Assignment of the solid state spectra of the group VI hexacarbonyls by inelastic neutron scattering spectroscopy.

The solid state vibrational spectra of M(CO)6, (M = Cr, Mo, W) in the region below 800 cm-1 have been assigned by a combination of infrared, Raman and the first reported inelastic neutron scattering (INS) spectra from homoleptic metal carbonyls. This region comprises of the lattice modes, the OC-M-CO deformations, the M-C[triple bond, length as m-dash]O bends and the M-C stretches. Three modes that are forbidden in both the infrared and Raman spectra of the parent Oh symmetry gas phase molecule occur in this region. The absence of selection rules for INS spectroscopy means that all three modes are clearly seen for the first time, all previous work has relied on overtone and combination modes. Periodic density functional theory calculations of the complete orthorhombic structure support the assignments.

Muonium Chemistry at Diiron Subsite Analogues of [FeFe]-Hydrogenase.

The chemistry of metal hydrides is implicated in a range of catalytic processes at metal centers. Gaining insight into the formation of such sites by protonation and/or electronation is therefore of significant value in fully exploiting the potential of such systems. Here, we show that the muonium radical (Mu. ), used as a low isotopic mass analogue of hydrogen, can be exploited to probe the early stages of hydride formation at metal centers. Mu. undergoes the same chemical reactions as H. and can be directly observed due to its short lifetime (in the microseconds) and unique breakdown signature. By implanting Mu. into three models of the [FeFe]-hydrogenase active site we have been able to detect key muoniated intermediates of direct relevance to the hydride chemistry of these systems.

Achirality in the low temperature structure and lattice modes of tris(acetylacetonate)iron(iii).

Tris(acetylacteonate) iron(iii) is a relatively ubiquitous mononuclear inorganic coordination complex. The bidentate nature of the three acetylacteonate ligands coordinating around a single centre inevitably leads to structural isomeric forms, however whether or not this relates to chirality in the solid state has been questioned in the literature. Variable temperature neutron diffraction data down to T = 3 K, highlights the dynamic nature of the ligand environment, including the motions of the hydrogen atoms. The Fourier transform of the molecular dynamics simulation based on the experimentally determined structure was shown to closely reproduce the low temperature vibrational density of states obtained using inelastic neutron scattering.

Methyl tunnelling of adsorbed methoxy on alumina catalysts.

The aim of this paper is to investigate whether the methyl group of the adsorbed methoxy intermediate on industrial grade alumina catalysts undergoes rotational tunnelling on the wavenumber energy scale. The data show that this is clearly the case for a fraction of the methyl groups and potentially allows the subtle intermolecular interactions between adsorbed species and catalyst to be probed through the exponential dependence of the tunnel frequency on the rotational potential.

Rate of Molecular Transfer of Allyl Alcohol across an AOT Surfactant Layer Using Muon Spin Spectroscopy.

The transfer rate of a probe molecule across the interfacial layer of a water-in-oil (w/o) microemulsion was investigated using a combination of transverse field muon spin rotation (TF-µSR), avoided level crossing muon spin resonance (ALC-µSR), and Monte Carlo simulations. Reverse microemulsions consist of nanometer-sized water droplets dispersed in an apolar solvent separated by a surfactant monolayer. Although the thermodynamic, static model of these systems has been well described, our understanding of their dynamics is currently incomplete. For example, what is the rate of solute transfer between the aqueous and apolar solvents, and how this is influenced by the structure of the interface? With an appropriate choice of system and probe molecule, µSR offers a unique opportunity to directly probe these interfacial transfer dynamics. Here, we have employed a well characterized w/o microemulsion stabilized by bis(2-ethylhexyl) sodium sulfosuccinate (Aerosol OT), with allyl alcohol (CH2═CH-CH2-OH, AA) as the probe. Resonances due to both muoniated radicals, CMuH2-C*H-CH2-OH and C*H2-CHMu-CH2-OH, were observed with the former being the dominant species. All resonances displayed solvent dependence, with those in the microemulsion observed as a single resonance located at intermediate magnetic fields to those present in either of the pure solvents. Observation of a single resonance is strong evidence for interfacial transfer being in the fast exchange limit. Monte Carlo calculations of the ΔM = 0 ALC resonances are consistent with the experimental data, indicating exchange rates greater than 10(9) s(-1), placing the rate of interfacial transfer at the diffusion limit.

Muoniated spin probes in the discotic liquid crystal HHTT: rapid electron spin relaxation in the hexagonal columnar and isotropic phases.

Avoided level crossing muon spin resonance (ALC-µSR) spectroscopy was used to study radicals produced by the addition of the light hydrogen isotope muonium (Mu) to the discotic liquid crystal (DLC) 2,3,6,7,10,11-hexahexylthiotriphenylene (HHTT). Mu adds to the secondary carbon atoms of HHTT to produce a substituted cyclohexadienyl radical, whose identity was confirmed by comparing the measured hyperfine coupling constants with values obtained from DFT calculations. ALC-µSR spectra were obtained in the isotropic (I), hexagonal columnar (Col(h)), helical (H), and crystalline (Cr) phases. In the I and Col(h) phases the radicals, which are incorporated within the stacks of HHTT molecules as isolated paramagnetic defects, undergo extremely rapid electron spin relaxation, on the order of a hundredfold faster than in the H or Cr phases. The electron spin relaxation rate increases significantly with increasing temperature and appears to be caused by the liquidlike motion within the columns, which modulates the overlap between the π system of the radical and the π systems of the neighboring HHTT molecules, and hence, the hyperfine coupling constants. Rapid electron spin relaxation should occur for any π radical incorporated within the columns of a DLC, which may limit the utility of DLCs for future spin-based technologies.

Uptake and toxicity studies of poly-acrylic acid functionalized silicon nanoparticles in cultured mammalian cells.

Poly-acrylic acid (PAAc) terminated silicon nanoparticles (SiNPs) have been synthesized and employed as a synchronous fluorescent signal indicator in a series of cultured mammalian cells: HHL5, HepG2 and 3T3-L1. Their biological effects on cell growth and proliferation in both human and mouse cell lines have been studied. There was no evidence of in vitro cytotoxity in the cells exposed to PAAc terminated SiNPS when assessed by cell morphology, cell proliferation and viability, and DNA damage assays. The uptake of the nanocrystals by both HepG2 and 3T3-L1 cells was investigated by confocal microscopy and flow cytometry, which showed a clear time-dependence at higher concentrations. Reconstructed 3-D confocal microscope images exhibited that the PAAc-SiNPs were evenly distributed throughout the cytosol rather than attached to outer membrane. This study provides fundamental evidence for the safe application and further modification of silicon nanoparticles, which could broaden their application as cell markers in living systems and in micelle encapsulated drug delivery systems.

Muon spin spectroscopy of the discotic liquid crystal HAT6.

Avoided level crossing muon spin resonance (ALC-muSR) has been used to study the cyclohexadienyl-type radicals produced by the addition of muonium (Mu) to the discotic liquid crystal HAT6 (2,3,6,7,10,11-hexahexyloxytriphenylene) in the crystalline (Cr) phase, the hexagonal columnar mesophase (Col(h)) and isotropic (I) phase. In the Cr phase unpaired electron spin density can be transferred from the radical to neighboring HAT6 molecules depending on the overlap of their pi-systems and hence on the relative orientation of the triphenylene rings. The two Delta(1) resonances in the ALC-muSR spectra of the Cr phase indicate that the neighboring HAT6 molecules have two preferred orientations with respect to the radical: one which results in negligible spin density transfer and a second where 17% of the unpaired spin density is transferred. The ALC-muSR spectra in Col(h) and I phases are substantially different from those of the Cr phase in that there are two narrow resonances superimposed on an extremely broad and intense resonance. The narrow resonances are due to highly mobile radicals located in the aliphatic region between the columns and the broad resonance is due to radicals incorporated within the columns of HAT6 molecules. The large width and amplitude of this resonance indicates that the radicals within the columns are undergoing rapid electron spin relaxation but the mechanism that causes this relaxation is unknown.

Molecular structure, vinyl rotation barrier, and vibrational dynamics of 2,6-dichlorostyrene. A theoretical and experimental research.

The molecular structure of 2,6-dichlorostyrene has been analyzed at MP2 and DFT levels using different basis sets concluding in a nonplanar geometry. The influence of either the level of theory or the nature of the substituent has been assessed. The vinyl-phenyl torsion barrier has also been investigated as a function of level of theory. The ultimate factors responsible for the torsion barrier have been studied using two different partitioning schemes, i.e., the total electronic potential energy and the natural bond orbital, NBO. A topological analysis of the electron density within the atom-in-molecule, AIM, theory predicts soft intramolecular chlorine (ring)-hydrogen (vinyl) contacts when the system becomes planar. A first complete vibrational study has been performed using theoretical data and experimental vibrational frequencies from IR, Raman and, for the first time, inelastic neutron scattering, INS, spectra. The new assignment proposed is based on a scaled quantum mechanical, SQM, force field and the wavenumber linear scaling, WLS, approach.

Vibrational spectroscopic analysis of chlorosilanes and siloxane oligomers: implications for the spectra of polydimethylsiloxanes.

Inelastic neutron scattering spectra of the polydimethylsiloxane polymer models, methyltrichlorosilane (MTCS), dimethyldichlorosilane (DMDCS), trimethylchlorosilane (TMCS), hexamethyldisiloxane (HMDS), and octamethyltrisiloxane (OMTS), are reported and assigned by comparison with ab initio calculations. Also reported are the spectra of a series of polydimethylsiloxane samples with viscosities in the range 5-100 cSt, based on the model compounds and calculated spectra of a short polydimethylsiloxane, and the spectrum of polydimethylsiloxane is reassigned. Also reported are the infrared (IR) and Raman spectra of TMCS and OMTS.

Muon implantation of metallocenes: ferrocene.

Muon Spin Relaxation and Avoided Level Crossing (ALC) measurements of ferrocene are reported. The main features observed are five high field resonances in the ALC spectrum at about 3.26, 2.44, 2.04, 1.19 and 1.17 T, for the low-temperature phase at 18 K. The high-temperature phase at 295 K shows that only the last feature shifted down to about 0.49 T and a muon spin relaxation peak at about 0.106 T which approaches zero field when reaching the phase transition temperature of 164 K. A model involving three muoniated radicals, two with muonium addition to the cyclopentadienyl ring and the other to the metal atom, is postulated to rationalise these observations. A theoretical treatment involving spin-orbit coupling is found to be required to understand the Fe-Mu adduct, where an interesting interplay between the ferrocene ring dynamics and the spin-orbit coupling of the unpaired electron is shown to be important. The limiting temperature above which the full effect of spin-orbit interaction is observable in the muSR spectra of ferrocene was estimated to be 584 K. Correlation time for the ring rotation dynamics of the Fe-Mu radical at this temperature is 3.2 ps. Estimated electron g values and the changes in zero-field splittings for this temperature range are also reported.

Electronic interaction in an outer-sphere mixed-valence double salt: a polarized neutron diffraction study of K(3)(MnO(4))(2).

The mixed-valence double salt K(3)(MnO(4))(2) crystallizes in space group P2(1)/m with Z = 2. The manganese centers Mn1 and Mn2 constitute discrete "permanganate", [Mn(VII)O(4)](-), and "manganate", [Mn(VI)O(4)](2-), ions, respectively. There is a spin-ordering transition to an antiferromagnetic state at ca. T = 5 K. The spin-density distribution in the paramagnetic phase at T = 10 K has been determined by polarized neutron diffraction, confirming that unpaired spin is largely confined to the nominal manganate ion Mn2. Through use of both Fourier refinement and maximum entropy methods, the spin on Mn1 is estimated as 1.75 +/- 1% of one unpaired electron with an upper limit of 2.5%.

Solid state effects in the IR spectrum of octahydridosilasesquioxane.

Infrared spectrum of Octahydridosilasesquioxane, in the solid state, is reported and analyzed to show the presence of significant solid state effects. This is in marked contrast with the reported Raman spectroscopic studies of this compound in the solid state where no evidence for such a proliferation of these effects were found. It is found that the normal modes with radial motion of the atoms in their normal coordinate are distinguishable from other vibrations by the distinct correlations between the band intensity and the solid state splitting observed in the IR spectrum. This new insight suggests an interchange of the literature assignments of the two modes nu27 and nu29 in the IR spectrum and a different origin to the doublet of bands in the Raman spectrum at 883 and 897 cm(-1).

Nuclear inelastic scattering spectroscopy of iron-sulfur cubane compounds.

The potential of NIS spectroscopy to study the iron-sulfur clusters in metalloproteins is illustrated using model compounds. The origin of the intense low energy transfer bands is discussed.

Vibrational spectroscopic analysis of silicones: a Fourier transform-Raman and inelastic neutron scattering investigation.

An inelastic neutron scattering spectrum of a poly(dimethylsiloxane) (PDMS) is reported, and a spectrum simulated using a monomer molecular unit as a model for comparison. FT-Raman spectra of a series of PDMS derivatives are reported and structure spectra correlations are shown to exist for the estimation of (a) PDMS average chain length, (b) ratio of the number of monofunctional units to quadrifunctional units in silicone resins, and (c) the percentage weight of PDMS in silicone emulsions.

Molecular sensors using a resonance Raman template.

The potential use of resonance Raman spectroscopy as a molecular sensing tool is illustrated using a metalloporphyrin template and pyridine as an analyte. The equilibrium binding constant for the axial binding of pyridine to zinc tetraphenylporphyrin has been measured using resonance Raman spectroscopy. Although no new peaks are observed and the porphyrin peaks do not shift position, the quantification is made possible by the selective resonance enhancement of the template vibrations. The value for log k was determined by resonance Raman to be 3.65 +/- 0.32, which compares well with previously published values estimated using absorption data. Values for log k were determined for a series of related compounds, the picolines, and these also compare favourably with those previously reported.

Analysis of the Symmetry of Crystal Packing Forces by Methyl Proton Tunneling: A Strategy for the Unambiguous Assignment of the Magnetic Jahn - Teller Effect in Molecules.

Intrinsic and extrinsic forces behind the distortion in metal atom clusters can be readily distinguished provided that the clusters are embedded in a suitable ligand environment and that the tunneling of the protons in the peripheral ligands is then analyzed by inelastic neutron scattering. For the [Cr3 O(OOCCH3 )6 (H2 O)3 ]Cl⋅6 H2 O model system studied, the tunneling process is very sensitive to the local environment. Thus a tool is available to allow a better assessment of the cause of structural distortions.