The outcome of synthetic procedures for crystalline organic materials strongly depends on the first steps along the molecular self-assembly pathway, a process we know as crystal nucleation. New experimental techniques and computational methodologies have spurred significant interest in understanding the detailed molecular mechanisms by which nuclei form and develop into macroscopic crystals. Although classical nucleation theory (CNT) has served well in describing the kinetics of the processes involved, new proposed nucleation mechanisms are additionally concerned with the evolution of structure and the competing nature of crystallization in polymorphic systems. In this Review, we explore the extent to which CNT and nucleation rate measurements can yield molecular-scale information on this process and summarize current knowledge relating to molecular self-assembly in nucleating systems.

Quantitative analysis of complex amino acids and RGD peptides by X-ray photoelectron spectroscopy (XPS). Surface and Interface Analysis, 45 (8). 1238 -1246 . ISSN 0142-2421 https://doi.org/10.1002/sia.5261 eprints@whiterose.ac.uk https://eprints.whiterose.ac.uk/ Reuse Unless indicated otherwise, fulltext items are protected by copyright with all rights reserved. The copyright exception in section 29 of the Copyright, Designs and Patents Act 1988 allows the making of a single copy solely for the purpose of non-commercial research or private study within the limits of fair dealing. The publisher or other rights-holder may allow further reproduction and re-use of this version -refer to the White Rose Research Online record for this item. Where records identify the publisher as the copyright holder, users can verify any specific terms of use on the publisher's website. TakedownIf you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request. 1 Quantitative Analysis of Complex Amino Acids and RGD Peptides by X-ray Photoelectron Spectroscopy (XPS)Joanna

Recent studies suggested that X-ray photoelectron spectroscopy (XPS) sensitively determines the protonation state of nitrogen functional groups in the solid state, providing a means for distinguishing between co-crystals and salts of organic compounds. Here we describe how a new theophylline complex with 5-sulfosalicylic acid dihydrate was established as a salt by XPS prior to assignment with conventional methods. The presence of a C=NH(+) (N9) N1s peak in XPS allows assignment as a salt, while this peak is clearly absent for a theophylline co-crystal. The large low frequency shift for N9 observed by (15)N solid-state nuclear magnetic resonance spectroscopy (ssNMR) and corresponding density functional theory (DFT) calculations confirm that protonation has occurred. The crystal structure and further analytical studies confirm the conclusions reached with XPS and ssNMR. This study demonstrates XPS as an alternative technique for determining whether proton transfer has occurred in acid-base complexes.

The properties of nitrogen centres acting either as hydrogen-bond or Brønsted acceptors in solid molecular acid-base complexes have been probed by N 1s X-ray photoelectron spectroscopy (XPS) as well as (15)N solid-state nuclear magnetic resonance (ssNMR) spectroscopy and are interpreted with reference to local crystallographic structure information provided by X-ray diffraction (XRD). We have previously shown that the strong chemical shift of the N 1s binding energy associated with the protonation of nitrogen centres unequivocally distinguishes protonated (salt) from hydrogen-bonded (co-crystal) nitrogen species. This result is further supported by significant ssNMR shifts to low frequency, which occur with proton transfer from the acid to the base component. Generally, only minor chemical shifts occur upon co-crystal formation, unless a strong hydrogen bond is formed. CASTEP density functional theory (DFT) calculations of (15)N ssNMR isotropic chemical shifts correlate well with the experimental data, confirming that computational predictions of H-bond strengths and associated ssNMR chemical shifts allow the identification of salt and co-crystal structures (NMR crystallography). The excellent agreement between the conclusions drawn by XPS and the combined CASTEP/ssNMR investigations opens up a reliable avenue for local structure characterization in molecular systems even in the absence of crystal structure information, for example for non-crystalline or amorphous matter. The range of 17 different systems investigated in this study demonstrates the generic nature of this approach, which will be applicable to many other molecular materials in organic, physical, and materials chemistry.

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Ever since the discovery of heterogeneous gold catalysts for low-temperature oxidation reactions, the mechanism of O2 dissociation on these materials has been controversial. We report Au L3-edge X-ray absorption near-edge structure (XANES) data for an active Au/TiO2 catalyst which indicate that fully reduced metallic gold particles on a reducible support (TiO2) form activated gold-oxygen complexes in the absence of CO. It is possible that these play a role in the mechanism of CO oxidation. These results were obtained with a highly active powder catalyst and the conclusions agree with those derived from model catalysts, for which it was shown that reduced gold is the active component in the oxidation of CO.

Novel magnesium fluorides have been prepared by a new fluorolytic sol-gel synthesis for fluoride materials based on aqueous HF. By changing the amount of water at constant stoichiometric amount of HF, it is possible to tune the surface acidity of the resulting partly hydroxylated magnesium fluorides. These materials possess medium-strength Lewis acid sites and, by increasing the amount of water, Brønsted acid sites as well. Magnesium hydroxyl groups normally have a basic nature and only with this new synthetic route is it possible to create Brønsted acidic magnesium hydroxyl groups. XRD, MAS NMR, TEM, thermal analysis, and elemental analysis have been applied to study the structure, composition, and thermal behaviour of the bulk materials. XPS measurements, FTIR with probe molecules, and the determination of N(2)/Ar adsorption-desorption isotherms have been carried out to investigate the surface properties. Furthermore, activity data have indicated that the tuning of the acidic properties makes these materials versatile catalysts for different classes of reactions, such as the synthesis of (all-rac)-[alpha]-tocopherol through the condensation of 2,3,6-trimethylhydroquinone (TMHQ) with isophytol (IP).

Significant elemental segregation is shown to exist within individual hollow silver-gold (Ag-Au) bimetallic nanoparticles obtained from the galvanic reaction between Ag particles and AuCl4(-). Three-dimensional compositional mapping using energy dispersive X-ray (EDX) tomography within the scanning transmission electron microscope (STEM) reveals that nanoparticle surface segregation inverts from Au-rich to Ag-rich as Au content increases. Maximum Au surface coverage was observed for nanoparticles with approximately 25 atom % Au, which correlates to the optimal catalytic performance in a three-component coupling reaction among cyclohexane carboxyaldehyde, piperidine, and phenylacetylene.