Slater ice rules and H-bond dynamics in KDP-type systems

We report on the first observation of the dynamics of correlated hydrogen switching among the six Slater D2PO4 configurations, induced by unpaired D3PO4 and DPO4 Takagi group diffusion in a deuteron glass Rb0.5(ND4)0.5D2PO4. The results obtained by two-dimensional (2D) 31P exchange NMR prove the validity of the Slater-Takagi ice rules and allow for the direct determination of the correlation time for the unpaired Takagi group visits to a given D2PO4 group.     

Morphology and internal structure of barium sulfate--derivation of a new growth mechanism

Particle formation is the decisive step to control crystal morphology. Besides the classical primary processes, nucleation and molecular growth, the particle size can also increase by aggregation. The special case of self-assembled aggregation leads to the formation of highly ordered particles which often possess a porous internal structure. In the experiments of these studies the particle formation of barium sulfate has been investigated. SEM analysis shows a large variety of growth forms including plate-like, star-like, and spherical particles, whereas TEM exposures reveal the porous internal structure at all investigated supersaturation levels. The pore size and the volume fraction can be influenced by changing the supersaturation ratio. By means of a fast sampling technique in combination with cryo-TEM analysis it has been shown that the particles at the early stages of growth shortly after the beginning of nucleation consist of many small nanocrystallites which have aggregated in a highly ordered manner. The diffraction pattern indicates many small-angle grain boundaries, whereas the particles at the end of the precipitation process are monocrystalline. This leads to the conclusion that barium sulfate grows according to a self-assembled aggregation mechanism followed by a fast recrystallization process.     

Tacticity effects on the barriers to rotation of the ester methyl group in poly (methyl methacrylate): a deuteron magnetic resonance study

In isotactic poly(methyl methacrylate) (PMMA), we investigate the dynamics of the ester methyl groups by means of deuteron magnetic resonance (DMR) in a deuterated sample. We find that the motion of the CD(3)-group affects the deuteron spin-lattice relaxation as well as the DMR line shape in a characteristic way. Quadrupolar order spin lattice relaxation measurements between T=291 K and T=70 K reveal a broad temperature dependent probability distribution of autocorrelation times tau(c) for the 2pi/3 reorientation. This broad distribution corresponds to a temperature independent Gaussian distribution of activation energies rho(E(a)) with variance sigma(E(a) )=13.8+/-0.5 meV (1.33 kJ/mol). The line shape transition between T=70 K and T=23 K is explained with the freezing in of the methyl group reorientation. By comparing our results in an 88% isotactic sample with results obtained from a 50% syndiotactic, 30% atactic, and 20% isotactic sample of a previous investigation, we demonstrate the higher local order of the 88% isotactic sample, which corresponds to a ratio of 1.6 in the relative width sigma(E(a) )/E(a) of the E(a) distribution. We show that different stereospecific forms of PMMA can be easily distinguished by the characteristics of their line shape transition between T=70 K and T=23 K.     

Electrospray collision-induced dissociation of testosterone and testosterone hydroxy analogs

Complications with the gas chromatographic analysis of steroids prompted the use of alternative techniques for their identification. High-performance liquid chromatography/mass spectrometry with atmospheric pressure ionization allowed the collection of data for structural identification of these compounds. The objective of this study was to investigate the up-front collision-induced dissociation (UFCID) electrospray ionization (ESI) mass spectra of testosterone and monohydroxylated testosterones. The positive ion UFCID ESI mass spectrum of testosterone showed three significant ions at m/z 97, 109 and 123. The relative abundance of these ions in the UFCID ESI mass spectra of monohydroxylated testosterones varied with the position of the hydroxy group. Statistical data allowed the prediction of hydroxy group position on testosterone by evaluation of the relative abundance of the m/z 97, 109, 121 and 123 ions. Data from the ESI mass spectral analysis of testosterone in a deuterated solvent and from the analysis of cholestenone and 4-androstene-3 beta, 17 beta-diol indicated that the initial ionization of testosterone occurred at the 3-one position. CID parent ion monitoring analyses of the m/z 97, 109 and 123 ions indicated that each resulted from different fragmentation mechanisms and originated directly from the [M + H]+ parent ion. The elemental composition of these fragment ions is proposed based on evidence gathered from the CID analysis of the pseudo-molecular ions of [1,2-2H2]-, [2,2,4,6,6-2H5]-, [6,7-2H2]-, [7-2H]-, [19,19,19-2H3]- and [3,4-13C2]testosterone. The structure and a possible mechanism of formation of the m/z 109 and 123 ions is presented. The results of this study advance the understanding of the mechanisms of collision-induced fragmentation of ions.     

Quantum chemical calculation of vibrational spectra of large molecules--Raman and IR spectra for Buckminsterfullerene

In this work we demonstrate how different modern quantum chemical methods can be efficiently combined and applied for the calculation of the vibrational modes and spectra of large molecules. We are aiming at harmonic force fields, and infrared as well as Raman intensities within the double harmonic approximation, because consideration of higher order terms is only feasible for small molecules. In particular, density functional methods have evolved to a powerful quantum chemical tool for the determination of the electronic structure of molecules in the last decade. Underlying theoretical concepts for the calculation of intensities are reviewed, emphasizing necessary approximations and formal aspects of the introduced quantities, which are often not explicated in detail in elementary treatments of this topic. It is shown how complex quantum chemistry program packages can be interfaced to new programs in order to calculate IR and Raman spectra. The advantages of numerical differentiation of analytical gradients, dipole moments, and static, as well as dynamic polarizabilities, are pointed out. We carefully investigate the influence of the basis set size on polarizabilities and their spatial derivatives. This leads us to the construction of a hybrid basis set, which is equally well suited for the calculation of vibrational frequencies and Raman intensities. The efficiency is demonstrated for the highly symmetric C(60), for which we present the first all-electron density functional calculation of its Raman spectrum.     

Cyclopalladation of 1,1′-Azonaphthalene (= Di(naphthalen-1-yl)diazene): Isolation and characterization of two isomeric complexes

The cyclopalladation of 1,1′-azonaphthalene (= di(naphthalen-1-yl)diazene; 2 ) with bis(hexafluoroacetyl-acetonato)palladium(II) (3; [Pd(hfa)₂]) yields the ortho-palladated complex (1,1,1,5,5,5-hexafluoropentane-2,4-dionato-κ²O,O′)[1-(naphthalen-1-ylazo-κN²)naphthalen-2-yl-κC²]palladium(II) ( 4 ) as well as the peri-palladated complex (1,1,1,5,5,5-hexafluoropentane-2,4-dionato-κ²O,O′)[8-(naphthalen-1-ylazo-κN²)naphthalen-1-yl-κC¹]-palladium(II) ( 5 ); their structures were corroborated by X-ray analyses. The formation of the novel peri-metallated product 5 containing a six-membered palladacycle strongly depends upon the reaction conditions.     

Dynamic double lattice of 1-adamantaneselenolate self-assembled monolayers on Au{111}

We report a complex, dynamic double lattice for 1-adamantaneselenolate monolayers on Au{111}. Two lattices coexist, revealing two different binding modes for selenols on gold: molecules at bridge sites have lower conductance than molecules at three-fold hollow sites. The monolayer is dynamic, with molecules switching reversibly between the two site-dependent conductance states. Monolayer dynamics enable adsorbed molecules to reorganize according to the underlying gold electronic structure over long distances, which facilitates emergence of the self-organized rows of dimers. The low-conductance molecules assume a (7 × 7) all-bridge configuration, similar to the analogous 1-adamantanethiolate monolayers on Au{111}. The high-conductance molecules self-organize upon mild annealing into distinctive rows of dimers with long-range order, described by a (6√5 × 6√5)R15° unit cell.