New chiral phosphine-phosphite ligands in the enantioselective palladium-catalyzed allylic alkylation

A series of chiral phosphine-phosphite ligands 1-6 have been synthesized and used in the enantioselective palladium-catalyzed reaction of rac-1,3-diphenyl-2-propenyl acetate with dimethyl malonate as nucleophile. Ligands 1a, 2, 3, 5a, 6a, and 6b have been synthesized starting from racemic tert-butylphenylphosphinoborane. The use of dynamically resolved Li phosphide (-)-sparteine provided the optically pure ligands. Crystals of the allylpalladium (6a) complex were obtained, suitable for X-ray crystal structure determination. The X-ray crystal structure of the allylpalladium (6a) complex revealed a longer palladium-carbon bond distance trans to the phosphine moiety indicating that the attack of the nucleophile takes place at the carbon trans to the phosphine moiety. This was confirmed by the fact that the phosphine moiety did not affect the enantioselectivity directly. Under mild reaction conditions, enantioselectivities up to 83% were obtained (25 degrees C) with ligand 1e. Systematic variation of the ligand bridge and the phosphite moiety showed that the configuration of the product is controlled by the atropisomerism of the biphenyl substituent at the phosphite moiety. The conformation of the biphenyl group, in turn, is controlled by the substituent at the chiral carbon in the bridge. Ligands with large bite angles yielded higher enantioselectivities.     

Electromagnetic coupling in near-field scattering by small homogeneous and heterogeneous nanoaggregates

Progress in near-field optical spectroscopy research on metal nanoparticles demands a better understanding of the role played by particle-particle interactions and a deeper insight of the influence of the incident field wavelength. This is particularly true for scanning near-field optical microscopy (SNOM), where the mechanism by which some components of the evanescent illuminating field are transformed into propagating field components that carry information about the sample is at the core of the image formation and where the role played by the interactions between sample and tip remains a still open problem. In this perspective, we investigate numerically the optical behavior of small aggregates of spherical nanoparticles, taking into account the electromagnetic coupling between all particles and the apertureless tip. The tip is modeled as a sphere made of different materials characterized by appropriate dielectric functions. We find that the tip material affects both qualitatively and quantitatively the SNOM images; more important, from the analysis of the calculated scattering cross section, the resonance plasmon location of the whole (aggregate + tip) system undergoes detectable changes, if the tip is constituted of the same material of the sample, as the tip is situated in different positions. This modification of the plasmon frequencies induces a nontrivial variation of the near-field intensity as a function of the tip position and the resulting SNOM image can be distorted with respect to the actual shape of the sample. No simple arguments can be used to relate the value of the local field on the tip surface to the scattering cross section value; depending on the tip material, the comparison between these two measurements can help to clarify the role of basic interactions in the scattering mechanism.     

Intercalation of Benzamide into Kaolinite

Well-crystallized kaolinite (K) was initially reacted at 60 degrees C with a water/dimethylsulfoxide (DMSO) mixture and the resulting intercalation derivative (K-DMSO) was characterized by powder X-ray diffractometry (PXRD), thermal analysis (simultaneous TG and DSC), and Fourier-transformed infrared spectroscopy (FTIR). Benzamide crystals were then melted with the K-DMSO derivative at 140 degrees C for 4 days, when a gradual displacement of DMSO by benzamide was observed within the interlayer spacing of the modified kaolinite. The resulting material, after extensive washing with acetone, was characterized and compared to the results obtained previously for the K-DMSO composite. Benzamide intercalation proceeded by gradual displacement of DMSO molecules until completion. The structural stabilization of the K-BZ derivative was explained through the establishment of hydrogen bonds between the carbonyl oxygen atoms of the intercalated benzamide and aluminol groups present at the surface of the kaolinite layer. The interlamellar spacing of K-BZ was shown to be possibly occupied by benzamide molecules that were located at a 68 degrees orientation in relation to the layer surface. Unlike most intercalation molecules such as DMSO, variations in the interplanar spacing of kaolinite were consistent with the nonkeying of any other part of the molecule between the aluminosilicate interlayers. Copyright 2000 Academic Press.     

Theoretical study of electromagnetic scattering by metal nanoparticles

Progress in near-field optical spectroscopy research on metal nanoparticles demands a better understanding of the role of particle-particle and tip-sample interactions. In this perspective, we investigate theoretically, at a very moderate level of sophistication, the optical behavior of simple silver nanoparticle aggregates, in terms of a formalism involving a multipolar expansion of the fields involved, along with a simplified model for the optical behavior of nanostructures previously developed. In particular, the tip-sample interaction is taken into account roughly, treating the tip as an additional, single particle, characterized by proper dielectric behavior.     

Conventionalvs. Gauge-invariant quantum transition probabilities in simple systems. Morse oscillator

Summary Gauge-invariant transition and survival probabilities for a Morse oscillator initially prepared in its ground state are evaluated for the case of intense and short-duration pulses by a nonperturbative approach founded on the solution of the dynamical problem in terms of a Magnus formalism. False probabilities built up from ?conventional”,i.e. gaugedependent, amplitudes are also obtained and contrasted to the true transition probabilities as a test for the difference between them.

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Riassunto Il problema della gauge-invarianza delle probabilità di transizione calcolate in condizioni generali, che è stato riconsiderato da vari autori anche in anni recenti, si esamina dal punto di vista applicativo per il caso di un oscillatore di Morse sottoposto all’azione di un intenso impulso di radiazione di breve durata. Probabilità gauge-invarianti e (false) probabilità, ottenute da ampiezze ?convenzionali?, sono confrontate e discusse.

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Резюме Проблема калибровочной инвариантности вероятности перехода, вычисленной в общем случае, которая рассматривалась рядом авторов, исследуется применительно к осциллятору Морса, взаимодействующему с интенсивным импульсом излучения малой длительности. Сравниваются и обсуждаются калибровочно-инвариантные вероятности и (ошибочные) вероятности, полученные с помощью ?обычных?, т.е. зависящих от калибровки, амплитуд.

     

Use ofL 2-integrable wave functions in molecular photoionization: Application to a simple model

Summary One- and two-photon ionization of a simple model of diatomic molecule, consisting of a single electron moving one-dimensionally in the field of two attractive, fixed Dirac-delta wells, has been investigated in terms ofL ²-integrable wave functions. Emphasis is placed on the calculation of the two-photon ionization amplitude as far as the representability of the resolvent operator by means of aL ²-basis set is concerned. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Evaluation of fourier transform of two-center charge distribution for arbitrary slater-type orbitals

Fourier transform of two-center charge distributions corresponding to arbitrary Slater-type orbitals are evaluated by a Gaussian quadrature procedure without any preliminary series expansion of the integrand. Convergence and accuracy of the method are discussed and illustrated.     

Evaluation of molecular magnetic properties and some related approximations

Small magnetic terms in the molecular Hamiltonian are treated in a SCF perturbation scheme, using H.F. molecular orbitals as basis; some approximations are discussed and compared. In particular, the paramagnetic contributions to the susceptibility and nuclear shielding constants for LiH and N₂ are investigated and discussed.

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Zusammenfassung Es werden verschiedene Näherungen für Störungsrechnungen, kleine magnetische Terme eines molekularen Hamiltonoperatores betreffend, im H. F.-Schema diskutiert, speziell die Rechnungen für LiH- und N₂-Moleküle, die sich auf den paramagnetischen Teil der Suszeptibilität und die Abschirmungskonstante für verschiedene Kerne beziehen.

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Résumé Des approximations différentes qu'on peut choisir dans les calculs perturbatifs des éléments petits d'une Hamiltonienne moléculaire, sur une basis de Hartree-Fock, viennent examinées et comparées. En particulier, on présent les calculs rélatifs á la partie paramagnétique de la susceptibilité et de la constante d'écran nucléaire des molècules LiH et N₂.

     

Static polarizability of an exactly solvable model of diatomic molecule

Summary A very simple one-dimensional model for a one-electron diatomic molecule, under the influence of a weak static electric field, is investigated by perturbation theory. The dipole polarizabilities for the two only bound states supported by this molecular model are evaluated as a function of the internuclear distance.

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Riassunto Si studia perturbativamente l'influenza di un debole campo elettrico statico su un modello monodimensionale semplificato di molecola biatomica contenente un unico elettrone “attivo”. Si calcola la polarizzabilità di dipolo nei due soli stati legati supportati dal modello in questione, mettendo in evidenza la dipendenza dalla distanza internucleare.

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Резюме В рамках теории возмущений исследуется влияние слабого статического злектрического поля на улрощенную одномерную модель двухатомной молекулы с одним “активным” злектроном. Вычисляется дилольная поляризуемость для двух связанных состояний, долускаемых рассматриваемой моделью молекулы, как функция расстояния между ядрами.