Steve C.F. Au-Yeung

Light-scattering studies of poly(N-isopropylacrylamide) in tetrahydrofuran and aqueous solution

Poly(N-isopropylacrylamide) (PNIPAM) was synthesized by free-radical polymerization. High resolution n.m.r. was used to confirm its chain composition and linear chain configuration. The solution properties of both unfractionated and fractionated PNIPAM samples in THF and water at 20°C were investigated by laser light scattering (LLS). Owing to its polyelectrolyte nature, it is extremely difficult, or nearly impossible, to obtain a set of narrowly distributed PNIPAM fractions with Mw/Mn < 1.3. Therefore, a scaling relationship between the translational diffusion coefficient (D) and the molar mass (M) has not yet been established. In this study, a combination of Mw from static LLS and the translational diffusion coefficient distribution (G(D)) from dynamic LLS results enabled us to establish D (cm2 s−1)= 2.49 × 10−4M−0.540 for PNIPAM in THF at 20°C. Using this scaling relationship, we successfully calculated the molar mass distributions of a series of PNIPAM samples. For a given PNIPAM sample, the molar mass distributions obtained in different solvents are fairly comparable.

An Investigation of the Roles of Surface Aluminum and Acid Sites in the Zeolite MCM‐22

Ammonia adsorption studies reveal that the observed Lewis acidity in the zeolite MCM-22 is derived from at least two types of framework aluminum sites (AlF), that is, octahedral AlF and three-coordinate AlF. Comparative ammonia or trimethylphosphine (TMP) adsorption experiments with MCM-22 confirm that octahedral Al species gives rise to the signal at delta(iso) approximately 0 in the 27Al NMR spectrum; this is a superposition of two NMR signals from the different Al species on the water-reconstructed zeolite surface. A sharp resonance assigned to framework Al reversibly transforms on ammonia adsorption to delta(iso)27Al approximately 55 from tetrahedral AlF, while the broad peak is assigned to nonframework aluminum which results from hydrothermal treatment. This study also demonstrates the effectiveness of 27Al magic angle spinning (MAS) and multiple quantum (MQ) MAS NMR spectroscopy as a technique for the study of zeolite reactions.

Substituted metal carbonyls. Part 21. [M(CO)5(dppf-P)][M = Cr, Mo or W; dppf = Fe(C5H4PPh2)2] as a metalloligand in heteropolymetallic aggregates of AuI, PdII and PtII. Crystal and molecular structures of [PtCl2{(µ-dppf)W(CO)5}2] and [Mo(CO)5(dppf-P)]

The complexes [M(CO)5(dppf-P)][M = Cr, Mo or W; dppf = 1,1′-bis(diphenylphosphino)ferrocene] behave like a monodentate phosphine ligand and displace the labile ligands from [AuCl(SMe2)], trans-[PdCl2(PhCN)2] and cis-[PtCl2(dmso)2](dmso = dimethyl sulfoxide) to yield the corresponding dppf-bridged heteropolymetallic complexes of general formula [M′Clx{(µ-dppf)M(CO)5}y](M′= Au, x=y= 1; M′= Pd or Pt, x=y= 2). Only the trans isomers have been isolated for PdII and PtII. Isomerisation of the M′= Pt, M = Cr complex to the cis form, followed by partial elimination of [Cr2(CO)10(µ-dppf)] to form [PtCl2(dppf-P,P′)], after 3 d in CDCl3 was revealed by NMR spectroscopy. The solution characteristics of both geometrical isomers of the representative M′= Pt, M = Cr complex have been established by two-dimensional NMR studies. UV-Photolytic degradation of the M′= Pd or Pt, M complexes generally gave [M(CO)6], [M(CO)4(dppf-P,P′)], [M2(CO)10(µ-dppf)] and [M′Cl2(dppf)]. The molecular structures of trans-[PtCl2{(µ-dppf)W(CO)5}2] and [Mo(CO)5(dppf-P)] have been determined. The former represents a trimetallic pentanuclear aggregate and the latter a metalloligand with a pendant phosphine on a bimetallic complex. Cyclic voltammetry of all the complexes has been examined and generally reveals one chemically reversible phosphinoferrocene-based oxidation, followed by an irreversible oxidation of the complex.

Towards Guest–Zeolite Interactions: An NMR Spectroscopic Approach

Guest(metal)-zeolite interactions in a two component heterogeneous catalyst have been investigated by high-field and high-speed (27)Al MAS NMR, and two-dimensional (27)Al MQ MAS NMR experiments as well as ab initio DFT methods. It was established that strong interactions between guest and zeolite occur in a metal/zeolite system, with the metal anchored to the tetrahedral aluminum framework site through two oxygen bridges. It disturbs the tetrahedral environment of associated aluminum framework, changing AlO(4) geometry from near T(d) to C(2v); this enables us to resolve this species from the undisturbed aluminum framework species in high-field (27)Al MAS NMR and two-dimesional (27)Al MQ MAS NMR experiments.

Cobalt-59 NMR spectroscopy

Publisher Summary This chapter reviews the literature on solid-state cobalt-59 ( 59 Co) nuclear magnetic resonance (NMR) so as to complement the existing excellent reviews on solution-state work. A brief account of the recent progress in chemical shift interpretation and relaxation studies is presented, covering the relevant solution-state studies published in the 1990s, in which particular attention is given to controversial results. The chapter also summarizes the advances in the theoretical calculations of 59 Co NMR parameters. The focus is on what has been done for 59 Co rather than the “technical details” of the treatments. As very little has been reported for spin-spin coupling constants involving 59 Co nucleus, the solid-state data compilation was done only for chemical shifts and quadrupole coupling constants. The future trend will be to combine different strategies of solidstate and/or solution-state NMR to provide a unified description of the systems under investigation. In particular, it is anticipated that theoretical calculation will provide a new dimension in 59 Co NMR spectroscopy, enabling its application to more complicated systems.

Determination of the release of hydrolyzed demethylcantharidin from novel traditional Chinese medicine–platinum compounds with anticancer activity by gas chromatography

The present paper describes the development of a simple, accurate and reproducible gas chromatographic method for the determination of hydrolyzed demethylcantharidin release from a novel series of traditional Chinese medicine (TCM)-platinum compounds possessing potent anticancer and protein phosphatase 2A (PP2A)-inhibition properties. The salient features of the validated assay were a limit of detection (LOD) of 2 microg/mL, a limit of quantitation (LOQ) of 6 microg/mL, an intra- and inter-day precision of less than 11%, and an accuracy of more than 92%. The developed GC-flame ionization detection (FID) method was successfully utilized for the analysis of hydrolyzed demethylcantharidin, the TCM component that is slowly released from the novel compounds over 24 h, leading to PP2A inhibition. Further structural confirmation was achieved by GC-MS. The GC method is suitable for further mechanistic, pharmacokinetic and metabolic studies of the TCM-Pt compounds that might prove to be new anticancer agents with novel mechanisms of cytotoxic action.

A comparative study of the calculation of 59Co NMR shielding constants of hexacoordinated diamagnetic Co(III) complexes using DFT-IGLO and DFT-GIAO methods

A comparison between the DFT-IGLO and the DFT-GIAO shielding calculation methods is carried out using 59Co chemical shifts of hexacoordinated diamagnetic cobalt(III) complexes as probes. The results show that the DFT-GIAO method produces shielding constants in better agreement with the experimental values when the same exchange correlation (XC) functional (BECKE/PERDEW) and basis sets are used. It is also shown that the use of the B3LYP XC functional in DFT-GIAO improves the 59Co shielding calculation results for 13 Co(III) complexes to < 30% deviation to the experimental values for the first time. The calculated chemical shielding range covers 11 000 ppm.

SOS-DFPT-IGLO calculations of 59Co NMR shielding parameters of hexacoordinated diamagnetic Co(III) complexes

The 59Co chemical shieldings of several hexacoordinated Co(III) complexes have been calculated by the sum-over-states density-functional-perturbation-theoretical method using individual gauges localized orbital (SOS-DFPT-IGLO). The Co TZVP basis set employed in the shielding calculation is tested by calculating the molecular properties of CoH and CoO. The results obtained indicate that the SOS-DFT-IGLO method shows promise in the calculation of 59Co shielding tensors.

NMR evidence of the formation of surfactant micelles inside spherical poly(N-isopropylacrylamide) microgels

Abstract In recent years, water-soluble polymers, or hydrogels, with a lower critical solution temperature (LCST) have been intensively studied. One typical example is poly(N4sopropylacrylamide) (PNIPAM) and its gel. The influence of both the surfactant type and concentration on the volume phase transition of the PNIPAM/water system has been investigated by various methods, such as static and dynamic laser light scattering [l-3 swelling equilibrium [4-6], UV [a], cloud point [6], conductivity [5] and electrophoretic mobility [2], to name but a few. These studies show that the PNIPAM chain or gel network can swell more in the presence of anionic surfactants, and that its volume phase transition temperature increases. The binding of the hydrophobic tail to the PNIPAM chain or gel is believed to be responsible for the additional swelling, but this cannot satisfactorily explain why cationic surfactants with a similar hydrophobic tail have much less effect on swelling whereas nonionic surfactants cause no obse...

NTDB: Thermodynamic Database for Nucleic Acids

A new thermodynamic database for normal and modified nucleic acids has been developed. This Thermodynamic Database for Nucleic Acids (NTDB) includes sequence, structure and thermodynamic information as well as experimental methods and conditions. In this release, there are 1851 sequences containing both normal and modified nucleic acids. A user-friendly web-based interface has been developed to allow data searching under different conditions. Useful thermodynamic tools for the study of nucleic acids have been collected and linked for easy usage. NTDB is available at http://ntdb.chem.cuhk.edu.hk.