The molecular structure of gaseous 2-cyclopentene- 1,4-dione as determined by electron diffraction

The molecular structure of gaseous 2-cyclopentene-1,4-dione has been studied by electron diffraction. The molecule is planar to within the experimental error. The results obtained for some of the more important parameters with estimated uncertainties of 2σ are r(C-H) = 1.093 Å (0.013), r(C0) = 1.208 Å (0.002), r(CC) = 1.341 Å (0.005), r(CH-CO) = 1.493 Å (0.005), r(CO-CH₂) = 1.525 Å (0.005), ∠CC-C = 110.4° (0.3), ∠CH-CO = 124.9° (1.1), ∠CC-H. = 118.7° (5.8), ∠H-C-H = 113.2° (8.7) l(C-H) = 0.0853 A (0.0113), l(CO) = 0.0428 Å (0.0021), l(CC) = 0.0448 Å (0.0037) and l(C-C) = 0.0561 Å (0.0029). The structure is discussed in connection with the structures of related molecules.     

Matrix-dependent instability of selenium(IV) stored in teflon containers

Solutions of selenium(IV) standards with different acid matrices were stored in containers constructed of boosilicate glass, conventional polyethylene, and fluorinated ethylenepropylene (teflon FEP). After 50 days of storage in FEP, there were highly significant losses of Se(IV) from standards in either 5% HCl/5% H₂SO₄, or 5% H₂SO₄. Increasing the hydrochloric acid concentration, e.g., 15% HCl/5% H₂SO₄, greatly reduced this loss. Addition of selenium-75 (selenate-free) indicated that the losses did not result from physical adsorption onto container surfaces. It is shown that the losses were caused by oxidation of Se(IV) to Se(VI).     

Decoupling the effects of hydrophilic and hydrophobic moieties at the neuron–nanofibre interface

Peptide-based nanofibres are a versatile class of tunable materials with applications in optoelectronics, sensing and tissue engineering. However, the understanding of the nanofibre surface at the molecular level is limited. Here, a series of homologous dilysine–diphenylalnine tetrapeptides were synthesised and shown to self-assemble into water-soluble nanofibres. Despite the peptide nanofibres displaying similar morphologies, as evaluated through atomic force microscopy and neutron scattering, significant differences were observed in their ability to support sensitive primary neurons. Contact angle and labelling experiments revealed that differential presentation of lysine moieties at the fibre surface did not affect neuronal viability; however the mobility of phenylalanine residues at the nanofibre surface, elucidated through solid- and gel-state NMR studies and confirmed through tethered bilayer lipid membrane experiments, was found to be the determining factor in governing the suitability of a given peptide as a scaffold for primary neurons. This work offers new insights into characterising and controlling the nanofibre surface at the molecular level.

The mobility of hydrophobic moieties at a peptide nanofibre surface determines its suitability as a scaffold for sensitive primary cells.     

Stereochemical features of the physical and chemical interactions of singlet oxygen with enecarbamates

Oxazolidinone-substituted enecarbamates represent a mechanistically rich system for the study of stereoelectronic, steric, and conformational effects on stereoselectivity and mode selectivity in (1)O(2) [2 + 2] and ene reactions. Photooxygenation of these enecarbamates with (1)O(2) leads to diastereomerically pure dioxetanes that decompose to yield an oxazolidinone carbaldehyde and one of the two enantiomers of methyldesoxybenzoin in enantiomeric excess. Stereoselectivity originates at the allylic stereocenter, a result supported by quenching studies, computational analysis, and deuterium solvent isotope effects. [reaction: see text]     

Highly selective palladium catalyzed kinetic resolution and enantioselective substitution of racemic allylic carbonates with sulfur nucleophiles: asymmetric synthesis of allylic sulfides,allylic sulfones,and allylic alcohols

We describe the highly selective palladium catalyzed kinetic resolutions of the racemic cyclic allylic carbonates rac-1 a-c and racemic acyclic allylic carbonates rac-3 aa and rac-3 ba through reaction with tert-butylsulfinate, tolylsulfinate, phenylsulfinate anions and 2-pyrimidinethiol by using N,N'-(1R,2R)-1,2-cyclohexanediylbis[2-(diphenylphosphino)-benzamide] (BPA) as ligand. Selectivities are expressed in yields and ee values of recovered substrate and product and in selectivity factors S. The reaction of the cyclohexenyl carbonate 1 a (>/=99 % ee) with 2-pyrimidinethiol in the presence of BPA was shown to exhibit, under the conditions used, an overall pseudo-zero order kinetics in regard to the allylic substrate. Also described are the highly selective palladium catalyzed asymmetric syntheses of the cyclic and acyclic allylic tert-butylsulfones 2 aa, 2 b, 2 c, 2 d and 4 a-c, respectively, and of the cyclic and acyclic allylic 2-pyrimidyl-, 2-pyridyl-, and 4-chlorophenylsulfides 5 aa, 5 b, 5 ab, 6 aa-ac, 6 ba and 6 bb, respectively, from the corresponding racemic carbonates and sulfinate anions and thiols, respectively, in the presence of BPA. Synthesis of the E-configured allylic sulfides 6 aa, 6 ab, 6 ac and 6 bb was accompanied by the formation of minor amounts of the corresponding Z isomers. The analogous synthesis of allylic tert-butylsulfides from allylic carbonates and tert-butylthiol by using BPA could not be achieved. Reaction of the cyclopentenyl esters rac-1 da and rac-1 db with 2-pyrimidinethiol gave the allylic sulfide 5 c having only a low ee value. Similar results were obtained in the case of the reaction of the cyclohexenyl carbonate rac-1 a and of the acyclic carbonates rac-3 aa and rac-3 ba with 2-pyridinethiol and lead to the formation of the sulfides 5 ab, 6 ab, and 6 bb, respectively. The low ee values may be ascribed to the operating of a "memory effect", that is, both enantiomers of the substrate give the substitution product with different enantioselectivities. However, in the reaction of the racemic carbonate rac-1 a as well as of the highly enriched enantiomers 1 a (>/=99 % ee) and ent-1 a (>/=99 % ee) with 2-pyrimidinethiol the ee values of the substrates and the substitution product remained constant until complete conversion. Similar results were obtained in the reaction of the cyclic carbonates rac-1 a, ent-1 a (>/=99 % ee) and ent-1 c (>/=99 % ee) with lithium tert-butylsulfinate. Thus, in the case of rac-1 a and 2-pyrimidinthiol and tert-butylsulfinate anion as nucleophiles the enantioselectivity of the substitution step is, under the conditions used, independent of the chirality of the substrate; this shows that no "memory effect" is operating in this case. Hydrolysis of the carbonates ent-1 a-c, ent-3 aa and ent-3 ba, which were obtained through kinetic resolution, afforded the enantiomerically highly enriched cyclic allylic alcohols 9 a-c (>/=99 % ee) and acyclic allylic alcohols 10 a (>/=99 % ee) and 10 b (99 % ee), respectively.     

Elucidating interactions of ionic liquids with polymer films using confocal Raman spectroscopy

We report on the molecular interactions between room-temperature ionic liquids (RTILs) and Nafion and PDMS membranes, proving that in contact with these polymers RTILs behave like electrolytes rather than solvents.     

Water-soluble calix[4]resorcarenes as enantioselective NMR shift reagents for aromatic compounds

A tetra L-prolinylmethyl derivative of a tetra-sulfonated calix[4]resorcarene (1) is an effective chiral NMR solvating agent for water-soluble compounds with phenyl, pyridyl, bicyclic aromatic, or indole rings. These aromatic compounds form host-guest complexes with the calix[4]resorcarene in water. Complexation of substrates with the calix[4]resorcarene is likely promoted by hydrophobic effects, and bicyclic substrates have association constants with the calix[4]resorcarene larger than those of similar phenyl-containing compounds. Aromatic resonances of the substrates show substantial upfield shifts because of shielding from the aromatic rings of the calix[4]resorcarene, and several resonances in the 1H NMR spectra typically exhibit enantiomeric discrimination. The extent of enantiomeric discrimination depends in part on interactions of the substituent groups of the substrates with the prolinylmethyl groups of the calix[4]resorcarene. The effectiveness of a calix[4]resorcarene prepared from N-methyl-L-alanine (2) as a chiral NMR discriminating agent is compared to the L-prolinylmethyl derivative.     

Vibrationally resolved rate coefficients and branching fractions in the dissociative recombination of O2+

We have studied the dissociative recombination of the first three vibrational levels of O(2) (+) in its electronic ground X (2)Pi(g) state. Absolute rate coefficients, cross sections, quantum yields and branching fractions have been determined in a merged-beam experiment in the heavy-ion storage ring, CRYRING, employing fragment imaging for the reaction dynamics. We present the absolute total rate coefficients as function of collision energies up to 0.4 eV for five different vibrational populations of the ion beam, as well as the partial (vibrationally resolved) rate coefficients and the branching fractions near 0 eV collision energy for the vibrational levels v=0, 1, and 2. The vibrational populations used were produced in a modified electron impact ion source, which has been calibrated using Cs-O(2)(+) dissociative charge transfer reactions. The measurements indicate that at low collision energies, the total rate coefficient is weakly dependent on the vibrational excitation. The calculated thermal rate coefficient at 300 K decreases upon vibrational excitation. The partial rate coefficients as well as the partial branching fractions are found to be strongly dependent on the vibrational level. The partial rate coefficient is the fastest for v=0 and goes down by a factor of two or more for v=1 and 2. The O((1)S) quantum yield, linked to the green airglow, increases strongly upon increasing vibrational level. The effects of the dissociative recombination reactions and super elastic collisions on the vibrational populations are discussed.     

Non planar silylium, germylium and stannylium ions

Quantum mechanical calculations at the B3LYP/6-311+G(d,p) and MP2/6-311+G(d,p) level of theory reveal that higher congeners of the aromatic imidazolium ion, e.g. 2-E-imidazolium ions (E = Si, Ge, Sn), adopt either planar or pyramidal structures, depending on the substituent R ² attached to the element and on the group 14 element itsself. In the case of 2-silaimidazolium ions chemically significant energy differences in favour of non-planar cations are predicted only for strongly σ-electron withdrawing substituents R ² such as F or CF₃. The pyramidalization computed for the germanium and tin analogues are however significant for all investigated substituents R ² and are accompanied by a substantial stabilization compared to the corresponding planar structures. A detailed bonding analysis reveals that the non-planar cations are best described as complexes of monovalent group 14 element cations R ²E⁺ with the diazabutadiene ligand.     

A new palladium catalyst system for the cyanation of aryl chlorides with K4[Fe(CN)6]

The development of a novel Pd-catalyzed synthesis of (hetero)aromatic nitriles from the corresponding aryl chlorides and potassium hexacyanoferrate(II) is described. This novel protocol avoids the use of highly toxic alkali cyanides and proceeds in the presence of small amounts of palladium catalysts. High yields and selectivities of the corresponding aryl nitriles were achieved applying di(1-adamantyl)-1-butylphosphine (cataCXium^® A) as the ligand.