Evidence for the decay D0-->K(-)pi(+)pi(-)e(+)nu(e)

Using a 281 pb{-1} data sample collected at the psi(3770) with the CLEO-c detector, we present the first absolute branching fraction measurement of the decay D0-->K(-)pi(+)pi(-)e(+)nu(e) at a statistical significance of about 4.0 standard deviations. We find 10 candidates consistent with the decay D0-->K(-)pi(+)pi(-)e(+)nu(e). The probability that a background fluctuation accounts for this signal is less than 4.1 x 10{-5}. We find B(D0-->K(-)pi(+)pi(-)e(+)nu(e)) = [2.8{-1.1}{+1.4}(stat)+/-0.3(syst)]x10{-4}. By restricting the invariant mass of the hadronic system to be consistent with K1(1270), we obtain the product of branching fractions B(D{0}-->K{1}{-}(1270)e{+}nu{e})xB(K1-(1270)-->K{-}pi{+}pi{-})=[2.5{-1.0}{+1.3}(stat)+/-0.2(syst)]x10{-4}. Using B(K1-(1270)-->K{-}pi{+}pi{-})=(33+/-3)%, we obtain B(D{0}-->K{1}{-}(1270)e{+}nu{e})=[7.6{-3.0}{+4.1}(stat)+/-0.6(syst)+/-0.7]x10{-4}. The last error accounts for the uncertainties in the measured K1-(1270)-->K{-}pi{+}pi{-} branching fractions.     

Target‐Based Whole‐Cell Screening by 1H NMR Spectroscopy

An NMR‐based approach marries the two traditional screening technologies (phenotypic and target‐based screening) to find compounds inhibiting a specific enzymatic reaction in bacterial cells. Building on a previous study in which it was demonstrated that hydrolytic decomposition of meropenem in living Escherichia coli cells carrying New Delhi metallo‐β‐lactamase subclass 1 (NDM‐1) can be monitored in real time by NMR spectroscopy, we designed a cell‐based NMR screening platform. A strong NDM‐1 inhibitor was identified with cellular IC₅₀ of 0.51 μM , which is over 300‐fold more potent than captopril, a known NDM‐1 inhibitor. This new screening approach has great potential to be applied to targets in other cell types, such as mammalian cells, and to targets that are only stable or functionally competent in the cellular environment.     

Determination of dissociation temperature for ArO in inductively coupled plasma-mass spectrometry: Effects of excited electronic states and dissociation pathways

The method of comparing experimental and calculated ion ratios to determine a gas kinetic temperature (T_gas) characteristic of the origin of a polyatomic ion in inductively coupled plasma-mass spectrometry (ICP-MS) is applied to ArO⁺. Repeated measurements of ion ratios involving this species yield erratic T_gas values. Complications arise from the predicted presence of a low-lying excited electronic state (²Π) above the ⁴Σ ground state. Omission of this excited state yields unreasonably high temperatures (> 10,000 K) for nine out of nineteen trials. Inclusion of the excited electronic state in the partition function of ArO⁺ causes temperatures to increase further. The problem appears to be related to the prediction that ArO⁺ in the ²Π excited state dissociates into Ar⁺ and O, different products than ArO^{+ 4}Σ which dissociates into Ar and O⁺. Adjustments to the calculations to account for these different products yield reasonable temperatures (2100 to 3500 K) that are consistent from day-to-day and similar to those seen for other weakly-bound polyatomic ions.     

Stable heteroleptic complexes of divalent lanthanides with bulky pyrazolylborate ligands--iodides, hydrocarbyls and triethylborohydrides

The reaction of YbI(2) with KTp(Me2) gives (Tp(Me2))YbI(THF)(2) (1-Yb) as a thermally unstable product. Use of the more hindered KTp(tBu,Me) gave (Tp(tBu,Me))LnI(THF)(n) (Ln = Sm, n = 2, 2-Sm; Ln = Yb, n = 1, 2-Yb). The crystal structures of both these compounds are reported. Adducts with neutral ligands such as pyridines and isonitriles can be prepared and the crystal structures of [(Tp(tBu,Me))YbIL(n)] (L = CN(t)Bu, n = 1; L = 3,5-lutidine, n = 2) are described. 2-Sm can be oxidized using AgBPh(4) to give [(Tp(tBu,Me))SmI(THF)(2)]BPh(4). Compounds 2-Sm and 2-Yb are useful starting materials for the preparation of heteroleptic compounds by metathesis with appropriate potassium reagents. The preparations and characterization of the hydrocarbyls (Tp(tBu,Me))Ln{CH(SiMe(3))(2)} (Ln = Sm, 5-Sm; Yb, 5-Yb) and [(Tp(tBu,Me))Ln{CH(2)(SiMe(3))}(THF)] (Ln = Yb, 6a-Yb) and the triethylborohydrides [(Tp(tBu,Me))Ln(HBEt(3))(THF)(n)] (Ln = Sm, n = 0, 7-Sm; Yb, n = 1, 7-Yb) are reported, as well as the crystal structures of 5-Sm and 5-Yb, and the THF adducts 6a-Yb and [(Tp(tBu,Me))Sm{CH(SiMe(3))(2)}(THF)], 5a-Sm.     

Comparison of structure and reactivity of phosphine-amido and hemilabile phosphine-amine chelates of rhodium

A series of mono- and binuclear rhodium(I) complexes bearing ortho-phosphinoanilido and ortho-phosphinoaniline ligands has been synthesized. Reactions of the protic monophosphinoanilines, Ph(2)PAr or PhPAr(2) (Ar = o-C(6)H(4)NHMe), with 0.5 equiv of [Rh(μ-OMe)(COD)](2) result in the formation of the neutral amido complexes, [Rh(COD)(P,N-Ph(2)PAr(-))] or [Rh(COD)(P,N-PhP(Ar(-))Ar)] (Ar(-) = o-C(6)H(4)NMe(-)), respectively, through stoichiometrically controlled deprotonation of an amine by the internal methoxide ion. Similarly, the binuclear complex, [Rh(2)(COD)(2)(μ-P,N,P',N'-mapm(2-))] (mapm(2-) = Ar(Ar(-))PCH(2)P(Ar(-))Ar), can be prepared by reaction of the protic diphosphinoaniline, mapm (Ar(2)PCH(2)PAr(2)), with 1 equiv of [Rh(μ-OMe)(COD)](2). An analogous series of hemilabile phosphine-amine compounds can be generated by reactions of monophosphinoanilines, Ph(2)PAr' or PhPAr'(2) (Ar' = o-C(6)H(4)NMe(2)), with 1 equiv of [Rh(NBD)(2)][BF(4)] to generate [Rh(NBD)(P,N-Ph(2)PAr')][BF(4)] or [Rh(NBD)(P,N-PhPAr'(2))][BF(4)], respectively, or by reactions of diphosphinoanilines, mapm or dmapm (Ar'(2)PCH(2)PAr'(2)), with 2 equiv of the rhodium precursor to generate [Rh(2)(NBD)(2)(μ-P,N,P',N'-mapm)][BF(4)](2) or [Rh(2)(NBD)(2)(μ-P,N,P',N'-dmapm)][BF(4)](2), respectively. Displacement of the diolefin from [Rh(COD)(P,N-Ph(2)PAr(-))] by 1,2-bis(diphenylphosphino)ethane (dppe) yields [Rh(P,P'-dppe)(P,N-Ph(2)PAr(-))] which, while unreactive to H(2), reacts readily and irreversibly with oxygen to form the peroxo complex, [RhO(2)(P,P'-dppe)(P,N-Ph(2)PAr(-))], and with iodomethane to yield [RhI(CH(3))(P,P'-dppe)(P,N-Ph(2)PAr(-))]. Hemilabile phosphine-amine compounds can also be prepared by reactions of [Rh(P,P'-dppe)(P,N-Ph(2)PAr(-))] with Me(3)OBF(4) or HBF(4)·Et(2)O, resulting in (thermodynamic) additions at nitrogen to form [Rh(P,P'-dppe)(P,N-Ph(2)PAr')][BF(4)] or [Rh(P,P'-dppe)(P,N-Ph(2)PAr)][BF(4)], respectively. The nonlabile phosphine-amido and hemilabile phosphine-amine complexes were tested as catalysts for the silylation of styrene. The amido species do not require the use of solvents in reaction media, can be easily removed from product mixtures by protonation, and appear to be more active than their hemilabile, cationic congeners. Reactions catalyzed by either amido or amine complexes favor dehydrogenative silylation in the presence of excess olefin, showing modest selectivities for a single vinylsilane product. The binuclear complexes, which were prepared in an effort to explore possible catalytic enhancements of reactivity due to metal-metal cooperativity, are in fact somewhat less active than mononuclear species, discounting this possibility.     

Excited-state processes in first-generation phenyl-cored thiophene dendrimers

First generation dendrimers with three oligothiophene arms (meta-arranged, 3G1-nS) and four arms (ortho- and para-arranged, 4G1-nS) connected to a central phenyl core were investigated spectroscopically in solution. In all dendrimers, on an ultrafast time scale (<10 ps), two "cooling" processes convert the initially generated, "hot" exciton into the geometrically relaxed, "cold" exciton. A decrease in the triplet yield, particularly evident for the 4-arm dendrimers; intersystem crossing rate; and nonradiative triplet decay time with increasing number of bridging thiophene units n all meet with expectations from prior studies on linear oligothiophenes. A relatively fast internal conversion process (>0.6 ns(-1)) is observed in both dendrimer series, possibly due to increased twisting about the phenyl core that reduces the triplet yields considerably with respect to oligothiophenes. An anomalous shifting of the triplet-triplet absorption spectra characterizes the 4G1-nS dendrimers as unique from the 3G1-nS series in terms of the hindrance of torsional motion and confinement of excited states enforced by the arrangement of dendrons.     

A spectroscopic study of the intermolecular photodimerization of 1-(9-anthryl)-3-(1-naphthyl)propane

Single crystals of the intermolecular photodimer of 1-(9-anthryl)-3-(1-naphthyl)propane have been prepared. The photophysical properties of this molecule as well as the sandwich dimer obtained by photocleavage in methylcyclohexane at 77 K have been recorded. A qualitative study of the photochemical properties of the sandwich dimer in single crystals of the photodimer host shows that the photochemical mechanism is similar to those of the anthracene sandwich pair and the anthracene-naphthalene pair.