell which agrees within 7.5% with that deduced by diffraction. DFT including dispersion force corrections (DFT+D) calculations help to identify the nature of the intermolecular bonding. The overlayer interactions are principally van der Waals, with a smaller contribution from weak C-H···N hydrogen bonds. This behaviour is compared with that of 4,4′-bipyridyl.

The structures of 1-bromoheptane and 1-bromononane monolayers adsorbed on the surface of graphite

X-ray diffraction and differential scanning calorimetry (DSC) have been used to confirm the formation of the solid monolayers of 1-bromoheptane (C7Br) and 1-bromononane (C9Br) on the surface of graphite. The X-ray diffraction patterns of these two species can be fitted by two possible monolayer structures with symmetries Pgb and P2. For one of these (C7Br) this ambiguity can be lifted using neutron diffraction and the Pgb plane group can be identified as the correct structural solution. Both of the unit cells contain two molecules forming an approximately 90o zig-zag chain of bromine atoms with an inter-bromine distance of approximately 3.70 Å, which is similar to twice the Van der Waals radii of the bromine, suggesting rather little non-covalent interaction. The dipole moments in two adjacent chains are opposed leading to no net dipole for the layer as a whole.