A method for calibrating the relative gamma-ray light yield of plastic scintillators.

Currently we are investigating the inclusion of organotin compounds in new polystyrene scintillator materials to improve full gamma-ray energy sensitivity. Accurate calibration of the relative light yield from the newly developed scintillators is crucial to assess merits of compounds and chemical processes used in the scintillators' development. The full energy gamma-ray peak in a measured gamma-ray spectrum is commonly used in calibrating the relative light yield.

Electronic Properties of Vinylene-Linked Heterocyclic Conducting Polymers: Predictive Design and Rational Guidance from DFT Calculations.

The band structure and electronic properties in a series of vinylene-linked heterocyclic conducting polymers are investigated using density functional theory (DFT). In order to accurately calculate electronic band gaps, we utilize hybrid functionals with fully periodic boundary conditions to understand the effect of chemical functionalization on the electronic structure of these materials. The use of predictive first-principles calculations coupled with simple chemical arguments highlights the critical role that aromaticity plays in obtaining a low band gap polymer.

Improved synthesis of bis(borano)hypophosphite salts.

A synthesis of the bis(borano)hypophosphite anion with various counterions has been developed to make use of more benign and commercially available reagents. This method avoids the use of potentially dangerous reagents used by previous methods and gives the final products in good yield. Details of the crystal structure determination of the sodium salt in space group Ama2 are given using a novel computational technique combined with Rietveld refinement.

Coumarin dyes for dye-sensitized solar cells: A long-range-corrected density functional study.

The excited-state properties in a series of coumarin solar cell dyes are investigated with a long-range-corrected (LC) functional which asymptotically incorporates Hartree-Fock exchange. Using time-dependent density functional theory (TDDFT), we calculate excitation energies, oscillator strengths, and excited-state dipole moments in each of the dyes as a function of the range-separation parameter mu. To investigate the acceptable range of mu and to assess the quality of the LC-TDDFT formalism, an extensive comparison is made between LC-BLYP excitation energies and approximate coupled-cluster singles and doubles calculations.

A synthetic cycle for the ruthenium-promoted formation of 1H-phosphindoles from phosphaalkynes.

Beginning with inexpensive and commercially available starting materials, a rational synthesis for the new phosphaalkyne Ph3C-C[triple bond]P (1) is presented. Coordination of 1 to group 8 transition metal centers furnishes the eta1-complexes [MH(dppe)2(Ph3CC[triple bond]P)]OTf, where M = Fe (3) or Ru (4) (dppe = bis-1,2-diphenylphosphinoethane). Treatment of 3 or 4 with a strong acid cyclizes the coordinated phosphaalkyne and is the first example of an electrophilic aromatic substitution reaction in which the electrophile is a low coordinate phosphorus.

Dissociation of carbanions from acyl iridium compounds: an experimental and computational investigation.

Instead of reductive elimination of aldehyde, or decarbonylation to give a trifluoroalkyl hydride, heating Cp(PMe(3))Ir(H)[C(O)CF(3)] (1) leads to the quantitative formation of Cp(PMe(3))Ir(CO) (2) and CF(3)H. Kinetic experiments, isotope labeling studies, solvent effect studies, and solvent-inclusive DFT calculations support a mechanism that involves initial dissociation of trifluoromethyl anion to give the transient ion-pair intermediate [Cp(PMe(3))Ir(H)(CO)](+)[CF(3)](-). Further evidence for the ability of CF(3)(-) to act as a leaving group came from the investigation of the analogous methyl and chloride derivatives Cp(PMe(3))Ir(Me)[C(O)CF(3)] and Cp(PMe(3))Ir(Cl)[C(O)CF(3)].

Elimination of R-H from iridium acyl hydrides without loss of CO: evidence for the intervention of metal cation/carbanion pairs.

Instead of reductive elimination of aldehyde, or decarbonylation to give a trifluoroalkyl hydride, heating Cp*(PMe3)Ir(H)[C(O)CF3] leads to the quantitative formation of Cp*(PMe3)Ir(CO) and CF3H. Kinetic experiments, isotope-labeling studies, solvent effect studies, and DFT calculations support a mechanism which involves dissociation of trifluoromethyl anion to give the transient ion-pair intermediate [Cp*(PMe3)Ir(H)(CO)]+[CF3]-. Further evidence for the ability of CF3 to act as a leaving group came from investigation of the analogous methyl and chloride derivatives Cp*(PMe3)Ir(Me)[C(O)CF3] and Cp*(PMe3)Ir(Cl)[C(O)CF3].

Synthesis of mono-substituted 2,2'-bipyridines.

The rapid synthesis of 4-aryl-2,2'-bipyridines is described leading to some previously reported compounds in good yields in addition to some new functionalized bipyridines.

Chiral recognition phenomena probed by steady-state luminescence measurements: Stern-Volmer analysis of chiral discriminatory behavior.

The chiral recognition phenomenon observed in enantioselective excited-state energy transfer processes currently requires the use of chiroptical spectroscopic techniques to probe the magnitude and sense of the discriminatory interactions. The use of chiroptical spectroscopic techniques limits the study of chiral recognition to those molecular species with strong absorption or emission dissymmetry factors. This study presents the theoretical and experimental methodology to determine the magnitude of chiral discriminatory interactions with unpolarized, steady-state luminescence measurements.

Mortality of sea lions along the central California coast linked to a toxic diatom bloom.

Over 400 California sea lions (Zalophus californianus) died and many others displayed signs of neurological dysfunction along the central California coast during May and June 1998. A bloom of Pseudo-nitzschia australis (diatom) was observed in the Monterey Bay region during the same period. This bloom was associated with production of domoic acid (DA), a neurotoxin that was also detected in planktivorous fish, including the northern anchovy (Engraulis mordax), and in sea lion body fluids.

Zidovudine pharmacokinetics in HIV-positive women during different phases of the menstrual cycle.

Study Objective: To examine the pharmacokinetics of zidovudine during the menstrual cycle in human immunodeficiency virus- (HIV-) positive women.

Design: Open, unblinded study.

Setting: A women's clinic for acquired immunodeficiency syndrome (AIDS) at a large medical center.

Transgenic mice as future tools in risk assessment.

Historically, mice have served a routine and useful purpose in the research, development, and testing of biologicals, chemicals, and drugs for efficacy, toxicity, and carcinogenic risk. The literature is replete with examples using mice to study organic compounds both in short-term tests involving tumor initiation and promotion and in long-term experiments dealing with fertility, reproduction, and teratology. During the past two decades, a virtual explosion of advances has occurred in modern biology that includes the discoveries of retroviruses, oncogenes, DNA restriction enzymes, nucleotide sequence analyses, and microinjection techniques.

Fosfomycin resistance: selection method for internal and extended deletions of the phosphoenolpyruvate:sugar phosphotransferase genes of Salmonella typhimurium.

Selection for resistance to the antibiotic fosfomycin (FOS; L-cis 1,2-epoxypropylphosphonic acid, a structural analogue of phosphoenolpyruvate) was used to isolate mutants carrying internal and extended deletions of varying lengths within the ptsHI operon of Salmonella typhimurium. Strains carrying "tight" ptsI point mutations and all mutants in which some or all of the ptsI gene was deleted were FOS resistant. In contrast, strains carrying ptsH point mutations were sensitive to FOS.

Promoter-like mutation affecting HPr and enzyme I of the phosphoenolpyruvate: sugar phosphotransferase system in Salmonella typhimurium.

A promoter-like mutation, ptsP160, has been identified which drastically reduces expression of the genes specifying two proteins, HPr and enzyme I, of the phosphoenolpyruvate:sugar phosphotransferase system (PTS) in Salmonella typhimurium. This mutation lies between trzA, a gene specifying susceptibility to 1,2,4-triazole, and ptsH, the structural gene for HPr. It leads to a loss of active transport of those sugars that require the PTS for entry into the cell.

Deletion mapping of the genes coding for HPr and enzyme I of the phosphoenolpyruvate: sugar phosphotransferase system in Salmonella typhimurium.

Sugars transported by a bacterial phosphoenolpyruvate:sugar phosphotransferase system (PTS) require two soluble proteins: HPr, a low-molecular-weight phosphate-carrier protein, and enzyme I. The structural genes coding for HPr (ptsH) and Enzyme I (ptsI) are shown to be cotransducible in Salmonella typhimurium. The gene order of this region of the Salmonella chromosome is cysA-trzA-ptsH-ptsI.