Large structural modulations in incommensurate Te-III and Se-IV

The high-pressure phase of tellurium, Te-III, is found to have an incommensurate monoclinic structure, superspace group I(')2/m(0q0)s0, of a type previously unknown in the elements. Te-III is stable from 4.5(2) to 29.2(7) GPa; the previously reported transition to a distinct Te-IV phase at 10.6 GPa is not observed. The incommensurate wave vector of Te-III is strongly pressure dependent and varies in a strongly nonlinear way. Se-IV is found to be isostructural with Te-III.     

Continuous-wave and passively Q-switched cladding-pumped planar waveguide lasers

Greater than 12 W of average output power has been generated from a diode-pumped Yb:YAG cladding-pumped planar waveguide laser. The laser radiation developed is linearly polarized and diffraction limited in the guiding dimension. A slope efficiency of 0.5 W/W with a peak optical-optical conversion efficiency of 0.31 W/W is achieved. In a related structure, greater than 8 W of Q -switched average output power has been generated from a Nd:YAG cladding-pumped planar waveguide laser by incorporation of a Cr(4+): YAG passive Q switch monolithically into the waveguide structure. Pulse widths of 3 ns and pulse-repetition frequencies as high as 80 kHz have been demonstrated. A slope efficiency of 0.28 W/W with a peak optical-optical conversion efficiency of 0.21 W/W is achieved.     

Is the multilayer segregation of tin to the iron (100) surface an equilibrium phenomenon?

Iron crystals doped with tin are known to exhibit segregation of more than one monolayer of tin to the (100) surface when heated in ultra-high vacuum. This unusually large coverage raises the question of whether the multilayer enrichment is due to equilibrium segregation or to some kind of metastable phenomenon. In the latter case, the presence of the bulk equilibrium second phase, FeSn, would be expected to cause a reduction in the surrounding tin coverage. This issue was addressed by the use of chemical-vapor deposition to seed the surface of a tin-doped crystal with this intermetallic compound. It was found that the presence of this second phase did not alter the equilibrium surface coverage, which returned quickly to its previous level by surface diffusion from the FeSn crystallites after surface cleaning by sputtering. The exact structure of this unique surface phase remains to be clarified.     

Molecular structure of [(tBu)2Al(μ-Cl)]2

The crystal and molecular structure of [(^tBu)₂Al(μ-Cl)]₂ has been determined. The bond lengths and geometry about aluminum are similar to that in [Me₂Al(μ-Cl)]₂ and [(Mes)₂Al(μ-Cl)]₂ suggesting that the geometry about the aluminum is defined by the steric repulsion between the alkyl groups on adjacent aluminum centers and not steric congestion between ligands on individual aluminum centers. Crystal data: Triclinic, $P\bar 1$ ,a=6.514(1),b=8.628(2),c-11.236(2)c=11.236(2) Å, α=102.05(3), β=105.32(3), γ=103.47(3)°,V=567.2(3) ų,Z=1,R=0.063,R _w =0.067.     

Fingerprint vibrational spectra of protonated methyl esters of amino acids in the gas phase

Infrared spectra of the protonated monomers of glycine, alanine, valine, and leucine methyl esters are presented. These protonated species are generated in the gas phase via matrix assisted laser desorption ionization (MALDI) within the cell of a Fourier transform ion cyclotron resonance spectrometer (FTICR) where they are subsequently mass selected as the only species trapped in the FTICR cell. Alternatively, they have also been generated by electrospray ionization and transferred to a Paul ion-trap mass spectrometer where they are similarly isolated. In both cases IR spectra are then derived from the frequency dependence of the infrared multiple photon dissociation (IRMPD) in the mid-infrared region (1000-2200 cm(-1)), using the free electron laser facility Centre de Laser Infrarouge d'Orsay (CLIO). IR bands are assigned by comparison with the calculated vibrational spectra of the lowest energy isomers using density functional theory (DFT) calculations. There is in general good agreement between experimental IRMPD spectra and calculated IR absorption spectra for the lowest energy conformer which provides evidence for conformational preferences. The two different approaches to ion generation and trapping yield IRMPD spectra that are in excellent agreement.     

Investigation of proton transport tautomerism in clusters of protonated nucleic acid bases (cytosine, uracil, thymine, and adenine) and ammonia by high-pressure mass spectrometry and ab initio calculations

The energetics of the ion-molecule interactions and structures of the clusters formed between protonated nucleic acid bases (cytosine, uracil, thymine, and adenine) and ammonia have been studied by pulsed ionization high-pressure mass spectrometry (HPMS) and ab initio calculations. For protonated cytosine, uracil, thymine, and adenine with ammonia, the measured enthalpies of association with ammonia are -21.7, -27.9, -22.1, and -17.5 kcal mol-1, respectively. Different isomers of the neutral and protonated nucleic acid bases as well as their clusters with ammonia have been investigated at the B3LYP/6-31+G(d,p) level of theory, and the corresponding binding energetics have also been obtained. The potential energy surfaces for proton transfer and interconversion of the clusters of protonated thymine and uracil with ammonia have been constructed. For cytosine, the experimental binding energy is in agreement with the computed binding energy for the most stable isomer, CN01-01, which is derived from the enol form of protonated cytosine, CH01, and ammonia. Although adenine has a proton affinity similar to that of cytosine, the binding energy of protonated adenine to ammonia is much lower than that for protonated cytosine. This is shown to be due to the differing types of hydrogen bonds being formed. Similarly, although uracil and thymine have similar structures and proton affinities, the binding energies between the protonated species and ammonia are different. Strikingly, the addition of a single methyl group, in going from uracil to thymine, results in a significant structural change for the most stable isomers, UN01-01 and TN03-01, respectively. This then leads to the difference in their measured binding energies with ammonia. Because thymine is found only in DNA while uracil is found in RNA, this provides some potential insight into the difference between uracil and thymine, especially their interactions with other molecules.     

Organic glass-forming materials: 1,3,5-tris(naphthyl)benzene derivatives

The organic glass-forming materials 1,3-bis(1-naphthyl)-5-(2-naphthyl)benzene (2) and its partially deuterated analogue, 1,3-bis(1-naphthyl-d(7))-5-(2-naphthyl)benzene (2-d(14)), have been synthesized on a gram scale using Suzuki coupling reactions. Detailed spectroscopic studies afford complete NMR assignments (1H, 2H, 13C) for both compounds. Modest energy barriers for the interconversion of atropisomers (ca. 15 kcal/mol) result in a propensity for these materials to form supercooled liquids and glasses, rather than undergoing crystallization. The preparation of these materials enables detailed studies of physical properties of organic glasses and molecular diffusion in condensed phases.     

Crystal structures of dense lithium: a metal-semiconductor-metal transition

Ab initio random structure searching and single-crystal x-ray diffraction have been used to determine the full structures of three phases of lithium, recently discovered at low temperature above 60 GPa. A structure with C2mb symmetry, calculated to be a poor metal, is proposed for the oC88 phase (60-65 GPa). The oC40 phase (65-95 GPa) is found to have a lowest-enthalpy structure with C2cb symmetry, in excellent agreement with the x-ray data. It is calculated to be a semiconductor with a band gap of ～1 eV at 90 GPa. oC24, stable above 95 GPa, has the space group Cmca, and refined atomic coordinates are in excellent agreement with previous calculations.     

On the Electride Nature of Na-hP4

Early quantum mechanical models suggested that pressure drives solids towards free-electron metal behavior where the ions are locked into simple close-packed structures. The prediction and subsequent discovery of high-pressure electrides (HPEs), compounds assuming open structures where the valence electrons are localized in interstitial voids, required a paradigm shift. Our quantum chemical calculations on the iconic insulating Na-hP4 HPE show that increasing density causes a 3s→3pd electronic transition due to Pauli repulsion between the 1s2s and 3s states, and orthogonality of the 3pd states to the core. The large lobes of the resulting Na-pd hybrid orbitals point towards the center of an 11-membered penta-capped trigonal prism and overlap constructively, forming multicentered bonds, which are responsible for the emergence of the interstitial charge localization in Na-hP4. These multicentered bonds facilitate the increased density of this phase, which is key for its stabilization under pressure.