Gaseous CF(3)(+) interchanges F(+) for O with simple carbonyl compounds. CF(3)(+) reacts with propionaldehyde in the gas phase to produce (CH(3))(2)CF(+) via two competing pathways. Starting with 1-(13)C-propionaldehyde, the major pathway (80%) produces (CH(3))(2)CF(+) with the carbon label in one of the methyl groups. The minor pathway (20%) produces (CH(3))(2)CF(+) with the carbon label in the central position. The relative proportions of these two pathways are measured by (19)F NMR analysis of the neutral CH(3)CF=CH(2) produced by deprotonation of (CH(3))(2)CF(+) at <10(-)(3) Torr in an electron bombardment flow (EBFlow) reactor. Formation of alkene in which carbon is directly bonded to fluorine means that (in the minor product, at least) an F(+) for O transposition occurs via adduct formation followed by 1,3-atom transfer and then isomerization of CH(3)CH(2)CHF(+) to the more stable (CH(3))(2)CF(+). Use of CF(4) as a chemical ionization (CI) reagent gas leads to CF(3)(+) adduct ions for a variety of ketones, in addition to isoelectronic transposition of F(+) for O. Metastable ion decompositions of the adduct ions yield the metathesis products. Decompositions of fluorocycloalkyl cations formed in this manner give evidence for the same kinds of rearrangements as take place in CH(3)CH(2)CHF(+). Density functional calculations confirm that F(+) for O metathesis takes place via addition of CF(3)(+) to the carbonyl oxygen followed by transposition via a four-member cyclic transition state. A computational survey of the effects of different substituents in a series of aldehydes and acyclic ketones reveals no systematic variation of the energy of the transition state as a function of thermochemistry, but the Hammond postulate does appear to be obeyed in terms of progress along the reaction coordinate. Bond lengths corresponding to the central barrier correlate with overall thermochemistry of the F(+) for O interchange, but in a sense opposite to what might have been expected: the transition state becomes more product-like as the metathesis becomes increasingly exothermic. This reversal of the naive interpretation of the Hammond postulate is accounted for by the relative positions of the potential energy wells that precede and follow the central barrier.
Primary cations of the form XCH2CH2 + (1) are unstable with respect to the bridged isomer cyclo-(CH2)2X+ (2) or the hydride-shift isomer CH3CHX+ (3).Two independent hydrogen isotope experiments for X = OH demonstrate that gaseous HOCH2CH2 + (1a) gives less of its bridged isomer, protonated oxirane (2a), than of its hydride-shift isomer, protonated acetaldehyde (3a).Metastable ion decompositions of ionized HOCH2CH2OPh show that the radical cation decomposes via an ion−neutral complex containing 3a and phenoxy radical. Deuterium labeling studies exhibit no detectable interconversion of the two sp3-carbons, implying that complexes containing 2a are not formed to a measurable extent. An independent neutral product study looks at the radioactive oxirane and acetaldehyde produced when tritium on the methyl group of gaseous CH2TCHTOH undergoes radioactive decay. Loss of a beta-particle and a helium atom forms transient, tritiated 1a. Ions from isomerization of this radiolabeled primary cation were deprotonated by Me3N and gave a >97% yield of acetaldehyde. Quantitative assessment of possible routes to acetaldehyde (including rearrangement of excited 2a) implies that 1 undergoes hydride shift at least 5 times faster than bridging by neighboring oxygen.
The structure and spectroscopy of β-fluorophenetole (2-phenoxy-1-fluoroethane, FCH2CH2OPh) have been studied by X-ray crystallography and Raman scattering of the solid and by resonance-enhanced multiphoton ionization (REMPI) excitation spectra of a supersonic-jet-cooled gaseous sample, as well as by ab initio calculations. Fluorine and oxygen are synclinal (with an FCCO torsion angle near 70°) in the dominant conformational isomer for both the crystalline and gas phases. The minor conformer observed in the gas phase has antiperiplanar substituents (FCCO torsion angle = 180°), with a relative abundance comparable to that previously inferred from NMR measurements in solution. Hartree−Fock-based computations, as well as second-order Møller−Plesset and density functional geometry optimizations, predict the structural features closely, and the computed (unscaled) normal modes ≤350 cm-1 have frequencies not far from those measured by vibrational spectroscopy. CI singles calculations give reasonable estimates of the isomeric differences in the UV absorptions and fit the observed overtones well, though they err in predicting the absolute wavelengths. Ab initio calculations of the electronic ground states do not give a useful ordering of the relative energies of the conformational isomers, for they predict high stability for a highly nonplanar structure for which no experimental evidence is seen. Atoms-in-molecules analysis of theoretical electron densities correlates the preferences for synclinal versus antiperiplanar geometries (in 1-phenoxypropane as well as β-fluorophenetole) with double bowing of the bond paths between two methylene carbons, which (in-planar conformations with C s symmetry) cut across the lines of centers. Time-of-flight mass spectrometry of isotopically substituted analogues ionized by REMPI shows that deuterium substitution does not decelerate the rate of decomposition of radical cations nor do different conformers manifest any differences in their fragmentation patterns.
biochemical and metal separations, to highlight recent advances in the field and discuss their applications to real world problems in biological and metal ion separations. This reviewer recalls the benefit from combining diverse groups in such a manner. A Gordon Conference on Separations in the late 1980s served to introduce him to "centrifugal partition chromatography" (CPC), which earlier had been developed for and used by biotechnologists. Since then, CPC separations have been applied to many metals separations.Aqueous two-phase systems have been used for the partitioning of cellular particles, biological macromolecules, and smaller organic molecules since the pioneering work of Albertsson. The first experiments on partition of cells and cell particles (bacteria, algae, chloroplast fragments, cell walls, and starch grains) in aqueous polymer (polyethylene glycol) phase systems were carried out in 1955 and published in 1956. In the last several years, a new group of international scientists has shown such systems also have remarkable utility for the separation of metal ions. Aqueous biphasic separation systems have high potential for cleaner, cheaper, and safer separations of metal ions as well as for gentle nondenaturing separations of molecules of biological interest.This book draws on the expertise gathered for the symposium and presents the now fairly diverse work united by the use of aqueous biphasic systems. Chapters include material on metal ion separations, mass transfer effects, affinity partitioning, protein partitioning and refolding, and cell partitioning. It offers an excellent introduction to this interesting and still evolving separation technique.Henry Freiser, UniVersity of Arizona
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