192,128 research outputs found
Synthesis of indoles via alkylidenation of acyl hydrazides
Indoles have been synthesised via alkylidenation of acyl phenylhydrazides using phosphoranes and the Petasis reagent, followed by in situ thermal rearrangement of the product enehydrazines. The Petasis reagent provides an essentially neutral equivalent of the [acid-catalysed] Fischer indole synthesis, but with acyl phenylhydrazides as starting substrates. Alkylidene triphenylphosphoranes convert aroyl phenylhydrazides to indoles, but acyl phenylhydrazides derived from aliphatic carboxylic acids undergo a Brunner reaction to form indolin-2-ones
Valence electronic structure of Mn in undoped and doped lanthanum manganites from relative K x-ray intensity studies
Relative x-ray intensities of in , , and
( = , , and ) systems have been
measured following excitation by 59.54 keV -rays from a 200 mCi
Am point-source. The measured results for the compounds deviate
significantly from the results of pure . Comparison of the experimental
data with the multiconfiguration Dirac-Fock (MCDF) effective atomic model
calculations indicates reasonable agreement with the predictions of ionic model
for the doped {manganites except} that the electron doped
and hole doped compounds
show some small deviations. The results of and deviate
considerably from the predictions of the ionic model. Our measured
ratio of in cannot be explained
as a linear superposition of ratios of for the end
members which is in contrast to the recent proposal by Tyson et al. from their
spectra.Comment: 14 pages, 4 figures. to appear in NIM-B.Please send an e-mail for
figure
D-Brane Charges in Gepner Models
We construct Gepner models in terms of coset conformal field theories and
compute their twisted equivariant K-theories. These classify the D-brane
charges on the associated geometric backgrounds and therefore agree with the
topological K-theories. We show this agreement for various cases, in particular
the Fermat quintic.Comment: 25 pages, 2 figures. LaTeX. v2: typos and references corrected. v3:
reference adde
Tensor Product and Permutation Branes on the Torus
We consider B-type D-branes in the Gepner model consisting of two minimal
models at k=2. This Gepner model is mirror to a torus theory. We establish the
dictionary identifying the B-type D-branes of the Gepner model with A-type
Neumann and Dirichlet branes on the torus.Comment: 26 page
K-theoretical boundary rings in N=2 coset models
A boundary ring for N=2 coset conformal field theories is defined in terms of
a twisted equivariant K-theory. The twisted equivariant K-theories K_H(G) for
compact Lie groups (G, H) such that G/H is hermitian symmetric are computed.
These turn out to have the same ranks as the N=2 chiral rings of the associated
coset conformal field theories, however the product structure differs from that
on chiral primaries. In view of the K-theory classification of D-brane charges
this suggests an interpretation of the twisted K-theory as a `boundary ring'.
Complementing this, the N=2 chiral ring is studied in view of the isomorphism
between the Verlinde algebra V_k(G) and twisted K_G(G) as proven by Freed,
Hopkins and Teleman. As a spin-off, we provide explicit formulae for the ranks
of the Verlinde algebras.Comment: 22 pages, harvmac (b); reference added, table 2 beautifie
Boundary Rings and N=2 Coset Models
We investigate boundary states of N=2 coset models based on Grassmannians
Gr(n,n+k), and find that the underlying intersection geometry is given by the
fusion ring of U(n). This is isomorphic to the quantum cohomology ring of
Gr(n,n+k+1), and thus can be encoded in a ``boundary'' superpotential whose
critical points correspond to the boundary states. In this way the intersection
properties can be represented in terms of a soliton graph that forms a
generalized, Z_{n+k+1} symmetric McKay quiver. We investigate the spectrum of
bound states and find that the rational boundary CFT produces only a small
subset of the possible quiver representations.Comment: 40p, 5 figs, refs added, typos and minor errors correcte
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