Conformational interconversions in supersonic jets: Matrix IR spectroscopy and model calculations

Terminal conformer populations in supersonic molecular beams have been measured by use of matrix IR spectroscopy. the experimental technique is based on trapping of the beam molecules in to a cryogenic matrix. The ratio of conformational isomers is determined by comparing intensity ratios of infrared absorption bands with those found in analogous experiments with thermal effusive molecular beams. supersonic beams of pure 1,2-difluoroethane a considerable depopulation fo the less stable trans conformer is found, the lowest terminal conformational temperature reached being T_c^t8 = 207 (3) K. In seeded argon beams the cooling of the conformational distribution was found to be weaker. In supersonic beams of 1,2-dichloroethane and of methyl nitrite no significant conformational cooling was found. The experimental results are discussed in terms of a kinetic model of conformational interconversion in the flow field of a continuum free jet. The calculation indicate that conformational cooling by supersonic expansion may be expected only for molecules with a low energy barrier to internal rotation.     

Coisotropic submanifolds in Poisson geometry and branes in the Poisson sigma model

General boundary conditions (branes') for the Poisson sigma model are studied. They turn out to be labeled by coisotropic submanifolds of the given Poisson manifold. The role played by these boundary conditions both at the classical and at the perturbative quantum level is discussed. It turns out to be related at the classical level to the category of Poisson manifolds with dual pairs as morphisms and at the perturbative quantum level to the category of associative algebras (deforming algebras of functions on Poisson manifolds) with bimodules as morphisms. Possibly singular Poisson manifolds arising from reduction enter naturally into the picture and, in particular, the construction yields (under certain assumptions) their deformation quantization.     

Modular covariance of minimal model correlation functions

We prove that one-point functions of all scaling fields in minimal left-right diagonal models of conformal field theory are modular covariant. This consistency condition should allow one to extend these minimal models to Riemann surfaces of arbitrary genus.Supported in part by NSF grant DMS 8610730     

Structure and low temperature properties of SrB6

A detailed study of the room temperature structure of high quality single crystalline SrB6 has been made. Minute changes in interatomic distances may drastically affect the electronic spectrum of this compound. Electronic transport properties both at zero and non-zero frequencies above ⁴He temperatures indicate that SrBP₆ is close to be a semiconductor. However, at very low temperatures this compound enters a metallic state with a low concentration of itinerant charge carriers. Possible correlation effects are indicated by anomalous temperature dependences of the electrical resistivity and the specific heat below 1 K.     

Amphiphilic Fullerene‐Cholesterol Derivatives: Synthesis and Preparation of Langmuir and Langmuir‐Blodgett Films

Amphiphilic fullerene bis‐adducts 11 and 14 containing two and four cholesterol moieties, respectively, were prepared starting from the corresponding bis‐malonate derivatives. In a systematic study, their spreading behavior at the air‐water interface was compared to that of bis‐adduct 6 with no polar head‐group. Compared to 6 , for which some three‐dimensional aggregation occurs, the polar head‐group in 11 and 14 is responsible for an attractive interaction with the aqueous subphase, forcing the molecules towards the water surface into a two‐dimensional arrangement. Even if homogeneous Langmuir films were obtained with both 11 and 14 , only the films of 14 show a reversible compression/expansion behavior. This suggests that, by increasing the number of cholesterol subunits, the encapsulation of the C‐sphere in its addend is more efficient, thus preventing fullerene‐fullerene interactions and aggregation phenomena. The Langmuir films of 11 and 14 were also efficiently transferred onto hydrophilic quartz slides, yielding Langmuir‐Blodgett films.     

Salimabromide: Unexpected Chemistry from the Obligate Marine Myxobacterium Enhygromxya salina

Marine myxobacteria (Enhygromyxa, Plesiocystis, Pseudoenhygromyxa, Haliangium) are phylogenetically distant from their terrestrial counterparts. Salimabromide is the first natural product from the Plesiocystis/Enhygromyxa clade of obligatory marine myxobacteria. Salimabromide has a new tetracyclic carbon skeleton, comprising a brominated benzene ring, a furano lactone residue, and a cyclohexane ring, bridged by a seven‐membered cyclic moiety. The absolute configuration was deduced from experimental and calculated CD data. Salimabromide revealed antibiotic activity towards Arthrobacter cristallopoietes.     

Topological representations of the quantum groupU q(sl 2)

We define a topological action of the quantum groupU _q(sl ₂) on a space of homology cycles with twisted coefficients on the configuration space of the punctured disc. This action commutes with the monodromy action of the braid groupoid, which is given by theR-matrix ofU _q(sl ₂).Currently visiting the Institute for Theoretical Physics, University of California, Santa Barbara, CA 93106, USA. Supported in part by the NSF under Grant No. PHY89-04035, supplemented by funds from the NASA