Makale 2000-2009 yılları
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Browsing Makale 2000-2009 yılları by All Authors "Bıyık, Recep"
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Item An EPR study on tea: Identification of paramagnetic species, effect of heat and sweeteners(Elsevier, 2009-10) Bıyık, Recep; Tapramaz, Recep; 0000-0001-7929-4395; TAEK-ÇNAEMTea (Camellia Sinensis) is the most widely consumed beverage in the world, and is known to be having therapeutic, antioxidant and nutritional effects. Electron paramagnetic resonance (EPR) spectral studies made on the tea cultivated along the shore of Black Sea, Turkey, show Mn2+ and Fe3+ centers in green tea leaves and in black tea extract. Dry black tea flakes and dry extract show additional sharp line attributed to semiquinone radical. The origins of the paramagnetic species in black tea are defined and discussed. Effect of humidity and heat are investigated. It is observed that dry extract of black tea melts at 100 ◦C and the semiquinone radical lives up to 140 ◦C while Mn2+ sextet disappears just above 100 ◦C in tea extract. Natural and synthetics sweeteners have different effects on the paramagnetic centers. White sugar (sucrose) quenches the Mn2+ and semiquinone lines in black tea EPR spectrum, and glucose, fructose, lactose and maltose quench Fe3+ line while synthetic sweeteners acesulfam potassium, aspartame and sodium saccharine do not have any effect on paramagnetic species in tea.Item EPR and optical absorption studies of VO2+ doped KH2PO4 and KH3C4O8·2H2O single crystals(Verlag der Zeitschrift fur Naturforschung, 2006) Bıyık, Recep; Tapramaz, Recep; 0000-0001-7929-4395; TAEK-ÇNAEMVO2+ doped potassium dihydrogen phosphate (KH2PO4) and potassium tetraoxalate (KH3C4O8 · 2H2O) single crystals and powders are examined by electron paramagnetic resonance and optical absorption spectroscopy. Angular variations of KH2PO4 and KH3C4O8 ·2H2O single crystals show four and two different VO2+ sites, respectively. The local symmetry of VO2+ complexes is nearly axial for both host crystals. The optical absorption spectra show three bands. Spin Hamiltonian parameters are measured and molecular orbital coefficients are calculated by correlating EPR and optical absorption data for the central vanadyl ion.Item EPR spectra of Cu2+ in KH2PO4 single crystals(Elsevier, 2007-03-18) Bıyık, Recep; Tapramaz, Recep; TAEK-ÇNAEMCu2+ doped single crystals of KH2PO4 were investigated using EPR technique at room temperature. The spectra of the complex contains large number of overlapping lines. Five sites are resolved and four of them are compatible with the tetragonal symmetry, and the fifth one belongs to an interstitial site. The results are discussed and compared with previous studies. Detailed investigation of the EPR spectra indicate that Cu2+ substitute with K+ ions. The principal values of the g and hyperfine tensors and the ground state wave function of Cu2+ ions are obtained.