Publications by authors named "Alexander K Yuen"

13 Publications

  • Page 1 of 1

Structural and Magnetic Studies of O-Type Ruthenium and Osmium Oxides.

Inorg Chem 2020 Mar 14;59(5):2791-2802. Epub 2020 Feb 14.

School of Chemistry, The University of Sydney, Sydney, New South Wales 2006, Australia.

Oxides of the form O with = K, Rb, Cs and = Ru and Os have been synthesized and characterized by diffraction and magnetic techniques. For = K the oxides adopted the tetragonal (4/) scheelite structure. RbOsO, which crystallizes as a scheelite at room temperature, underwent a continuous phase transition to 4/ near 550 K. RbRuO and CsOsO were found to crystallize in the orthorhombic () pseudoscheelite structure, and both displayed discontinuous phase transitions to 4/ at high temperatures. CsOsO was determined to undergo a phase transition to a 2/ structure below 140 K. CsRuO crystallizes with a baryte-type structure at room temperature. Upon heating CsRuO a first order phase transition to the scheelite structure in 4/ is observed at 400 K. A continuous phase transition is observed to 222 below 140 K. DC magnetic susceptibility data is consistent with long-range antiferromagnetic ordering at low temperatures for all compounds except for CsOsO, which is paramagnetic to 2 K. The effective magnetic moments are in agreement with the spin only values for an = 1/2 quantum magnet. Effective magnetic moments calculated for Os compounds were lower than their Ru counterparts, reflective of an enhanced spin orbit coupling effect. A magnetic structure is proposed for RbRuO consisting of predominately antiferromagnetic (AFM) ordering along the 001 direction, with canting of spins in the 100 plane. A small ordered magnetic moment of 0.77 μ was determined.
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http://dx.doi.org/10.1021/acs.inorgchem.9b03118DOI Listing
March 2020

Hydrothermal Liquefaction of α-O-4 Aryl Ether Linkages in Lignin.

ChemSusChem 2020 Apr 27;13(8):2002-2006. Epub 2020 Feb 27.

Laboratory of Advanced Catalysis for Sustainability, School of Chemistry, The University of Sydney, Sydney, 2006, Australia.

By using lignin model compounds with relevant key characteristic structural features, the reaction pathways of α-O-4 aryl ether linkages under hydrothermal conditions are elucidated. Experimental results and computational modeling suggest that the α-O-4 linkages in lignin undergo catalyzed hydrolysis and elimination to give phenolic and alkenylbenzene derivatives as major products in subcritical water. The decreased relative permittivity of water at these high temperatures and pressures facilitates the elimination reactions. The alkyl group on the α-carbon and the methoxy groups on the phenyl rings both have positive effects on the rate of conversion of α-O-4 linkages in native lignin.
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http://dx.doi.org/10.1002/cssc.201903263DOI Listing
April 2020

Structural and magnetic studies of KOsO, a 5d quantum magnet oxide.

Phys Chem Chem Phys 2019 Apr;21(14):7261-7264

School of Chemistry, The University of Sydney, Sydney, NSW 2006, Australia.

The quantum magnet KOsO4 has been characterized by a combination of X-ray and neutron diffraction techniques. The tetrahedrally coordinated Os7+ 5d1S = 1/2 cations were determined to order antiferromagnetically along the c axis below 35 K. A miniscule ordered magnetic moment of 0.46(18) μB was determined per Os7+ cation.
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http://dx.doi.org/10.1039/c9cp00448cDOI Listing
April 2019

Unravelling Some of the Key Transformations in the Hydrothermal Liquefaction of Lignin.

ChemSusChem 2017 05 21;10(10):2140-2144. Epub 2017 Apr 21.

Laboratory of Advanced Catalysis for Sustainability, School of Chemistry, The University of Sydney, Sydney, 2006, Australia.

Using both experimental and computational methods, focusing on intermediates and model compounds, some of the main features of the reaction mechanisms that operate during the hydrothermal processing of lignin were elucidated. Key reaction pathways and their connection to different structural features of lignin were proposed. Under neutral conditions, subcritical water was demonstrated to act as a bifunctional acid/base catalyst for the dissection of lignin structures. In a complex web of mutually dependent interactions, guaiacyl units within lignin were shown to significantly affect overall lignin reactivity.
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http://dx.doi.org/10.1002/cssc.201700528DOI Listing
May 2017

Masked N-Heterocyclic Carbene-Catalyzed Alkylation of Phenols with Organic Carbonates.

ChemSusChem 2016 09 16;9(17):2312-6. Epub 2016 Aug 16.

Laboratory of Advanced Catalysis for Sustainability, School of Chemistry, The University of Sydney, Sydney, NSW, 2006, Australia.

An easily prepared masked N-heterocyclic carbene, 1,3-dimethylimidazolium-2-carboxylate (DMI-CO2 ), was investigated as a "green" and inexpensive organocatalyst for the alkylation of phenols. The process made use of various low-toxicity and renewable alkylating agents, such as dimethyl- and diethyl carbonate, in a focused microwave reactor. DMI-CO2 was found to be a very active catalyst and excellent yields of a range of aryl alkyl ethers were obtained under relatively benign conditions. The observed difference in the conversion behavior of phenol methylation, in the presence of either the carbene or 1,8-diazabicycloundec-7-ene (DBU) catalyst, was rationalized on the basis of mechanistic investigations. The primary mode of action for the N-heterocyclic carbene is nucleophilic catalysis. Activation of the dialkyl carbonate electrophile results in concomitant evolution of an organo-soluble alkoxide, which deprotonates the phenolic starting material. In contrast, DBU is initially protonated by the phenol and thus consumed. Subsequent regeneration and participation in nucleophilic catalysis only becomes significant after some phenolate alkylation occurs.
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http://dx.doi.org/10.1002/cssc.201600493DOI Listing
September 2016

Ionic liquids are compatible with on-water catalysis.

Chem Commun (Camb) 2013 Sep;49(75):8347-9

School of Chemistry F11, University of Sydney, Sydney 2006, Australia.

A major limitation of on-water catalysis has been the need for liquid reactants to enable emulsification. We demonstrate that ionic liquids are compatible with on-water catalysis, enabling on-water catalysed reactions for otherwise unreactive solid-solid systems. The unique solvation properties of ionic liquids dramatically expands the scope of on-water catalysis.
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http://dx.doi.org/10.1039/c3cc44150dDOI Listing
September 2013

A flexible, bolaamphiphilic template for mesoporous silicas.

Phys Chem Chem Phys 2013 Aug;15(32):13343-53

Laboratory of Advanced Catalysis for Sustainability, School of Chemistry F11, The University of Sydney, Sydney 2006, Australia.

A novel symmetrical bolaamphiphile, containing two N-methylimidazolium head-groups bridged by a 32-methylene linker, was synthesized and characterized. A variety of mesoporous silicas was prepared using the bolaamphiphile as a "soft template". The effects of absolute surfactant concentration and synthesis conditions upon the morphologies of these silicas were investigated. For a given surfactant concentration, particle morphology; pore size; and pore ordering were modified through control of the template to silica-precursor ratio and synthesis conditions. Observed morphologies included: lenticular core-shell nanoparticles and decorticated globules, truncated hexagonal plates, and sheets. In all cases the mesopores are aligned along the shortest axis of the nanomaterial. Decorticated materials displayed surface areas of up to 1200 m(2) g(-1) and pore diameters (D(BJH)) of 24-28 Å. Small-angle X-ray diffraction and transmission electron microscopy measurements revealed that the majority of the materials has elliptical pores arranged in rectangular lattices (c2mm). Adoption of this symmetry group is a result of the template aggregate deformation from a regular hexagonal phase of cylindrical rods to a ribbon phase under the synthetic conditions.
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http://dx.doi.org/10.1039/c3cp51546jDOI Listing
August 2013

Exploring the myth of nascent hydrogen and its implications for biomass conversions.

Chem Asian J 2012 Nov 5;7(11):2629-37. Epub 2012 Sep 5.

School of Chemistry, The University of Sydney, NSW 2006, Australia.

Iron (and to a lesser extent manganese) in the wall of a 316 stainless steel (SS) reactor is responsible for the hydrogenation of cyclohexanone to cyclohexanol when using an aqueous formic acid solution under high temperature and pressure water (HTPW) conditions. However, not only dilute formic acid but also aqueous solutions of several other organic and mineral acids in the presence of iron are active in this reaction covering a range of aldehydes and ketones, even under ambient conditions. The stoichiometry, kinetics, and the possible mechanisms of both dihydrogen production as well as of the hydrogenation of the model compound cyclohexanone were examined. The reduction is essentially stoichiometric with respect to metallic iron, and the conversions are highly dependent on the speed of stirring as well as temperature and reactant concentrations. Importantly, it is established unequivocally that water participates in dihydrogen gas formation (hydrogen atoms originate from both the acid and water molecules) and facilitates substrate reduction.
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http://dx.doi.org/10.1002/asia.201200557DOI Listing
November 2012

The role of the reactor wall in hydrothermal biomass conversions.

Chem Asian J 2012 Nov 5;7(11):2638-43. Epub 2012 Sep 5.

School of Chemistry, The University of Sydney, NSW 2006, Australia.

The processing of renewable feedstocks to platform chemicals and, to a lesser degree, fuels is a key part of sustainable development. In particular, the combination of lignocellulosic biomass with hydrothermal upgrading (HTU), using high temperature and pressure water (HTPW), is experiencing a renaissance. One of the many steps in this complicated process is the in-situ hydrogenation of intermediate compounds. As formic acid and related low-molecular-weight oxygenates are among the species generated, it is conceivable that they act as a hydrogen source. Such hydrogenations have been suggested to be catalyzed by water, by bases like NaOH, and/or to involve "reactive/nascent hydrogen". To achieve the temperatures and pressures required for HTU, it is necessary to conduct the reactions in high-pressure vessels. Metals are typical components of their walls and/or internal fittings. Here, using cyclohexanone as a model compound for more complex biomass-derived molecules, iron in the wall of high-pressure stainless steel reactors is shown to be responsible for the hydrogenation of ketones with low-molecular-weight oxygenates acting as a hydrogen source in combination with water.
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http://dx.doi.org/10.1002/asia.201200556DOI Listing
November 2012

The interplay of catechol ligands with nanoparticulate iron oxides.

Dalton Trans 2012 Mar 12;41(9):2545-59. Epub 2012 Jan 12.

Laboratory of Advanced Catalysis for Sustainability, School of Chemistry F11, The University of Sydney, Sydney, Australia.

The unique properties exhibited by nanoscale materials, coupled with the multitude of chemical surface derivatisation possibilities, enable the rational design of multifunctional nanoscopic devices. Such functional devices offer exciting new opportunities in medical research and much effort is currently invested in the area of "nanomedicine", including: multimodal imaging diagnostic tools, platforms for drug delivery and vectorisation, polyvalent, multicomponent vaccines, and composite devices for "theranostics". Here we will review the surface derivatisation of nanoparticulate oxides of iron and [email protected] core-shells. They are attractive candidates for MRI-active therapeutic platforms, being potentially less toxic than lanthanide-based materials, and amenable to functionalisation with ligands. However successful grafting of groups onto the surface of iron-based nanoparticles, thus adding functionality whilst preserving their inherent properties, is one of the most difficult challenges for creating truly useful nanodevices from them. Functionalised catechol-derived ligands have enjoyed success as agents for the masking of superparamagnetic iron-oxide particles, often so as to render them biocompatible with medium to long-term colloidal stability in the complex chemical environments of biological milieux. In this perspective, the opportunities and limitations of functionalising the surfaces of iron-oxide nanoparticles, using coatings containing a catechol-derived anchor, are analysed and discussed, including recent advances using dopamine-terminated stabilising ligands. If light-driven ligand to metal charge transfer (LMCT) processes, and pH-dependent ligand desorption, leading to nanoparticle degradation under physiologically relevant conditions can be suppressed, colloidal stability of samples can be maintained and toxicity ascribed to degradation products avoided. Modulation of the redox behaviour of iron catecholate systems through the introduction of an electron-withdrawing substituent to the aromatic π-system of the catechol is a promising approach towards achieving these goals.
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http://dx.doi.org/10.1039/c2dt11864eDOI Listing
March 2012

The use of high dose letrozole in ovulation induction and controlled ovarian hyperstimulation.

ISRN Obstet Gynecol 2011 17;2011:242864. Epub 2011 Nov 17.

Wisconsin Fertility Institute, 3146 Deming Way, Middleton, WI 53562, USA.

Letrozole, an aromatase inhibitor, has been demonstrated to be effective as an ovulation induction and controlled ovarian hyperstimulation agent. However, dose administration has generally been limited to 5 days at 2.5 to 7.5 mg daily. We undertook a retrospective review of over 900 treatment cycles using letrozole in doses as high as 12.5 mg per day. Results indicate that such doses do indeed offer benefit to patients; in that there is increased follicular growth and a higher number of predicted ovulations with higher doses of the drug. However, increasing doses does not produce a detrimental effect upon endometrial thickness. High-dose letrozole may be of value in women who fail to respond adequately to lower doses. Furthermore, randomized trials are needed to determine whether high-dose letrozole might actually be optimal as a starting dose for certain treatment groups.
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http://dx.doi.org/10.5402/2011/242864DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3236406PMC
August 2012

Measurement of long-range interatomic distances by solid-state tritium-NMR spectroscopy.

J Am Chem Soc 2010 Feb;132(6):1734-5

CEA, iBiTecS, Service de Chimie Bioorganique et de Marquage, F-91191 Gif sur Yvette, France.

For the structural determination of a ligand bound to an amorphous macromolecular system, solid-state NMR can be used to provide interatomic distances. It is shown here that selective labeling in discrete locations with tritium enables accurate measurement of long-range distances owing to the high gyromagnetic ratio of this nucleus, without structural modification of the molecule. This approach gives access to the largest NMR distance ever measured between two nuclei (14.4 A). (3)H MAS NMR appears to be a promising tool for structural applications in the biological and material sciences.
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http://dx.doi.org/10.1021/ja908915vDOI Listing
February 2010

Estimating subglottal pressure via airflow redirection.

Laryngoscope 2007 Aug;117(8):1491-5

Department of Surgery, Division of Otolaryngology-Head and Neck Surgery, University of Wisconsin School of Medicine and Public Health, Madison, Wisconsin 53792-7375, USA.

Subglottal pressure (SGP) is a valuable parameter in the research and clinical assessment of laryngeal function. The lungs serve as a constant pressure source during sustained phonation, and that pressure, SGP, can be used to determine the efficiency with which the larynx converts aerodynamic power to acoustic power. As the larynx serves as an aerodynamic transducer, the vocal efficiency (Ve) coefficient, defined as acoustic power (dB) divided by aerodynamic power (SGP x glottal airflow) has been shown to reliably reflect vocal health. However, current SGP measurement techniques are hesitantly used because of either an invasive nature or the requirement of intensive patient training. This study tests a novel device that has been designed to noninvasively estimate SGP through mechanical airflow redirection, producing a numeric output on completion of the trial, which lasts only a few seconds. The novelty of this design lies in the ease of use for both the patient and the clinician. Multiple mechanical airflow redirections occlude the airway for only 135 ms, which is predicted to limit the effect of confounding laryngeal reflexes that may occur during the trials. Additionally, the airflow redirection into a retention device allows for the pneumatic in-trial comparison of the estimated SGP with the pressure achieved by the patient, providing a numeric output to the clinician on completion.
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http://dx.doi.org/10.1097/mlg.0b013e318063e89eDOI Listing
August 2007
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