The Dewar Isomer of 1,2‐Dihydro‐1,2‐azaborinines: Isolation,Fragmentation, and Energy Storage

The photochemistry of 1,2‐dihydro‐1,2‐azaborinine derivatives was studied under matrix isolation conditions and in solution. Photoisomerization occurs exclusively to the Dewar valence isomers upon irradiation with UV light (>280 nm) with high quantum yield (46 %). Further photolysis with UV light (254 nm) results in the formation of cyclobutadiene and an iminoborane derivative. The thermal electrocyclic ring‐opening reaction of the Dewar valence isomer back to the 1,2‐dihydro‐1‐tert‐butyldimethylsilyl‐2‐mesityl‐1,2‐azaborinine has an activation barrier of (27.0±1.2) kcal mol^?1. In the presence of the Wilkinson catalyst, the ring opening occurs rapidly and exothermically (ΔH=(?48±1) kcal mol^?1) at room temperature.     

Determination of the strange quark content of the nucleon from a next-to-leading-order QCD analysis of neutrino charm production

We present the first next-to-leading-order QCD analysis of neutrino charm production, using a sample of 6090 ^– and -induced opposite-sign dimuon events observed in the CCFR detector at the Fermilab Tevatron. We find that the nucleon strange quark content is suppressed with respect to the non-strange sea quarks by a factor =0.477 _–0.053 ^+0.063 , where the error includes statistical, systematic and QCD scale uncertainties. In contrast to previous leading order analyses, we find that the strange seax-dependence is similar to that of the non-strange sea, and that the measured charm quark mass,m _c =1.70±0.19 GeV/c², is larger and consistent with that determined in other processes. Further analysis finds that the difference inx-distributions betweenxs(x) and is small. A measurement of the Cabibbo-Kobayashi-Maskawa matrix element |V _cd |=0.232 _–0.020 ^+0.018 is also presented.     

Laminated morphology of nontwisting beta-sheet fibrils constructed via peptide self-assembly

A synthetic peptide has been de novo designed that self-assembles into beta-sheet fibrils exhibiting a nontwisted, stacked morphology. The stacked morphology is constituted by 2.5 nm wide filaments that laterally associate to form flat fibril laminates exceeding 50 nm in width and micrometers in length. The height of each fibril is limited to the length of exactly one peptide monomer in an extended beta-strand conformation, approximately 7 nm. Once assembled, these highly ordered, 2-D structures are stable over a wide range of pH and temperature and exhibit characteristics similar to those of amyloid fibrils. Furthermore, the rate of assembly and degree of fibril lamination can be controlled with kinetic parameters of pH and temperature. Finally, the presence of a diproline peptide between two beta-sheet-forming strands in the peptide sequence is demonstrated to be an important factor in promoting the nontwisting, laminated fibril morphology.     

A study of wrong-sign single muon production inv μ-nucleon interaction

We report on a search forv _μ-induced events where the single emerging muon carries lepton number opposite that of the incident neutrino. The rate and kinematic quantities of the candidate events are compared with known backgrounds from \(\bar v_\mu \) -induced charged current interactions and ν-induced interactions that produce dileptons. We derive an upper limit on the rate of wrong-sign single muon production relative to the rate ofv _μ charged current interactions to be 1.6×10^?4 fory<0.5 and 3.1×10^?4 fory>0.5 (90% CL). These upper limits enable us to constrain exotic sources of wrong-sign muons such as the charm component of the nucleon sea, flavor changing neutral currents and lepton number violating processes. Finally, the rate and kinematic properties of these events are compared with those of the neutrino-induced opposite-sign dimuon events.     

在18-冠-6存在下苯乙烯无乳化剂乳液聚合

本文研究了在苯乙烯不外加乳化剂的乳液聚合体系中,加入王冠醚18-冠-6对聚合反应及其产物的影响。发现18-冠-6能起到相转移催化作用,并提出两相粒子成核机理,对实验结果作出了合理的解释。     

Advantages of a two-detector system in automated substoichiometric radioisotope dilution analysis

Automation widens the scope of substoichiometric radioisotope dilution analysis. This is because the very strict requirement of the manual method-chemical separation of exactly equal quantities of the test substance-need no longer be fulfilled: reproducibility of the determination is reached by means of automated operation and activity measurement. The theory given in this paper shows how the choice of suitable chemical reactions is widened and why the reliability and the advantages of isotope dilution analysis are secured by the use of a two-detector system.     

Modeling styrene-styrene interactions

This study is the first step in the systematic investigation of substituted (carboxyl) polystyrene nanoparticles. Understanding the fundamental interactions between the p-carboxyl styrene monomers, where an ethyl group is used instead of a vinyl group (referenced, for convenience, as "p-carboxyl styrene"), provides the basic information needed to construct potentials for nanoparticles composed of these monomers. In this work, low-energy isomers of p-carboxyl styrene dimer were studied. The dimer structures and their relative and binding energies were determined using both M?ller-Plesset second-order perturbation theory (MP2) and the general effective fragment potential (EFP2) method. Sections of the intermolecular potential energy surface (PES) of the p-carboxylated styrene dimer in its global minimum orientation were also determined. As expected, double hydrogen bonding between the two carboxylic groups provides the strongest interaction in this system, followed by isomers with a single H-bond and strong benzene ring-benzene ring (pi-pi) type interactions. Generally, the EFP2 method reproduces the MP2 geometries and relative energies with good accuracy, so it appears to be an efficient alternative to the correlated ab initio methods, which are too computationally demanding to be routinely used in the study of the more-complex polymeric systems of interest.     

PKa of cytosine on the third strand of triplex DNA: Preliminary Poisson–Boltzmann calculations

The energetics of formation of a triple-helical structure in homopurine–homopyrimidine mixtures has been modeled using Poisson–Boltzmann calculations. Oligomers with the sequence d(TC)_n and d(AG)_n form hydrogen-bonded triple-helical structures of the form d(TC)_n⋅d(AG)_n⋅d(TC⁺)_n. The third base, a pyrimidine in this case, forms Hoogsteen-type hydrogen bonds with the purine, requiring that the cytosine residues of the third strand protonate at N3. The pK_a of cytosine, 4.3 in the isolated solvated molecule, is raised by the strong electrostatic field in the triple helix. We have done calculations of the effective pK_a of this cytosine and compared the results with experimental studies of triple-helix formation as a function of pH. This provides a test of various models of the dielectric constant for triplex DNA and its local environment. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 1177–1184, 1998