A computational study of the nondissociative mechanisms that interchange apical and equatorial atoms in square pyramidal molecules

The lowest energy transition state for the nondissociative apical/equatorial atom exchange mechanism for three square pyramidal AEX5 molecular species was calculated (CCSD(T)/pVTZ; B3LYP/pVTZ, aug-cc-pV5Z) to have a hemidirected geometry with C(s) symmetry for BrF5, IF5, and XeF5+. In contrast, holodirected C2v-symmetric transition states for this process were located for the AEX5 square pyramidal molecules ClF5, ICl5, and IBr5. Imaginary frequencies were calculated and examined in a visual/dynamic fashion to gain insight into these fluxional processes. Although both mechanisms exchange one apical for one equatorial atom in each cycle of motion, processes that pass through C2v transition states have characteristic features of the well-known Berry pseudorotation and Lever mechanisms while those which pass through transition states of C(s) symmetry have features that are a mixture of Berry, Lever, and turnstile-like character. Two periodic trends are observed: as the atomic number on the central atom increases (same terminal atoms), the barrier for apical/equatorial exchange and the value of the imaginary frequency both decrease. Similarly, as the atomic number of the terminal atoms increase (same central atom), the barrier for apical/equatorial exchange decreases, as does the computed imaginary frequency.     

Structure and mobility of lipid membranes on a thermoplastic substrate

Supported lipid membranes constitute one of the most important model systems for cell membranes. The properties of lipid membranes supported by the hydrophobic solid polymer cyclic olefin copolymer (COC) were investigated. Lipid layers consisting of varying amounts of 1,2-dioleoyl-3-trimethylammonium propane (DOTAP, cationic) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC, neutral) prepared by vesicle fusion and solvent exchange were compared. All lipid mixtures coated the COC surface homogeneously forming a fluid membrane as verified by fluorescence microscopy and fluorescence recovery after photobleaching (FRAP). The exact structure of the supported membranes was determined by synchrotron reflectivity experiments using a microfluidic chamber. The X-ray data are in agreement with a compressed (head-to-head distance = 29 angstroms) and less densely packed bilayer.     

Suzuki-Miyaura, alpha-ketone arylation and dehalogenation reactions catalyzed by a versatile N-heterocyclic carbene-palladacycle complex

[reaction: see text] The activity of the complex (IPr)PdCl(eta2-N,C-C12H7NMe2), 1 [IPr = (N,N'-bis(2,6-diisopropylphenyl)imidazol)-2-ylidene], in the Suzuki-Miyaura cross-coupling reaction involving unactivated aryl chlorides and triflates with arylboronic acids at room temperature in technical grade 2-propanol is described. These conditions allow for the synthesis of di- and tri-ortho-substituted biaryls in very short reaction times. This complex also displays very high activity for alpha-ketone arylation and dehalogenation reactions of activated and unactivated aryl chlorides.     

Two New Retigerane‐Type Sesterterpenoids from the Lichen Leprocaulon microscopicum

Two new sesterterpenes, 1 and 2 , have been isolated from the lichen Leprocaulon microscopicum. In addition to classic chromatographic methods, a liquid‐liquid chromatography technique, namely centrifugal partition chromatography (CPC) was applied for the purification of compound 2 . The structures were determined by analyses of mass spectrometry and 1D‐ and 2D‐NMR data. The relative configuration of the isolated compounds was assigned on the basis of 2D‐NOESY experiments. The two compounds possess a rare pentacyclic carbon skeleton typical for lichen metabolism, and quite unusual in the vegetal kingdom.     

Combined Infrared Multiphoton Dissociation with Ultraviolet Photodissociation for Ubiquitin Characterization

Herein we report the successful implementation of the consecutive and simultaneous photodissociation with high (213 nm) and low (10.6 μm) energy photons (HiLoPD, high-low photodissociation) on ubiquitin in a quadrupole-Orbitrap mass spectrometer. Absorption of high-energy UV photon is dispersed over the whole protein and stimulates extensive C–C_α backbone fragmentation, whereas low-energy IR photon gradually increases the internal energy and thus preferentially dissociates the most labile amide (C–N) bonds. We noticed that simultaneous irradiation of UV and IR lasers on intact ubiquitin in a single MS/MS experiment provides a rich and well-balanced fragmentation array of a/x, b/y, and z ions. Moreover, secondary fragmentation from a/x and z ions leads to the formation of satellite side-chain ions (d, v, and w) and can help to distinguish isomeric residues in a protein. Implementation of high-low photodissociation in a high-resolution mass spectrometer may offer considerable benefits to promote a comprehensive portrait of protein characterization.

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The infrared spectrum of DCCF in the 320–850 cm−1 region: bending states up to υ4+υ5 = 3

Infrared spectra of deuterated monofluoroacetylene, DCCF, have been recorded in the region between 320 and 850 cm^?1 at an effective resolution ranging from 0.0024 to 0.0031 cm^?1. In total, 6650 rotation vibration transitions were assigned to 37 bands involving the bending states with v₄ + v₅ and |l₄+l₅|, respectively, up to 3, allowing the characterisation of the ground state and of 18 vibrationally excited states. The vν₅ bending fundamental has been studied for the first time. In addition, the difference band v₃ ← v₄ has been detected and analysed. All the assigned transitions have been fitted simultaneously by adopting a model Hamiltonian that takes into account the vibration and rotation l?type resonances. Rotational transitions in the ground and in bending excited states reported in the literature have been included in the global analysis. The set of 57 derived spectroscopic parameters reproduces 6130 infrared and 90 microwave and millimetre?wave transitions satisfactorily with root mean square values of 5.3 × 10^?4 cm^?1 and 77 kHz, respectively.     

Determination of Chlorinated Solvent Sorption by Porous Material—Application to Trichloroethene Vapor on Cement Mortar

Experiments have been performed to investigate the sorption of trichloroethene (TCE) vapor by concrete material or, more specifically, the cement mortar component. Gas-flow experiments were conducted using columns packed with small pieces of cement mortar obtained from the grinding of typical concrete material. Transport and retardation of TCE at high vapor concentrations (500 mg L \(^{-1})\) was compared to that of a non-reactive gas tracer (Sulfur Hexafluoride, SF \(_{6})\) . The results show a large magnitude of retardation (retardation factor \(=\) 23) and sorption (sorption coefficient \(=\) 10.6 cm \(^{3}\)  g \(^{-1})\) for TCE, compared to negligible sorption for SF \(_{6}\) . This magnitude of sorption obtained with pollutant vapor is much bigger than the one obtained for aqueous-flow experiments conducted for water-saturated systems. The considerable sorption exhibited for TCE under vapor-flow conditions is attributed to some combination of accumulation at the air-water interface and vapor-phase adsorption, both of which are anticipated to be significant for this system given the large surface area associated with the cement mortar. Transport of both SF \(_{6}\) and TCE was simulated successfully with a two-region physical non-equilibrium model, consistent with the dual-medium structure of the crushed cement mortar. This work emphasizes the importance of taking into account sorption phenomena when modeling transport of volatile organic compounds through concrete material, especially in regard to assessing vapor intrusion.     

Pyridine ligands as promoters in Pd(II/0)-catalyzed C-H olefination reactions

Commercially available pyridine ligands can significantly enhance the rate, yield, substrate scope, and site selectivity of arene C-H olefination (Fujiwara-Moritani) reactions. The use of a 1:1 ratio of Pd/pyridine proved critical to maximize reaction rates and yields.     

Squaric acid is a suitable building-block in 4C-Ugi reaction: access to original bivalent compounds

The 4C-Ugi reaction requires an acidic component as one of the building-blocks. We report here the first use of squaric acid for this reaction to access original symmetrical compounds. The scope and conditions of the reaction were explored.