Synthesis and near infrared luminescence of a tetrametallic Zn2Yb2 architecture from a trinuclear Zn3L2 Schiff base complex

Near infrared luminescence is observed in tetrametallic [Zn2Yb2L2(mu-OH)2Cl4].2MeCN which is obtained from the Zn3 Schiff-base complex [Zn3L2(NO3)2].MeOH, (H2L =N,N'-bis(5-bromo-3-methoxysalicylidene)propylene-1,3-diamine).     

Zur Ermittlung der Äthylen-Sequenzlängenverteilung und der chemischen Uneinheitlichkeit in Copolymeren mit Propylen durch Pyrolyse-Gas-Chromatographie

Pyrolysis coupled with gas chromatography has been applied to investigate the sequence distribution in copolymers of ethylene and propylene. The copolymers analyzed have been prepared with soluble Ziegler-Natta-catalysts and show different degrees of crystallinity. In one series of samples the methyl group of the propylene units has been labeled with tritium. It was then possible to detect carbons by flame ionization detector and simultaneously the methyl groups by the radio-detector in parallel. Within the errors of these methods the results from both detectors are consistent with the calculations from kinetics. The accuracy of the data is best in copolymers with long ethylene sequences. By GPC fractionation a heterogeneity in the chemical composition can be observed. One component of the copolymers is of high molecular weight, while the other has a molecular weight of less than 1000 and is very rich in ethylene. The influence of chemical heterogeneity on sequence distribution derived from kinetics and pyrolysis has been discussed. A mechanism in which chains with different chemical composition are growing from different species of the catalyst has been proposed. The experimental results show that the heterogeneity is not critical for the sequence analysis by kinetics and pyrolysis.     

Effect of solution components on the termination mechanism in acrylamide microemulsion polymerizations

A study of the effect of the various solution components on the kinetics of the polymerization of acrylamide in water/oil (w/o) microemulsions has been performed. For the polymerizations with toluene as the continuous phase, both the rate of polymerization, R_p, and the molecular weight of the polyacrylamide were found to be first order in monomer concentration. Furthermore, for the low temperatures (10°C) involved in these experiments, nondegradative chain transfer to monomer appears to be insignificant. When the continuous-phase solvent was changed, an exponential dependence, X, of R_p on the incident light intensity in the order of toluene (X = 1.06) > heptane (X = 0.73) > benzene (X = 0.55) was found. Thus, the monoradical termination found in the toluene microemulsions is likely due to degradative transfer to toluene, forming a stable benzyl radical, while polymerization in benzene (no labile hydrogen atoms) leads to biradical termination     

Spectroscopy of hydrothermal reactions, 19: pH and salt dependence of decarboxylation of α‐alanine at 280–330°C in an FT‐IR spectroscopy flow reactor

The spontaneous decarboxylation of 0.5 m aqueous α‐alanine solutions as a function of pH (1–9 at 320°C, where neutrality is approximately 6) was determined with a flow reactor at 280–330°C and 275 bar by FT‐IR spectroscopy. The kinetics for the cationic and anionic forms have not been previously reported. The rate constants for the cationic form [CH₃(NH₃⁺)CHCO₂H], the anionic form [CH₃(NH₂)CHCO₂^?], and the zwitterion form [CH₃(NH₃⁺)CHCO₂^?] were obtained and followed the first‐order rate law. The rate of decarboxylation of the zwitterion is three times greater than that of the cationic and anionic forms in the temperature and pH ranges of study. The corresponding Arrhenius parameters were determined and compared with previously reported data. The addition of KCl (1 and 2 m) at the natural pH of α‐alanine resulted in a reduction of the decarboxylation rate, suggesting that the transition state is less polar than the zwitterion and/or that the activity of the zwitterion has been reduced. The α‐alanine solution is therefore somewhat more robust in solutions of high ionic strength, such as seawater, than it is in pure water. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 271–277, 2002     

Synthesis of Acyclic Nitroazole Nucleosides and Their Incorporation into Oligonucleotides,and Their Duplex and Triplex Formation

Acyclic nucleosides of 4‐nitro‐1H‐imidazole and 4‐nitropyrazole have been synthesized by nucleophilic addition of the appropriate 4‐nitroazole to (?)‐(S)‐(hydroxymethyl)oxirane in the presence of a catalytic amount of potassium carbonate. (+)‐(R)‐3‐(4‐nitro‐1H‐imidazol‐1‐yl)propane‐1,2‐diol and (+)‐(R)‐3‐(2‐methyl‐4‐nitro‐1H‐imidazol‐1‐yl)propane‐1,2‐diol were also obtained in an independent reaction starting from appropriate 1,4‐dinitro‐1H‐imidazole and (+)‐(R)‐3‐aminopropane‐1,2‐diol. (+)‐(R)‐3‐(4‐Nitropyrazol‐1‐yl)propane‐1,2‐diol was also obtained by direct noncatalyzed addition of 4‐nitropyrazole to (?)‐(S)‐(hydroxymethyl)oxirane, whereas the (S)‐enantiomer was obtained by reaction of 4‐nitropyrazole with (+)‐(S)‐1,2‐O‐isopropylideneglycerol under Mitsunobu reaction conditions, followed by a cleavage of the isopropylidene group with 80% AcOH. Racemization during any of these syntheses has not been observed. 3‐(4‐Nitroazol‐1‐yl)propane‐1,2‐diols were incorporated into a 26‐mer oligonucleotide. UV Thermal melting studies of duplexes of the oligonucleotides with 4‐nitropyrazole or 4‐nitro‐1H‐imidazole paired with four natural bases showed moderately decreased stabilities of the duplexes. A narrow range of melting temperatures, typically being within 2° for each acyclic nucleoside, fulfill one of the requirements of using acyclic 4‐nitroazoles as general bases. Single incorporation of 4‐nitroazoles into a 14‐mer triplex forming oligonucleotide resulted in considerably decreased triplex stabilities.     

Use of U-shaped donor-bridge-acceptor molecules to study electron tunneling through nonbonded contacts

A systematic determination of electronic coupling matrix elements in U-shaped molecules is demonstrated. The unique architecture of these systems allows for the determination of the electronic coupling through a pendant molecular moiety that resides between the donor and acceptor groups; this moiety quantifies the efficiency of electron tunneling through nonbonded contacts. Experimental electron-transfer rate constants and reaction free energies are used to calibrate a molecular-based model that describes the solvation energy. This approach makes it possible to experimentally determine electronic couplings and compare them with computational values.     

Model complexes for metallated polythiophenes: gold(I) and palladium(II) complexes of bis(diphenylphosphino)oligothiophenes

The preparations of two new phosphinothiophene ligands, 3,3'-bis(diphenylphosphino)-2,2'-bithiophene (dppbt; 1) and 3,3' "-dihexyl-3',3' '-bis(diphenylphosphino)-2,5':2',2' ':5' ',2' "-quaterthiophene (hdppqt; 2) are reported. Oxidation of 1 gives 3,3'-bis(diphenylphosphine oxide)-2,2'-bithiophene (3), and the crystal structure of this compound was determined. Pd(II) and Au(I) complexes of these ligands have been synthesized and characterized. Crystal structures of [(dppbt)PdCl(2)] (1-Pd), [(hdppqt)PdCl(2)] (2-Pd), [(dppbt)(AuCl)(2)] (1-Au), and [(hdppqt)(AuCl)(2)] (2-Au) were obtained. [(dppbt)(AuCl)(2)] crystallized in two solid-state forms; crystals grown from CH(2)Cl(2)/Et(2)O show a gold-gold interaction of 3.3221(4) A, but from CH(2)Cl(2)/toluene, the molecule crystallizes as a toluene adduct (1-Au-tol) and does not show any gold-gold interaction. All the complexes were characterized via UV-vis spectroscopy and cyclic voltammetry, and the effect of the metal on the energy of the pi-pi transition and oxidation potential was determined. These data are correlated to the interannular torsion angles in the oligothienyl groups from the crystal structure studies.     

Mechanism of thermal energy charge transfer reactions: rare gas ions reacting with NH3 and PH3

Absolute rate constants and approximate product distributions are presented for the reactions of He⁺, Ne⁺, Ar⁺, Kr⁺, Xe⁺, CO⁺ and CO⁺₂ with NH₃ and PH₃. In all cases, electron transfer is the dominant reaction channel. Hydrogen atom transfer is observed in several systems, but only as a minor product, even when this channel is very exothermic. The magnitude of the absolute rate constants can be correlated with the Franck-Condon factors associated with the reactions in most cases. Several exceptions to this general rule are observed that could not be readily predicted a priori. It is speculated that these reactions proceed via a collision complex.     

Dendrimeric organochalcogen catalysts for the activation of hydrogen peroxide: origins of the "dendrimer effect" with catalysts terminating in phenylseleno groups

Several scenarios were evaluated to explain the large "dendrimer effect" observed in the bromination of cyclohexene with H(2)O(2) and NaBr catalyzed by the addition of Frechét-type dendrimers terminating in -O(CH(2))(3)SePh groups. Although phenylseleninic acid was an efficient catalyst for the oxidation of NaBr with H(2)O(2), first-order rate constants for the selenoxide elimination were too small to produce PhSeO(2)H at a rate sufficient to explain the rates of catalysis and no dendrimer effect was observed in the rates of selenoxide elimination. An induction period was observed using 1-SePh as a catalyst for the oxidation of Br(-) with H(2)O(2). The addition of preformed selenoxide 1-Se(=O)Ph gave immediate catalysis with no induction period. However, rates of oxidation of the selenides with H(2)O(2) under homogeneous or biphasic conditions or with t-BuOOH under homogeneous conditions were too slow to account for the rates of catalysis, and no dendrimer effect was observed in the rates of oxidation. The primary oxidant for converting selenides to selenoxides was "Br(+)" produced initially by the uncatalyzed background reaction of H(2)O(2) with NaBr and then produced catalytically following formation of selenoxide groups. Autocatalysis is observed, and the rate of oxidation increases with the number of SePh groups. Autocatalysis is the source of the large dendrimer effect observed with the SePh series of catalysts.     

Flash photolysis resonance fluorescence investigation of the reactions of OH radicals with OCS and CS2

Rate constants for the reaction of OH radicals with OCS and CS₂ have been determined at 296 K using the flash photolysis resonance fluorescence technique. The values derived from this study are k_{OH + OCS} = (5.66 ± 1.21) × 10^?14 cm³ molecule^?1 s^?1 and k_{OH + CS2} = (1.85 ± 0.34) × 10^?13 cm³ molecule^?1 s^?1, where the uncertainties are 95% confidence limits making allowance for possible systematic errors.