No description available.

<p> The preparation of compounds containing polyatomic cations of sulphur, selenium and tellurium has been investigated by using oxidising agents such as AsF5, SbF5, S2O6F2 and SO3. New compounds, containing polyatomic cations Se8 2+, Se4 2+, Te3n n+, Te4 2+ Ten n+, S16 2+, S8 2+ and S4 2+, and anions of very strong acids such as Sb2F11- and AsF6- were isolated and characterised by a combination of the stoichiometry of the preparation reactions, UV-visible spectrophotometry, Infrared and Raman spectroscopy and magnetic susceptibility measurements.</p> <p> Solutions of sulphur in various concentrations of oleum were investigated in detail by UV-visible spectrophotometry and e.s.r. spectroscopy. Evidence is presented for the formation of the sulphur cations S16 2+, S8 2+ and S4 2+ in these media, and for the presence of low concentrations of the radical cations S8+ and S4+.</p> / Thesis / Doctor of Philosophy (PhD)

published_or_final_version / abstract / Chemistry / Doctoral / Doctor of Philosophy

No description available.

In previous research, ranitidine has formed high yields of the disinfection by-product N-Nitrosodimethylamine (NDMA) upon chloramination. In the current research, bench-scale experiments were conducted to investigate the impact of cations (i.e., Ca2+, Mg2+, and Na+) on NDMA formation from ranitidine in three water matrices (Milli-Q® water, Lake Ontario water, and Otonabee River water) under practical chloramine disinfection conditions. In Milli-Q® water, excess cations did not change the yields of NDMA. NDMA formation kinetic profiles monitored in the lake and river water also indicated that elevating the cation concentrations did not affect the ultimate NDMA formation from ranitidine, but then did affect the observed rates of NDMA formation; the rates underwent an initial decrease and a subsequent increase as the cation concentrations were increased. The lowest reaction rates were observed in the lake and river water samples when they have a hardness level of 240 and 203 mg/L as CaCO3, respectively.

The kinetics of electron transfer between the radical cations of a series of anthracene derivatives and a variety of amine electron donors were studied on the surface of silica gel. The anthracene derivatives were excited with 355 nm radiation and formed radical cations by multi-photon ionisation. The radical cation absorption decay was measured by diffuse reflectance laser flash photolysis at wavelengths between 680 and 780 nm, depending upon the identity of the substituent of the anthracene. The decay kinetics of the radical cations were complex and non-exponential. The addition to the surface of a co-adsorbed electron donor, with an oxidation potential below that of the anthracene derivative resulted in an increase in the rate of the radical cation decay because of the electron transfer from the donor species. The kinetics of electron transfer were fitted with using the dispersive model of Albery.

The rational design, synthesis and complexation characteristics of several monovalent cation-selective ligands are described. Molecular modeling employing a combination of dynamics, mechanics (AMBER94) and electrostatics was used to design ligands for the complexation of ammonium, potassium, sodium and lithium ions. A modular technique was used to synthesize an ammonium selective ionophore based on a cyclic depsipeptide structure (8). The ionophore was incorporated into a planar ion selective electrode (ISE) sensor format and the selectivity tested versus a range of metal cations. It was found that the membrane containing the polar plasticizer NPOE (nitrophenyloctylether) in the absence of ionic additive exhibited near-Nernstian behavior (slope = 60.1 mV/dec @ 37¢ªC) and possessed high selectivity for ammonium ion over lithium and the divalent cations, calcium and magnesium (logK = -7.3, -4.4, -7.1 for lithium, calcium and magnesium ions, respectively). The same membrane also exhibited sodium and potassium selectivity that was comparable to that reported for nonactin (logK = -2.1, -0.6 for sodium and potassium, respectively, compared to -2.4, -0.9 in the case of nonactin). N-(9-methylanthracene)-25,27-bis(1-propyloxy)calix[4]arene-azacrown-5 (10) was synthesized and tested as a fluoroionophore for the selective detection of potassium ions. Compound 10 acts as an ¡°off-on¡± fluorescent indicator for ion complexation as a result of photoinduced intramolecular electron transfer (PET). Studies demonstrate that 10 is selective for potassium over other alkali metal cations, with excellent selectivity over sodium and lithium (log K ¡ -3.5) and moderate selectivity over rubidium and cesium (log K ~ -1). N-(9-methylanthracene)-25,27-bis(1-propyloxy)-4-tert-butylcalix[4]arene-azacrown-3 (11) was synthesized and tested as a fluoroionophore for the selective detection of lithium cations. When exposed to lithium ions in a 75:25 dichloromethane/THF solvent mixture, the molecule, which operates on PET, exhibited a >106-fold enhancement in fluorescence emission intensity. Selectivity studies demonstrated that 11 effectively discriminates against sodium and potassium ions log K ¡ -3.8 and log K ¡ -2.3. A fluorescent sodium optode based on a fluoroionophore consisting of aminorhodamine B covalently-linked through an amide bond to a calix[4]arene has also been developed (12). The optode, fashioned by incorporation of the fluoroionophore into a single component polymer matrix, operates on the basis of PET. The fluorescence intensity increased linearly with increasing sodium ion concentration in the range 0.01 M to 2.0 M, exhibiting a three-fold enhancement over this range. The optode provides selectivity for sodium ions compared to potassium ions that is sufficient for clinical determinations of sodium ion concentration.

Thesis (Ph. D.)--University of Texas at Austin, 2002. / Vita. Includes bibliographical references. Available also from UMI Company.

Thesis (Ph. D.)--University of Texas at Austin, 2002. / Vita. Includes bibliographical references. Available also from UMI Company.

Understanding the dissociation of odd-electron peptide radical cations is of great importance for the analytical applications of biological mass spectrometry because their diverse array of fragmentation pathways provides structural information to supplement that from even-electron protonated peptides—allowing peptide sequencing and, ultimately, protein identification. Nevertheless, the mechanisms of peptide radical formation and dissociation remain largely unexplored. In the studies reported in this Thesis, peptide radical cations (M?+) were generated through one-electron transfer (ET) in collision-induced dissociation (CID) of [CuII(L)M]?2+ (L = auxiliary ligand; M = peptide) complexes. Competitive dissociative pathways were circumvented experimentally through judicious selection of the macrocyclic auxiliary ligand, allowing the formation of a broad range of M?+ species. Chapter 3.1 examines the competition between proton transfer (PT) and ET within [CuII(L)His]?2+ complexes with L = dien (an open-chain ligand), or L = 9-aneN3 (the macrocyclic analogue of dien). Density functional theory (DFT) calculations revealed that macrocyclic ligand (9-aneN3) facilitates M?+ formation by maintaining similar ET barriers with open-chain ligand (dien), but substantially increasing PT barriers. Studying and understanding the fragmentations of M?+ species is fundamentally important and a formidable challenge—both charge-directed and radical-driven fragmentations play important roles, in a competitive manner, in the dissociations of M?+ species. Chapters 3.2-3.4 were built upon successful gas phase syntheses of a wide variety of M?+ species. Chapter 3.2 reports the novel Cβ–Cγ bond cleavage of tryptophan residues in the dissociations of various tryptophan-containing M?+ species, resulting in a neutral 116-Da loss; this process is an α-radical–induced fragmentation. Substitution of the tryptophan residue by a 1-methyltryptophan residue revealed that the 116-Da neutral species is a radical with an unpaired electron on the indole nitrogen atom. Chapter 3.3 describes a systematic examination of tryptophan-containing model systems, both with and without basic residues, to unveil the mechanisms of Cβ–Cγ bond cleavages. M?+ species containing non-basic residues undergo protonation of the γ-carbon atom of the tryptophan residue, thereby weakening the Cβ–Cγ bond and facilitating its cleavage. The formation of [1H-indole]?+ (m/z 117) or [M – CO2 – 116]+ ions is a competition between two incipient fragments for the proton in a dissociating proton-bound dimer. In basic residue containing M?+ species, the proton is tightly sequestered by the basic side chain, resulting in more accessible radical migration barriers prior to subsequent bond cleavages; DFT calculations supported the notion that the charge-remote radical-driven pathway is more favorable than the proton-driven process by 6.2 kcal/mol. Selective radical-induced fragmentations were then used to investigate the radical propagation processes occurring via hydrogen atom transfers—in particular, for Cα–C bond cleavages leading to the formation of an+ ions. The energetics and kinetics of the dissociations of [RGn–2FG7–n – CO2]?+ (n = 2–6) with well-defined C-terminal α-radicals were determined by RRKM modeling of surface-induced dissociation experiments and DFT calculations, revealing that radical propagations in peptide radical cations are not necessarily stepwise processes. / published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy