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Stereochemistry of atropisomeric 9,10-dihydrophenanthrene dimers from Pholidota chinensis
No full textSix bis-9,10-dihydrophenanthrene and 9,10-dihydrophenanthrene/(dihydro)stilbene derivatives, phochinenins G-L (1-6), were isolated from the whole plant of Pholidota chinensis. Their structures were elucidated on the basis of extensive spectroscopic investigations (1D, 2D NMR and HR-EIMS). Owing to the sterically hindered rotation around the biaryl axis, some of these biaryl compounds can exist as a pair of enantiomers, but were isolated as racemates. Computed inversion barriers of selected atropisomeric derivatives suggested that phochinenins K (5), gymconpin C (7) and flavanthrin (9) have optically stable atropisomers. Their racemates were separated by HPLC on an optically active stationary phase, and stereochemically characterized on-line by circular dichroism (CD) spectroscopy (LC-CD coupling), in conjunction with quantum-mechanics CD calculations
Leggere nella società della conoscenza
No full textRIASSUNTO Ogni cittadino nella società della conoscenza deve sapere utilizzare criticamente le molteplici informazioni che lo circondano. E' ormai appurato che la lettura individuale e di gruppo permette lo sviluppo di capacità legate all'autonomia di pensiero e alla crescita del livello di creatività, che ben contrastano quel tipo di cultura passiva prodotta dal condizionamento dei media. SUMMARY Every citizen into knowledge society must be able to use critically the surrounding informations. It's known that individual and group reading allows to develop abilities as autonomous thought and creativity, in contrast with the passive culture produced by medias' conditioning
Evaluating scientific products by means of citation-based models: a first analysis and validation
No full textSome integrated models for ranking scientific publications together with authors and journals are presented and analyzed. The models rely on certain adiacency matrices obtained from the relations of citation,authorship and publication, which concurr to forming a suitable irreducible stochastic matrix whose Perron vector provides the ranking. Some perturbation theorems concerning the Perron vector of nonnegative irreducible matrices are proved. These theoretical results provide a validation of the consistency and effectiveness of our models. Several paradigmatic examples are reported together with some results obtained on a real set of data
A combined approach for evaluating papers, authors and scientific journals
No full textAn integrated model for ranking scientific publications together with authors and journals recently presented in [Bini, Del Corso, Romani,ETNA 2008]is closely analyzed. The model, which relies on certain adjacency matrices and obtained from the relations of citation,authorship and publication, provides the ranking by means of the Perron vector of a stochastic matrix obtained by combining and . Some perturbation theorems concerning the Perron vector previously introduced by the authors are extended to more general cases and a counterexample to a property previously addressed by the authors is presented. The theoretical results confirm the consistency and effectiveness of our model. Some paradigmatic examples are reported together with some results obtained on a real set of data
Universal dependence on disorder of 2D randomly diluted and random-bond +-J Ising models
No full textWe consider the two-dimensional randomly site diluted Ising model and the
random-bond +-J Ising model (also called Edwards-Anderson model), and study
their critical behavior at the paramagnetic-ferromagnetic transition. The
critical behavior of thermodynamic quantities can be derived from a set of
renormalization-group equations, in which disorder is a marginally irrelevant
perturbation at the two-dimensional Ising fixed point. We discuss their
solutions, focusing in particular on the universality of the logarithmic
corrections arising from the presence of disorder. Then, we present a
finite-size scaling analysis of high-statistics Monte Carlo simulations. The
numerical results confirm the renormalization-group predictions, and in
particular the universality of the logarithmic corrections to the Ising
behavior due to quenched dilution
The critical behavior of three-dimensional Ising spin glass models
No full textWe perform high-statistics Monte Carlo simulations of three-dimensional Ising
spin-glass models on cubic lattices of size L: the +- J (Edwards-Anderson)
Ising model for two values of the disorder parameter p, p=0.5 and p=0.7 (up to
L=28 and L=20, respectively), and the bond-diluted bimodal model for
bond-occupation probability p_b = 0.45 (up to L=16). The finite-size behavior
of the quartic cumulants at the critical point allows us to check very
accurately that these models belong to the same universality class. Moreover,
it allows us to estimate the scaling-correction exponent \omega related to the
leading irrelevant operator: \omega=1.0(1). Shorter Monte Carlo simulations of
the bond-diluted bimodal models at p_b=0.7 and p_b=0.35 (up to L=10) and of the
Ising spin-glass model with Gaussian bond distribution (up to L=8) also support
the existence of a unique Ising spin-glass universality class. A careful
finite-size analysis of the Monte Carlo data which takes into account the
analytic and the nonanalytic corrections to scaling allows us to obtain precise
and reliable estimates of the critical exponents \nu and \eta: we obtain
\nu=2.45(15) and \eta=-0.375(10)
Masking Patterns in Sequences: a New Class of Motif Discovery with Don't Cares
No full textIn this paper, we introduce a new notion of motifs, called
\emph{masks}, that succinctly represent the repeated patterns for an
input sequence of symbols drawn from an alphabet .
We show how to build the set of all maximal masks of length~ and
quorum~, in time and space in the worst case. We
analytically show that our algorithms perform better than
constant-time enumerating and checking all the potential
candidate patterns in~ after a polynomial-time
preprocessing of . Our algorithms are also cache-friendly,
attaining block transfers, where
is the cache oblivious complexity of sorting
items
Hyperbolic-parabolic singular perturbation for mildly degenerate Kirchhoff equations: time-decay estimates
No full textWe consider the second order Cauchy problem and the first order limit problem where , is a Hilbert space, is a self-adjoint nonnegative operator on with dense domain , and is a function of class . We prove decay estimates (as ) for solutions of the first order problem, and we show that analogous estimates hold true for solutions of the second order problem provided that is small enough. We also show that our decay rates are optimal in many cases. The abstract results apply to parabolic and hyperbolic partial differential equations with non-local nonlinearities of Kirchhoff type
Optical pumping and vibrational cooling of molecules
No full textSUMMARY The methods producing cold molecules from cold atoms tend to leave molecular
ensembles with substantial residual internal energy. For instance, Cs2
molecules initially formed via photoassociation of cold Cs atoms are in several
vibrational levels, v, of the electronic ground state. Here we apply a
broadband femtosecond laser that redistributes the vibrational population in
the ground state via a few electronic excitation - spontaneous emission cycles.
The laser pulses are shaped to remove the excitation frequency band of the v =
0 level, preventing re-excitation from that state. We observe a fast and
efficient accumulation, about 70% of the initially detected molecules, in the
lowest vibrational level, v = 0, of the singlet electronic state. The validity
of this incoherent depopulation pumping method is very general and opens
exciting prospects for laser cooling and manipulation of molecules
Trap modulation spectroscopy of the Mott-insulator transition in optical lattices
No full textWe introduce a new technique to probe the properties of an interacting cold
atomic gas that can be viewed as a dynamical compressibility measurement. We
apply this technique to the study of the superfluid to Mott insulator quantum
phase transition in one and three dimensions for a bosonic gas trapped in an
optical lattice. Excitations of the system are detected by time-of-flight
measurements. The experimental data for the one-dimensional case are in good
agreement with the results of a time-dependent density matrix renormalization
group calculation