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.     

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.     

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.     

A gas-phase anionic analog of the wittig reaction. An ion cyclotron resonance study of the gas-phase ion chemistry of silyl carbanions

Gas-phase ion–molecule reactions of a variety of fluorosilyl carbanions with compounds containing double bonds to oxygen, X?O, have been examined using pulsed ion cyclotron resonance spectroscopy. The predominant reaction channel observed for species not containing acidic hydrogens is a Wittig-like process involving Si? O bond formation and elimination of X?CH₂ species. The gas-phase acidity of F₃Si(CH₃) has been determined and those of F₂Si(CH₃)₂ and FSi(CH₃)₃ have been estimated. From the fluoride transfer reactions of F₃SiCH₂^? the fluoride affinity of F₂Si?CH₂ has been estimated and limits on the π bond strength in this silaethene obtained. Potential analytical applications of the Wittig reactivity have been discussed.     

Effects of ethynyl substitution on cyclobutadiene

The effects of ethynyl substitution on cyclobutadiene are explored via density functional theory and coupled-cluster calculations. The computed singlet-triplet gaps indicate a monotonic dependence on the degree of ethynyl substitution, which differentially stabilizes the triplet relative to the singlet ground state and reduces the gap. A series of isodesmic, homodesmotic, and hyperhomodesmotic equations are employed to quantify the stabilization upon ethynyl substitution. Analyses that rely on a simple isodesmic equation and/or B3LYP/6-31G(d) values are found to be problematic. Analyses that rely on homodesmotic or hyperhomodesmotic equations, in conjunction with CCSD/cc-pVDZ values, are more robust. Using a hyperhomodesmotic equation to assess the stabilization enthalpies of tetra-substituted singlet cyclobutadienes, our analysis predicts tetramethylcyclobutadiene (ΔH(0)(rxn) = -17.3 kcal/mol) to be more stable than tetraethynylcyclobutadiene (ΔH(0)(rxn) = -11.7 kcal/mol), which, in turn, is substantially more stable than tetracyanocyclobutadiene (ΔH(0)(rxn) = +12.7 kcal/mol).