Charge reversal of three alkylamide ions: Free nitrenium ions

The methylnitrenium, ethylnitrenium and dimethylnitrenium ions are prepared by charge reversal collisional activation (CR CA) of the corresponding negative ions; their collisional activation mass spectra are shown to support the assigned structures. MINDO/3 energies are used to evaluate relative energies of [CH₄N]⁺ and [C₂H₆N]⁺ isomers, and to determine whether unstable forms rearrange spontaneously to stable ones. As in other examples, charge reversal here generates cations that do not exist in an energy well, but their transient existence is established because their fragmentation is more rapid than their rearrangement to a more stable form.     

NMR studies of conformations and dynamic processes—II : [26]Bicyclophanes with ethylene bridges

The conformations and dynamic processes in two bicyclophanes have been analysed on the basis of temperature-dependent ¹H NMR spectra. Both bicyclophanes are suggested to have a lowest-energy conformation of D₃ symmetry in which the substituents at all ethylene bridges are gauche⁺ (or gauche^?) oriented. The interconversion of the mirror image conformers of each bicyclophane equilibrates the two hydrogens in each methylene group, the barriers being ca 36 and 37 kJ mol^?1, respectively, as determined by line-shape analysis.     

Analysis of carboxylic acids by gas chromatography of acetonyl esters. Application to aromatic,dicarboxylic and higher fatty acids

Summary The gas chromatographic analysis of the acetonyl esters of aromatic, dicarboxylic and higher fatty acids is reported and discussed together with their electron impact and chemical ionisation mass spectra. An improved method for the preparation of acetonyl esters is discussed.     

PCILO calculation of intermolecular energies: The water dimer

The PCILO (perturbative configuration interaction using localized orbitals) method for approximating the electronic structure of molecules has been used with some success for calculating intramolecular interactions in large molecules where intramolecular hydrogen bonding is involved. In this note we show that the PCILO method may be used to calculate the energy of interaction between two water molecules in selected configurations.     

CHEMISTRY OF SINGLET OXYGEN—XXVI. PHOTOOXYGENATION OF PHENOLSy†

Abstract— The aerobic dye-sensitized photooxygenation of monohydric phenols proceeds by way of singlet oxygen under the conditions studied. Various phenols give different proportions of reaction with and quenching of singlet oxygen. Para-substituted 2,6-di-t-butylphenols show a linear correlation between the log of the total rate of singlet oxygen removal and their halfwave oxidation potentials; the same correlation is given for certain phenol methyl ethers. A Hammett plot using s?⁺ gives ρ - 1.72 ± 0.12, consistent with development of some charge in the quenching step. Reaction of photo-chemically generated singlet oxygen with 2,4,6-triphenylphenol gives 2,4,6-triphenylphenoxy radical as an intermediate in singlet oxygen quenching, although no overall reaction occurs. Kinetic analysis indicates that the radical is derived exclusively from the interaction of 2,4,6-triphenylphenol with singlet oxygen. A charge-transfer mechanism for quenching of singlet oxygen by phenols is proposed.     

New Molybdenyl Iodates: Hydrothermal Preparation and Structures of Molecular K2MoO2(IO3)4 and Two-Dimensional β-KMoO3(IO3)

Two new molybdenyl iodates, K₂MoO₂(IO₃)₄ (1) and β-KMoO₃(IO₃) (2), have been prepared from the reactions of MoO₃ with KIO₄ and NH₄Cl at 180°C in aqueous media. The structure of 1 consists of molecular [MoO₂(IO₃)₄]²⁻ anions separated by K⁺ cations. The Mo(VI) centers are ligated by two cis-oxo ligands and four monodentate iodate anions. Both terminal and bridging oxygen atoms of the iodate anions form long ionic contacts with the K⁺ cations. β-KMoO₃(IO₃) (2) displays a two-dimensional layered structure constructed from ²_∞[(MoO₃(IO₃)]¹⁻ anionic sheets separated by K⁺ cations. These sheets are built from one-dimensional chains formed from corner-sharing MoO₆ octahedra that run along the b-axis that are linked together through bridging iodate groups. K⁺ cations separate the layers from one another and form long contacts with oxygen atoms from both the iodate anions and molybdenyl moieties. Crystallographic data: 1, monoclinic, space group C2/c, a=12.8973(9) Å, b=6.0587(4) Å, c=17.694(1) Å, β=102.451(1)°, Z=4, MoKα, λ=0.71073, R(F)=2.64% for 97 parameters with 1584 reflections with I>2σ(I); 2, monoclinic, space group P2₁/n, a=7.4999(6) Å, b=7.4737(6) Å, c=10.5269(8) Å, β=109.023(1)°, Z=4, MoKα, λ=0.71073, R(F)=2.73% for 83 parameters with 1334 reflections with I>2σ(I).     

De novo structural prediction of transition metal complexes: application to technetium

De novo structural prediction of transition metal complexes is investigated. Technetium complexes are chosen given their importance in medical imaging and nuclear waste remediation and for the chemical diversity they display. A new conformational searching algorithm (LIGB) for transition metals is described that allows one to search for different conformational and geometric isomers within a single simulation. In the preponderance of cases, both conformational searching techniques (LIGB and high-temperature molecular dynamics/simulated annealing) provide comparable results, while LIGB is superior for macrocyclic complexes. A genetic algorithm-optimized PM3(tm) parametrization for Tc is compared with the standard implementation and found to yield a significant improvement in predictive ability for the most prevalent Tc structural motifs. The utility of a coupled molecular mechanics-semiempirical quantum mechanics protocol is demonstrated for very rapid, efficient, and effective de novo prediction of transition metal complex geometries.     

Site-directed photochemical disruption of the actin cytoskeleton by actin-binding Rose Bengal-conjugates

The in situ light-induced, non-enzymatic digestion of cytoskeletal actin by a xanthene dye conjugated to heavy meromyosin, anti-actin antibodies and/or anti-myosin antibodies is reported. The dye Rose Bengal was conjugated to either anti-actin antibodies, anti-myosin antibodies or heavy meromyosin. Under our experimental conditions, visible light induced the non-enzymatic breakdown of cytoskeletal actin when mammalian tissue culture cells were probed either with Rose Bengal-conjugated anti-actin and/or anti-myosin antibodies. Similar results were obtained when tissue culture cells were probed with Rose Bengal-conjugated heavy meromyosin before irradiation with visible light. The in situ photochemical reaction depended on the presence of actin-binding Rose Bengal-conjugates.     

Size-dependent chemistry: properties of nanocrystals

Properties of materials determined by their size are indeed fascinating and form the basis of the emerging area of nanoscience. In this article, we examine the size dependent electronic structure and properties of nanocrystals of semiconductors and metals to illustrate this aspect. We then discuss the chemical reactivity of metal nanocrystals which is strongly dependent on the size not only because of the large surface area but also a result of the significantly different electronic structure of the small nanocrystals. Nanoscale catalysis of gold exemplifies this feature. Size also plays a role in the assembly of nanocrystals into crystalline arrays. While we owe the beginnings of size-dependent chemistry to the early studies of colloids, recent findings have added a new dimension to the subject.     

The utilization of chemical shift and spin-lattice (T1) relaxation time data for the discrimination of isomeric indenoisoxazoles

Treatment of 2-pivaloyl-1,3-indandione with hydroxylamine leads to the formation of a pair of isomeric indenoisoxazoles, the product formed dependent upon the cyclization conditions. Under acidic conditions, 8-t-butylindeno[1,2-c]isoxazol-7-one ( 5 ) is formed while under neutral or basic conditions, an oxime, 2 , is generated which may then be cyclized under acidic conditions to give 3-t-butylindeno[1,2-c]isoxazol-4-one ( 4 ). Although these isomeric indenoisoxazoles may be discriminated by chemical means, we were interested in developing an unequivocal method for distinguishing these and potentially other isomeric pairs by spectroscopic means. A ¹³C-nmr based method for the discrimination of these isomers which is based on the utilization of chemical shift arguments and spin-lattice relaxation data is thus presented.