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Publication Metadata only Computational screening of ZIFs for CO2 separations(Taylor & Francis Ltd, 2015) N/A; N/A; Department of Chemical and Biological Engineering; Yılmaz, Gamze; Özcan, Aydın; Keskin, Seda; Master Student; PhD Student; Faculty Member; Department of Chemical and Biological Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; N/A; 40548Using molecular simulations, we studied a diverse collection of zeolite-imidazolate frameworks (ZIFs) to evaluate their performances in adsorption- and membrane-based gas separations. Molecular simulations were performed for both single-component gases (CH4, CO2, H-2 and N-2) and binary gas mixtures (CO2/CH4, CO2/N-2, CO2/H-2 and CH4/H-2) to predict the intrinsic and mixture selectivities of ZIFs. These two selectivities were compared to discuss the importance of multi-component mixture effects on making predictions about the separation performance of a material. Gas separation performances of ZIFs were compared with other nanoporous materials and our results showed that several ZIFs can outperform well-known zeolites and metal-organic frameworks in CO2 separations. Several other properties of ZIFs such as gas permeability, working capacity and sorbent selection parameter were computed to identify the most promising materials in adsorption- and membrane-based separation of CO2/CH4, CO2/N-2, CO2/H-2 and CH4/H-2.Publication Metadata only Conformational similarities in isomerization dynamics of clusters(Amer Chemical Soc, 2003) Department of Chemistry; N/A; Department of Chemical and Biological Engineering; Yurtsever, İsmail Ersin; Palazoğlu, Ahmet; Arkun, Yaman; Faculty Member; N/A; Faculty Member; Department of Chemistry; Department of Chemical and Biological Engineering; College of Sciences; N/A; College of Engineering; 7129; N/A; 108526A method for characterization of the isomerization dynamics from classical trajectories is presented. A measure function describing the topological distance between two clusters of atoms is first developed. Next, this measure is used to identify the regions of the potential energy surface visited by the trajectories. Unlike the commonly used techniques such as simulated annealing or quenching, the proposed method does not require repeated treatment of the trajectory and can be safely used to study the isomerization dynamics of large systems, especially those of monatomic clusters.Publication Metadata only Energy landscapes in photochemical dissociation of small peroxides(Amer Chemical Soc, 2019) N/A; N/A; N/A; Department of Chemistry; Tabriz, Meisam Farzalipour; Çizmeciyan, Melisa Natali; Birer, Özgür; Yurtsever, İsmail Ersin; Master Student; PhD Student; Researcher; Faculty Member; Department of Chemistry; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; N/A; College of Sciences; N/A; N/A; N/A; 7129Organic peroxides are known to have important roles in many chemical and biochemical processes such as intermediates in the oxidation of various hydrocarbons, as initiators of free-radical polymerization and cross-linking agents, etc. Consequently, the study of the organic peroxides and their radicals are of fundamental interest and importance. Although several reaction pathways after dissociation of organic peroxides have been successfully identified using time-resolved optical absorption spectroscopy, interpretation of the data can be complicated due to spectral overlap of parent molecules, intermediates, and products. Therefore, a reliable theoretical framework is necessary in case of complex or less studied systems. In this study, we investigated the plausible thermal dissociation pathways of diethyl peroxide, ditert butyl peroxide, and dicumyl peroxide by density functional theory with M06-2X hybrid functional and compared its results to coupled cluster single double and perturbative triple, CCSD(T), level energies. Our results indicate that methyl radical elimination is the main dissociation mechanism for all of the studied peroxides after O-O bond cleavage which has been also observed in experiments. The resulting relative energies of the M06-2X functional were found to have reasonable accuracy in comparison with the CCSD(T) method. We also show that time-dependent density functional theory (TD-DFT) with the M06-2X functional provides a suitable guide for interpretation of time-resolved optical absorption spectra of peroxides. The experimental transient absorption spectra of dicumyl peroxide are interpreted using the theoretically predicted pathways and transient radical species. Both results agree within experimental resolution and accuracy. We propose that the traditionally assigned visible absorption is not due to the cumuloxyl radical and the photodissociation of dicumyl peroxide involves other pathways with extremely short-lived radicals.Publication Metadata only Possible formation of metastable pah dimers upon pickup by helium droplets(Amer Chemical Soc, 2016) Calvo, F.; Department of Chemistry; N/A; Yurtsever, İsmail Ersin; Birer, Özgür; Faculty Member; Researcher; Department of Chemistry; N/A; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); College of Sciences; N/A; 7129; N/AUsing path-integral molecular dynamics simulations and two quantum-mechanical-based force fields, we have investigated the conformational stability of dimers of a polycyclic aromatic hydrocarbon, perylene (C20H12), produced under typical experimental conditions of successive pick-up under helium nanodroplet environment. The most stable configurations are found to be of the stacked form with different relative orientations of the main molecular axes, perpendicular or T-shaped dimers being energetically much disfavored; however, in the presence of helium our simulations suggest that the time for rearrangement and pi-stacking may be rather long and exceed hundreds of picoseconds. In addition, highly metastable dimers that are stacked but with a helium monolayer sandwiched between the two molecules are also found as likely products upon successive pickup. This stabilization occurs owing to the stronger localization of the helium atoms facing the aromatic rings, which is further enhanced in the dimer. The implications of the present results are discussed in the perspective of possible identification by spectroscopic methods.Publication Metadata only Single-molecule fluorescence of terrylene embedded in anthracene matrix: a room temperature study(Elsevier Science Bv, 2012) N/A; Department of Physics; Yüce, Mehdi Yavuz; Kiraz, Alper; PhD Student; Faculty Member; Department of Physics; Graduate School of Sciences and Engineering; College of Sciences; N/A; 22542We characterized room-temperature fluorescence properties of single terrylene molecules embedded in anthracene thin films that were prepared by spin-coating on glass substrates. Our results show that terrylene molecules embedded in anthracene matrix are oriented nearly parallel to the substrate. Typical measured fluorescence lifetime of terrylene was 3.6 ns and photon count rate at saturation was 750 kHz. Our analyses of 104 molecules indicate that we can detect on average 1.6 x 10(6) photons from a single terrylene molecule before it photobleaches. These results make terrylene-anthracene a promising guest-host system for room temperature single molecule experiments.Publication Metadata only Ultraviolet upconversion spectra of sonochemically synthesized doped NaYF4 crystals(Elsevier, 2014) N/A; Department of Chemistry; Yıldırım, Cansu; Birer, Özgür; Master Student; Researcher; Department of Chemistry; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); N/A; College of SciencesA facile sonochemical method was developed for the production of alpha- and beta-forms of Yb3+(20%)/Tm3+(2%) or Er3+(2%) doubly doped NaYF4 crystals. The reactions took place in aqueous solutions at low temperatures and employed oxide or salt precursors. Micrometer sized beta-NaYF4 and nanometer sized alpha-NaYF4 were obtained with upconversion property. The required activation energy for the solid-solid phase transition was overcome with the energy provided by cavitations. Upconversion emission properties of the Yb3+(20%), Er3+(2%) or Tm3+(2%) doubly doped, alpha- and beta-NaYF4 were investigated. The alpha-form with emission in the ultraviolet region had higher efficiency contradicting the literature. Several new peaks in the spectra, most likely due to crystal defects, were temporarily assigned.Publication Metadata only Vibrational spectroscopy of structural defects in oligothiophenes(Taylor & Francis Ltd, 2003) Department of Chemistry; N/A; Yurtsever, İsmail Ersin; Kırmızıaltın, Serdal; Faculty Member; Undergraduate Student; Department of Chemistry; College of Sciences; College of Sciences; N/A; N/AVibrational spectra of oligothiophenes with structural defects are calculated within the density-functional-theory methodology. The effects of the defective ab linkages on the infrared (IR) and Raman spectra are characterized from calculations of all isomers up to the hexamer. The signatures of ab linked monomers can be found in IR spectra from broken symmetry arguments which result in absorptions localized in the defective region. The positions of the absorption peaks in the Raman spectra seem to be unaffected by the presence of such defects; however, strong reductions in the intensities are observed because of the shortening of the conjugation length.