Pion correlations and resonance effects in \bar{p}p annihilation to 4\pi^0 at rest

We study correlations in the exclusive reaction at rest with complete reconstruction of the kinematics for each event. The inclusive distribution is fairly flat at small invariant mass of the pion pair while a small enhancement in the double differential distribution is observed for small invariant masses of both pion pairs. Dynamical models with resonances in the final state are shown to be consistent with the data while the stochastic HBT mechanism is not supported by the present findings. Received: 26 February 2002 / Revised version: 22 July 2002 / Published online: 30 August 2002     

N\bar N annihilation into two body channels

A simple model is constructed forN¯N annihilations into two non-strange mesons. Determining the ratio of isospin one to isospin zero annihilation by experimental information we predict the branching ratios for these channels. The agreement with experiment is surprisingly good.     

INTERACTION OF ENZYME-GENERATED SPECIES WITH CHLOROPHYLL-a AND PROBES BOUND TO SERUM ALBUMINS

Abstract— The interaction of serum albumin-bound acceptors with enzyme-generated and protected triplet species was studied in two types of systems. Chlorophyll-a bound to bovine and human serum albumins is efficiently excited by enzymatically generated triplet acetone and acetaldehyde. When the Chl-a concentration is much lower than that of the albumin the interaction occurs with chlorophyll in an aggregate in which one Chl-a is surrounded by several protein molecules. When the Chl-a concentration is higher than that of the protein, the aggregate contains the proteins and fluorescent chlorophylls in a 1:1 ratio. The excess chlorophylls, although able to interact with the donors, are not fluorescent.
In another study, probes bound to various specific sites of serum albumins were used as quenchers of the enzymatically generated triplet acetone. The efficiency of quenching by all the bound probes is equal and in one case even stronger than for the free probes.
A model for the interaction of the excited species contained in the enzyme with the acceptor(s) located in the protein is proposed.
The present results provide further evidence that enzyme-generated and protected triplet carbonyl species can interact through a collisional process with acceptors bound to or constituents of macromolecules.     

Übergangsmetall—methylen-komplexe: LXI. Übergangsmetall—vinyliden-komplexe: Neuartige synthese aus diazoalken-vorstufen

The diazoolefines of composition N₂CCR₂ (R/R = CH₃/CH₃ and(-CH₂-)₅) are suitable precursors of the corresponding vinylidene ligands CCR₂. Thus, treatment of the RhRh complex [(η⁵-C₅Me₅)Rh(μ-CO)]₂ (1) with the N-nitrosourethanes 2a and 2b, resp., in the presence of lithium t-butoxide yields the otherwise inaccessible μ-vinylidene complexes (μ-CCR₂)[(η⁵-C₅Me₅)Rh(CO)]₂ (R = CH₃ (3a), R,R = (-CH₂-)₅ (3b)). The analogous cobalt compound (μ-CCMe₂)[(η⁵-C₅Me₅)Co(CO)]₂ (5a) is obtained similarly. This procedure extends the well-documented diazoalkane method for the synthesis of μ-alkylidene complexes to the less stable diazoalkenes. A single-crystal X-ray diffraction study of the dimethylvinylidene derivative 3a shows the CMe₂ ligand to adopt an almost symmetrically metal-bridging position (d(RhC) 197.8(1) and 204.3(1) pm), with a rhodium-rhodium single bond completing a three-membered Rh₂C-metallacycle (d(RhRh) 268.4(0) pm) analogous with cyclopropane.     

Moment characterization of localized orbitals

The parallel between orbital first and second electric moments and statistical first and second central moments is noted. Three measures of orbital spatial distribution in terms of their moments are proposed, and applied to the LMO's in a series of ten-electron hydrides. Consistent differences between bond and lone pair distributions are found. Using the statistical interpretation, for each LMO an effective solid angle around the central atom is postulated.     

Synthesis and properties of crosslinked polyvinylformamide and polyvinylamine hydrogels in conjunction with silica particles

Polyvinylamine hydrogels with silica particles encapsulated (PVAm/silica) were produced by a two‐step synthesis. In the first step, polyvinylformamide/silica (PVFA/silica) hybrids were synthesized from vinylformamide (VFA) and 1,3‐divinylimidazolidin‐2‐one (1,3‐bisvinylethyleneurea, BVU), as the crosslinker, by radical copolymerization in silica/water suspensions using different compositions of VFA/BVU. The target product PVAm/silica was obtained by acidic hydrolysis of the PVFA/silica hydrogels in a second step. The chemical structures of both hydrogels, PVFA/silica and PVAm/silica, respectively, were revealed by solid‐state ¹³C(¹H) cross‐polarity/magic‐angle spinning NMR spectroscopy. Both hydrogels swelled significantly in water. The swelling capacity of the two systems was characterized by the correlation length ξ (or hydrodynamic blob size) of the network meshes with small‐angle neutron scattering experiments. ξ is significantly larger for PVAm/silica than for PVFA/silica, which corresponds to the observed higher swelling capacity of this polyelectrolyte material. Furthermore, the swelling behavior of the hybrid hydrogels was quantitatively described in terms of free swell capacity, centrifuge‐retention capacity, adsorption against pressure, and free swell rate as compared with values of the corresponding copolymer hydrogels. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3144–3152, 2002     

Transferability of some properties of localized molecular orbitals

The transferability of the first and second order electric moments of localized orbitals have been shown. Some moment characteristics have also been investigated. The standard deviation of first order localized moments is less than 2.0%. The standard deviation of second order electric moment components are larger: although they do not exceed 4.0% for bond orbitals, for lone pair ones they vary about 1.5–10.1%.     

Mechanistic insights into the photochromism of trans-10b,10c-dimethyl-10b,10c-dihydropyrene derivatives

A series of dimethyldihydropyrene derivatives was studied to elucidate the photochemical mechanism associated with the switching between the dimethyldihydropyrene (DHP, closed) and metacyclophanediene (CPD, open) forms of the molecule. Quantum yields of ring opening and closure, fluorescence quantum yields and lifetimes, as well as laser flash photolysis studies were performed to establish the effect of substituents on the switching efficiency. Ring opening of the DHPs occurs from the first singlet excited state. The low quantum yields for the ring opening reaction observed (< or =0.042) are a consequence of the low rate constant (< or =1.7 x 10(7) s(-1)) for this process. The quantum yields for ring closure of the CPD were determined for select compounds and were of the order of 0.1-0.4. These results show that the efficiency for ring opening of this class of compounds is intrinsically low, but can be modulated to some extent by the introduction of substituents. These properties should be taken into account when considering what type of photoswitching devices DHPs might be useful for.