Molecular structure and vibrational spectra of spin-crossover complexes in solution and colloidal media: resonance Raman and time-resolved resonance Raman studies

The spin-crossover system [Fe(btpa)](PF(6))(2) (btpa = N,N,N',N'-tetrakis(2-pyridylmethyl)-6,6'-bis(aminomethyl)-2,2'-bipyridine) and the predominantly low-spin species [Fe(b(bdpa))](PF(6))(2) ((b(bdpa) = N,N'-bis(benzyl)-N,N'-bis(2-pyridylmethyl)-6,6'-bis(aminomethyl)-2,2'-bipyridine) have been characterized by means of X-ray diffraction. The unit cell of [Fe(btpa)](PF(6))(2) contains two crystallographically independent molecules revealing octahedral low-spin and quasi-seven-coordinated high-spin structures. The unit cell of [Fe(b(bdpa))](PF(6))(2) contains two crystallographically independent molecules one of which corresponds to a low-spin structure, while the other reveals a disordering. On the basis of magnetic susceptibility and M?ssbauer measurements, it has been proposed that this disorder involves low-spin and high-spin six-coordinated molecules. The structures of [Zn(btpa)](PF(6))(2) and [Ru(btpa)](PF(6))(2) have been determined also. Pulsed laser photoperturbation, coupled here with time-resolved resonance Raman spectroscopy (TR(3)), has been used to investigate, for the first time by this technique, the relaxation dynamics in solution on nanosecond and picosecond time scales of low-spin, LS ((1)A) --> high-spin, HS ((5)T) electronic spin-state crossover in these Fe(II) complexes. For the nanosecond experiments, use of a probe wavelength at 321 nm, falling within the pi-pi transition of the polypyridyl backbone of the ligands, enabled the investigation of vibrational modes of both LS and HS isomers, through coupling to spin-state-dependent angle changes of the backbone. Supplementary investigations of the spin-crossover (SCO) equilibrium in homogeneous solution and in colloidal media assisted the assignment of prominent features in the Raman spectra of the LS and HS isomers. The relaxation data from the nanosecond studies confirm and extend earlier spectrophotometric findings, (Schenker, S.; Stein, P. C.; Wolny, J. A.; Brady, C.; McGarvey, J. J.; Toftlund, H.; Hauser, A. Inorg. Chem. 2001, 40, 134), pointing to biphasic spin-state relaxation in the case of [Fe(btpa)](PF(6))(2) but monophasic in the case of [Fe(b(bdpa))](PF(6))(2). The picosecond results suggest an early process complete in 20 ps or less, which is common to both complexes and possibly includes vibrational relaxation in the initially formed (5)T(2) state.     

A study of anisotropic water self-diffusion and defects in the lamellar mesophase

The correlation of molecular diffusion coefficients obtained via a novel two-dimensional pulsed gradient spin-echo (PGSE) NMR method has been shown to reveal detailed structural information on the mesophases of lyotropic liquid crystals. A four-component system containing both nonionic (pentaethylene glycol monododecyl ether) and ionic (sodium dodecyl sulfate) surfactants, water, and decane was prepared and left to equilibrate. In the temperature region around 309 K, a lamellar mesophase forms. A two-dimensional Laplace inverse transformation was performed on the (gammadeltag)2(delta - delta/3) domain data to separate any multiexponential behavior that resulted from local anisotropy. The results of the double PGSE experiment with contiguous gradient pulse pairs, applied both collinearly and orthogonally, clearly show the presence of local anisotropic self-diffusion of the water molecules and suggest a preferred orientation of the lamellae. Information about defects/domain size was obtained by the insertion of a mixing time (t(m)') between the successive gradient pulse pairs. This work highlights the value of this new NMR correlation method in the study of surfactant systems.     

The reaction of M(CO)3(Ph2PCH2CH2PPh2)(M = Fe, Ru) with parahydrogen: probing the electronic structure of reaction intermediates and the internal rearrangement mechanism for the dihydride products

The photochemical reaction of Ru(CO)(3)(dppe) and Fe(CO)(3)(dppe)(dppe = Ph(2)PCH(2)CH(2)PPh(2)) with parahydrogen has been studied by in situ-photochemistry resulting in NMR spectra of Ru(CO)(2)(dppe)(H)(2) that show significant enhancement of the hydride resonances while normal signals are seen in Fe(CO)(2)(dppe)(H)(2). This effect is associated with a singlet electronic state for the key intermediate Ru(CO)(2)(dppe) while Fe(CO)(2)(dppe) is a triplet. DFT calculations reveal electronic ground states consistent with this picture. The fluxionality of Ru(CO)(2)(dppe)(H)(2) and Fe(CO)(2)(dppe)(H)(2) has been examined by NMR spectroscopy and rationalised by theoretical methods which show that two pathways for ligand exchange exist. In the first, the phosphorus and carbonyl centres interchange positions while the two hydride ligands are unaffected. A second pathway involving interchange of all three ligand sets was found at slightly higher energy. The H-H distances in the transition states are consistent with metal-bonded dihydrogen ligands. However, no local minimum (intermediate) was found along the rearrangement pathways.     

Direct determination of cadmium in natural waters by electrothermal atomic absorption spectrometry without matrix modification

A procedure for the determination of cadmium in fresh, coastal and estuarine waters by polarized Zeeman-effect graphite-furnace atomic absorption spectrometry is validated by using lake waters and seawater. The limit of detection for freshwaters is <2 ng l^?1 cadmium. Undiluted seawater can be analyzed directly without the addition of matrix modifiers with the aid of a stabilized temperature platform. The instrument is calibrated with diluted NBS SRM 1643a (Trace Elements in Water). Analytical performance was tested extensively with fresh and brackish water samples and procedures were worked out to ensure that a high degree of accurately is achieved consistently.     

Electrokinetic potentials at the gas-aqueous interface by spinning cylinder electrophoresis

The natural buoyancy of gas bubbles has hampered conventional electrokinetic methods of evaluating charge at the gas-liquid interface. In this study, bubbles are held by centripetal force at the centre of a horizontal, rotating cylinder filled completely with liquid. Migration of the bubble along the axis of the cylinder can be achieved by applying a gravity force or an electrophoretic force. An expression for the drag coefficient has been developed and used to calculate surface charge densities and electrokinetic potentials. In the presence of potassium chloride, anionic and non-ionic surfactants, the electrokinetic potentials were negative (～2–5 mV) at the air-aqueous interface. A cationic surfactant rendered the zeta potential positive. A knowledge of the air-aqueous interfacial rheology is desirable if the drag coefficient, and consequently the zeta potential, at all types of gas-liquid interfaces is to be evaluated.     

Nuclear orientation of82Br implanted into Fe

Nuclear orientation measurements are used to determine the hyperfine hamiltonian for⁸²Br implanted into Fe single crystals (dose 5×10¹⁴/cm^–2 implant energy 80 keV). Using a model based upon channeling measurements a good fit to the temperature-dependent gamma anisotropy is obtained for a pure magnetic interaction experienced by the 36±5% of Br which implants substitutionally of magnitudeB _hf (substitutional)=840±120 kG, withB _hf (non-substitutional)<150 kG. This hamiltonian is used to deduce unknown multipole mixing ratios in the daughter⁸²Kr decay. Hyperfine field systematics are shown to indicate a substitutionalFeBr field of 1000 kG, and the origins of this field and the smaller interstitial interaction are discussed.     

The role of etched silicon channels on the pore ordering of mesoporous silica: The importance of film thickness on providing highly orientated and defect-free thin films

If mesoporous thin films (MTFs) are to be utilised in device applications it is important that we produce films which not only possess a single pore direction across large substrate areas (in the range of microns) but are also relatively defect free. In this paper we report the use of confining architectures in the form of topographically patterned rectangular section channels etched into native silicon substrates to promote ordering of the mesopores. We discuss the effects of the channels on films with different thicknesses. The film thickness is shown to be a critical parameter in defining highly orientated and defect-free films and the data demonstrate that it is possible to achieve a single mesoporous silica domain across macroscopic dimensions with thin film thicknesses of approximately 200 nm but that critically pore order can be lost in ultra thin and thicker films produced by these methods.     

Aging, yielding, and shear banding in soft colloidal glasses

Soft colloidal interactions in colloidal glasses are modeled using suspensions of multiarm star polymers. Using a preshearing protocol that ensures a reproducible initial state ("rejuvenation" of the system), we report here the evolution of the flow curve from monotonically increasing to one dominated by a stress plateau, demonstrating a corresponding shear-banded state. Phenomenological understanding is provided through a scalar model that describes the free-energy landscape.     

NMR characterisation of unstable solvent and dihydride complexes generated at low temperature by in-situ UV irradiation

Low temperature in-situ UV irradiation of toluene solutions containing bis(alkene)rhodium complexes and parahydrogen in conjunction with NMR monitoring enables the characterisation of unstable eta 2-solvent complexes and dihydrogen activation products.     

Nanoscale NMR velocimetry by means of slowly diffusing tracer particles

The resolution of NMR velocimetry is inherently limited by random displacements due to molecular self-diffusion, and has so far not extended below a few tens of microns. We report here an extension to the nanoscale domain, a result achieved by the use of slowly diffusing, NMR-visible core-shell latex particles. These particles comprise an oil core surrounded by a solid polymer shell, making spheres of diameter 370 nm. Using these particles in the annulus of a concentric cylinder Couette cell, we have measured flow-induced displacements down to a few hundreds of nanometers, allowing the observation of the solid-to-liquid transition of a glassy system. We envisage new possibilities for NMR velocimetry as an experimental tool for colloidal chemistry and physics.