Browsing by Subject "Exchange reactions"
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Item Application of picosecond time-correlated single photon counting technique to acid-base problems in chemistry(Texas Tech University, 1990-08) Yao, ShihuaThe behavior of the photon-initiated weak base 6-methoxyquinoline was compared in the neat solvents heptane, cyclohexane, acetonitrile, alcohols and water, and also in aqueous two-component solvent mixtures containing alcohol, dioxane and acetonitrile. In water, the strong intramolecular non-radiative decay rate k competes with the intermolecular proton abstraction rate k , . Lifetime and quantum yield measurements at various temperatures show that k , in aqueous solutions is not measurably sensitive to temperature, while k has an activation energy AE of about 1.25 kcal/mol. From the mixed solvent studies, a water cluster (H^O). - is found to be the effective proton donor. The significance of this specific water cluster as a proton donor and acceptor is discussed in terms of the microscopic structure of liquid water and the hydration of OH (H20)^ associated with the weak base. Surprisingly, perhaps, is the fact that proton transfer from a weak acid to water exactly parallels the mechanism of proton transfer to a weak base from water.Item Lewis acid-base exchange reactions of polypnicogens(Texas Tech University, 1984-12) Cribbs, Leonard VThe insertion of a perfluoroalkylphosphinidene moiety RfP from perfluoroalkyclcycloplyphosphines (RfP)n into the pnicogen-pnicogen bond of tetraalkyl dipnicogens appears to be general. Thus, a CFo unit from (CFoP)/ c reacts with R2P-PR2 (where R = Me, Et, Bu) to form the tripnicogen compounds R2P-P(CF3)-PR-2. These tripnicogens undergo neutral water hydrolysis surprisingly easily to form chiral dipnicogens. For example, Me2P-P(CF3)-PMe2 hydrolyzes immediately at ambient temperature with an equimolar quantity of water to yield the new chiral secondary phosphine Me2P-P(CF3)H plus Me2P(O)OH and Me2PH. A second equivalent of water hydrolyzes the dipnicogens to the primary perfluoroalkylphosphine. To further investigate the generality of the "insertion" reaction, cyclic polyarsines were studied to ascertain the possibility of (RfAs)n or (RAs)n providing a source of the arsinidene species RfAs or RAs. Addition of an equimolar quantity of Me2As-AsMe2 to (CF3As)4,5 in C6D6 does form the tripnictide Me2As-As(CF3)-AsMe2. The reaction of a secondary alkyl or aryl phosphine or arsine with the cyclic polyphosphine (CF3P)4,5 or cyclic polyarsine (CF3As)4,5 produces the chiral dipnictides of the type R2E-E(CF3)H, E = P or As. In addition, a number of other products are produced in a complex equilibrium mixture. The origin of the complex equilibrium has been studied by examining the reaction of pairs of reactants and/or products and a reaction mechanism has been deduced. Exchange reactions of the pnicogen-pnicogen bond have also been investigated. Thus, unsymmetrical diphosphines of the type R2P-PR2 typically disproportionate to form the symmetrical species R2P-PR2 and R2P-PR2. However, an exception to this behavior is noted when the relative basicities of the two pnicogen atoms in a dipnicogen differ dramatically. Generality of the Lewis Acid-Base exchange reactions was observed by studying molecules in which P-P bond breakage of the diphosphine was associated with scission of a P-P, P-H, and/or P-X bond in the second reactant. In all cases studied in which reaction occurred, product formation could be explained as proceeding through a four-centered intermediate in which nucleophilic attack by a relatively basic pnicogen atom occurs on a relatively acidic pnicogen atom. A variety of di- and tripnictides were prepared by this method and their NMR parameters determined.Item Structural characterization and enhanced detection of flavonoids by electrospray ionization mass spectrometry and molecular modeling(2004-08) Zhang, Junmei, 1970; Brodbelt, Jennifer S.Structural characterization and isomer differentiation of flavonoids was investigated by gas-phase hydrogen/deuterium (H/D) exchange and various metal complexation approaches using electrospray ionization quadrupole ion trap mass spectrometry (ESI-QIT-MS). Gas-phase H/D exchange was used to probe the conformations, gas-phase acidities, and sites of deprotonation of isomeric flavonoids. The structural factors that promote or prevent H/D exchange were identified and correlated with collisionally activated dissociation (CAD) patterns and/or molecular modeling data. The use of a transition metal and an auxiliary ligand to form flavonoid/transition metal/auxiliary ligand complexes was proved to be more effective than deprotonation for differentiating isomeric flavonoids using chalcones as an example (a subclass of flavonoids). The relative threshold dissociation energies of such transition metal complexes were measured by energy-variable CAD, and the structural features that influence the threshold energies were identified. The conformations, point charges, and helium (collision gas) accessible surface areas obtained by various computational means were used to rationalize the differences in threshold energies of isomeric flavonoids. Aluminum(III) was used to form strong complexes with flavonoids for more effective isomer differentiation of flavonoids. The CAD patterns of the aluminum complexes were used to differentiate flavonoids, and the structures and conformations of representative flavonoids were also identified by high level computational means. Silver complexation was used to conquer the limitation of the transition metal and aluminum complexation (flavonoids must have a 4-keto group and at least one adjacent hydroxyl group). Silver complexation could not only be used for isomer differentiation of individual flavonoids but could be adapted to characterize flavonoids in mixtures coupled with high performance liquid chromatography (HPLC). HPLC-ESI-MS was used as a sensitive method with high selectivity to detect flavonoids in kale and grapefruit juice and metabolites in urine. The major flavonoids in kale were detected and confirmed as quercetin and kaempferol by CAD and isotopic labeling. The growing conditions were found to significantly affect the flavonoid levels in kale. LC/MS and LC/MS/MS were used to identify the individual metabolites and to gain insight into the overall metabolite profiles in human urine after consumption of grapefruit juice.