A facile synthesis of bispyrroloquinone and bispyrroloiminoquinone ring system of marine alkaloids

Bispyrroloquinone and bispyrroloiminoquinone are two important polycyclic ring systems present in biologically active marine alkaloids such as Zyzzyanones, tsitsikammamines, and wakayin. A facile synthesis of these two ring systems starting from a 6-benzylamino indole-4,7-quinone or 6-benzylamino pyrroloiminoquinone is described here. This chemistry involves the construction of a pyrrole ring in a single step by treatment of the starting reagents with ethyl acetoacetate or phenylbutane-1,3-dione in the presence of ceric ammonium nitrate in MeOH/CH₂Cl₂ solvent.     

Mixing characterization of mechanically milled Fe75Si15M10 powders using Mössbauer spectroscopy

Iron nitrides are attractive as they show excellent magnetic properties which can be utilized as recording and permanent magnetic materials for potential applications. Due to the high saturation magnetization and chemical stability, γ ^′-Fe₄N compound is widely investigated as a promising high density magnetic recording material. γ ^′-Fe₄N particles were synthesized by conventional gaseous nitriding in a heated atmosphere containing ammonia as a source of nitrogen. X-ray diffraction, ⁵⁷Fe Mössbauer spectroscopy, vibrating sample magnetometer, scanning electron microscopy and transmission electron microscopy are used for the characterization of the as prepared sample.     

A density functional theoretic study of novel silicon-carbon fullerene-like nanostructures: Si40C20, Si60C20, Si36C24, and Si60C24

Fullerene-like silicon nanostructures with twenty and twenty-four carbon atoms on the surface of the Si₆₀ cage by substitution, as well as inside the cage at various orientations have been studied within the generalized gradient approximation to density functional theory. Full geometry optimizations have been performed without any symmetry constraints using the Gaussian 03 suite of programs and the LANL2DZ basis set. Thus, for the silicon atom, the Hay-Wadt pseudopotential with the associated basis set is used for the core electrons and the valence electrons, respectively. For the carbon atom, the Dunning/Huzinaga double zeta basis set is employed. Electronic and geometric properties of these nanostructures are presented and discussed in detail. Optimized silicon-carbon fullerene like nanostructures are found to have increased stability compared to the bare Si₆₀ cage and the stability depends on the number and the orientation of carbon atoms, as well as on the nature of silicon-carbon and carbon-carbon bonding.     

Quantum wavepacket ab initio molecular dynamics: generalizations using an extended Lagrangian treatment of diabatic states coupled through multireference electronic structure

We present a generalization to our previously developed quantum wavepacket ab initio molecular dynamics (QWAIMD) method by using multiple diabatic electronic reduced single particle density matrices, propagated within an extended Lagrangian paradigm. The Slater determinantal wavefunctions associated with the density matrices utilized may be orthogonal or nonorthogonal with respect to each other. This generalization directly results from an analysis of the variance in electronic structure with quantum nuclear degrees of freedom. The diabatic electronic states are treated here as classical parametric variables and propagated simultaneously along with the quantum wavepacket and classical nuclei. Each electronic density matrix is constrained to be N-representable. Consequently two sets of new methods are derived: extended Lagrangian-QWAIMD (xLag-QWAIMD) and diabatic extended Lagrangian-QWAIMD (DxLag-QWAIMD). In both cases, the instantaneous potential energy surface for the quantum nuclear degrees of freedom is constructed from the diabatic states using an on-the-fly nonorthogonal multireference formalism. By introducing generalized grid-based electronic basis functions, we eliminate the basis set dependence on the quantum nucleus. Subsequent reuse of the two-electron integrals during the on-the-fly potential energy surface computation stage yields a substantial reduction in computational costs. Specifically, both xLag-QWAIMD and DxLag-QWAIMD turn out to be about two orders of magnitude faster than our previously developed time-dependent deterministic sampling implementation of QWAIMD. Energy conservation properties, accuracy of the associated potential surfaces, and vibrational properties are analyzed for a family of hydrogen bonded systems.     

Characterization of an exception to the ‘even‐electron rule’ upon low‐energy collision induced decomposition in negative ion electrospray tandem mass spectrometry

Electrospray‐generated precursor ions usually follow the ‘even‐electron rule’ and yield ‘closed shell’ fragment ions. We characterize an exception to the ‘even‐electron rule.’ In negative ion electrospray mass spectrometry (ES‐MS), 2‐(ethoxymethoxy)‐3‐hydroxyphenol (2‐hydroxyl protected pyrogallol) easily formed a deprotonated molecular ion (M‐H)^? at m/z 183. Upon low‐energy collision induced decomposition (CID), the m/z 183 precursor yielded a radical ion at m/z 124 as the base peak. The radical anion at m/z 124 was still the major fragment at all tested collision energies between 0 and 50 eV (E_lab). Supported by computational studies, the appearance of the radical anion at m/z 124 as the major product ion can be attributed to the combination of a low reverse activation barrier and resonance stabilization of the product ions. Furthermore, our data lead to the proposal of a novel alternative radical formation pathway in the protection group removal of pyrogallol. Copyright © 2009 John Wiley & Sons, Ltd.