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Publication Metadata only A comprehensive study on the characteristic spectroscopic features of nitrogen doped graphene(Elsevier, 2019) Ogasawara, Hirohito; N/A; N/A; N/A; Department of Chemistry; Solati, Navid; Mobassem, Sonia; Kahraman, Abdullah; Kaya, Sarp; PhD Student; PhD Student; PhD Student; Faculty Member; Department of Chemistry; Koç University Research Center for Translational Medicine (KUTTAM) / Koç Üniversitesi Translasyonel Tıp Araştırma Merkezi (KUTTAM); Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Sciences; N/A; N/A; N/A; 116541Despite significant methodical improvements in the synthesis of N-doped graphene, there are still unsolved questions regarding the control of content and the configuration of nitrogen species in graphene honeycomb network. A cross-examination of X-ray photoelectron spectroscopy and Raman spectroscopy findings indicates that the nitrogen dopant amount is graphene thicknesses dependent, but the various nitrogen dopant coordination can be obtained on both double- and few-layer graphene. Characteristic defect features (D') appearing in Raman spectra upon N-doping is sensitive to nitrogen dopant coordination, graphitic-pyridinic/nitrilic species and therefore the doping level can be identified. Pyridinic and nitrilic nitrogen as primary species turn graphene to p-type semiconductor after a mild thermal treatment.Publication Metadata only A post-HF study on the halogen bonding interaction of pyrene with diatomic halogen molecules(Wiley, 2016) Sütay, Berkay; Yurtsever, Mine; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129We present a detailed SCS-MP2 study on the potential energy curves (PEC) for interactions between diatomic halogen molecules and pyrene. BSSE corrected CCSD[T] energies at equilibrium distances are computed and compared to CCSD(T) energies. The most stable conformation of these weakly bound van der Waals complexes is almost linear in the perpendicular direction to the pyrene plane. The complexes of highly polarizable bromine and iodine molecules with pyrene are very stable and they carry rather large number of vibrational states. Despite its small size, F-2 also forms strong halogen bonding similar to Br-2 and I-2. The interaction between Cl-2 and pyrene is the weakest and it is attributed to the highest polarizability/molar mass ratio of chlorine among the others. I-2-pyrene is found to be the most stable complex due to the strongest mutual polarization effects and is carrying more than 60 vibrational states. Due to the rather large number of electrons in some complexes, the relativistic corrections are also considered.Publication Metadata only A profound analysis of Rb-2[PH] and Cs-2[PH] and the role of [PH](2-) ions during temperature-induced solid-solid phase transitions(Elsevier, 2014) Hochrein, Oliver; Zahn, Dirk; Department of Chemistry; Department of Chemistry; Somer, Mehmet Suat; Schnering, H. G. Von; Faculty Member; Other; Department of Chemistry; College of Sciences; College of Sciences; 178882; N/AThe temperature-induced solid-solid transformation of Rb-2[PH] and Cs-2[PH] is characterized from both experiment and theory. Neutron diffraction, IR-spectroscopy and ab-initio molecular dynamics simulations reveal an asymmetric shift of the lattice constants at 80 K. The molecular mechanism of the structural transformation as identified from IR-spectroscopy and ab-initio molecular dynamics simulations is closely connected to the orientation of the [PH](2-) moieties which undergo a partial order-disorder phase transition.Publication Metadata only A quantum mechanical study of the electrochemical polymerization of pyrrole(Elsevier Science Sa, 2001) Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129Mechanism for the electrochemical polymerization of pyrrole is studied using accurate density functional theory; (DFT) calculations. The primary emphasis is on the structures and stability of intermediates generated during various mechanisms. Structures of the radical cations, which play role in reactions, an optimized to elucidate radical-radical and radical-neutral pathways. The competing probabilities of reactions between various size oligomers are discussed in terms of their thermodynamical stability.Publication Metadata only A theoretical study of structural defects in conjugated polymers(Elsevier Science Sa, 1999) Yurtsever, Mine; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129Accurate ab-initio calculations are performed for pyrrole and thiophene oligomers bonded through alpha and beta carbons. The thermodynamical stabilitiy of all possible binding types including the branched forms of tetramers and pentamers are compared. Employing the probabilities obtained from these calculations, a Monte Carlo type growth scheme is applied to predict branching as functions of the chain length and temperature. A high degree of branching for polypyrrole is reported whereas the linear chains dominate the structure of polythiophene.Publication Open Access Ab initio quantum dynamics with very weak van der Waals interactions: structure and stability of small Li-2((1)Sigma(+)(g))-(He)(n) clusters(American Institute of Physics (AIP) Publishing, 2004) Bodo, E.; Sebastianelli, F.; Gianturco, F.A.; Yurtsever, M.; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129The potential energy surface (PES) for the interaction between Li-2((1)Sigma(g)(+)) and He-4 has been computed using an accurate, post-Hartree-Fock quantum calculation for its ground electronic state. The orientational anisotropy of the forces and the interplay between repulsive and attractive effects within the PES are analyzed to extract information on the possible existence of bound states in the triatomic system. The structures of a few of the Li-2(He)(n) small clusters are examined by comparing a classical approach with a full quantum one to generate bound configurations and to extract information on the possible spatial arrangements of the smaller clusters via a vis the location of the Li-2 dopant. Some significant consequences on the Li-2 behavior in larger clusters and droplets are drawn from the above findings.Publication Metadata only Alpha- and beta-na-2[BH4][NH2]: two modifications of a complex hydride in the system NaNH2-NaBH4; syntheses, crystal structures, thermal analyses, mass and vibrational spectra(Elsevier Science Sa, 2010) Höhn, Peter; Cardoso-Gil, Raul; Aydemir, Umut; Akselrud, Lev; Department of Chemistry; N/A; N/A; N/A; Somer, Mehmet Suat; Acar, Selçuk; Koz, Cevriye; Kokal, İlkin; Faculty Member; PhD Student; Master Student; Master Student; Department of Chemistry; College of Sciences; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; 178882; N/A; N/A; N/AThe title compound Na-2[BH4][NH2] Na2BNH6 exists in two different modifications: the low-temperature (LT) beta- and the high-temperature (HT) alpha-phase. The HT alpha-Na2BNH6 is accessible by single heating of the binaries NaNH2 and NaBH4 (molar ratio = 1:1) to the melting point of 492 K for I h in evacuated Pyrex ampoules. on longer annealing (T = 342 K, 10 days), the HT alpha-phase converts completely to the LT beta-phase. The beta -> alpha transition at 371 K is very fast, but the conversion alpha -> beta is quite slow. No further phase formation was detected in the system up to a molar ratio of 5:1 (NaNH2:NaBH4). The cubic alpha-Na2BNH6 crystallizes in the K3SO4F-type of structure (space group: Pm (3) over barm, Z = 1) with a = 4.6950(1) angstrom. beta-Na2BNH6 crystallizes in orthorhombic symmetry (space group: Pbcm, Z = 4) with a = 6.5384(2) angstrom. b = 6.4960(1) angstrom, c=9.8512(2)angstrom. Both structures can directly be derived from the perovskite type structure ABO(3), with the [BH4](-) and [NH2](-) moieties in the A and B sites, respectively. The charge balancing Na ions are occupying 2/3 of the O positions. In both cases, the presence of the two different anions in the solids was also confirmed by vibrational spectra. According to DTA and high temperature X-ray diffraction (HT-XRD) measurements, the educts NaNH2 and NaBH4 react at 430 K to form alpha-Na2BNH6 which remains stable until the melting point of 492 K. Between 492 and 573 K, no significant effects were detected in DTA and TG. Above 573 K, however, a sudden decomposition takes place in two steps (659 and 689 K) causing a total mass loss of 8%. The gaseous species formed during the decomposition process were followed by mass spectra (MS). The mass spectra obtained from 1:1 and 2:1 mixtures of NaNH2 and NaBH4 in the range 492 K < T < 773 K prove that the main gaseous product evolved during the thermal decomposition is H-2. NH3 and H2O were also detected, but their concentrations are quite low. These findings confirm that the mixture of the complex hydrides NaNH2-NaBH4 (molar ratio >= 1:1) - comparable to the LiNH2-LiBH4 system - are potential candidates for solid hydrogen storage materials. The composition of the solid residues after the thermal treatment depends on the molar ratio of the starting binaries NaNH2 and NaBH4. While Na2BNH6 (1:1) decomposes to a mixture of NaH, Na and an unidentified amorphous solid, mixtures >= 2:1 yielded exclusively Na3BN2. The vibrational spectra of the title compounds Na2BNH6 have been measured and interpreted based on the T-d and C-2v symmetry of the relevant [BH4](-) and [NH2](-) groups. Both the (N-H) and (B-H) frequencies exhibit small but significant shifts with respect to the pure binaries.Publication Open Access An integrated computational-experimental hierarchical approach for the rational design of an IL/UiO-66 composite offering infinite CO2 selectivity(Wiley, 2022) Department of Chemical and Biological Engineering; Department of Chemistry; Zeeshan, Muhammad; Gülbalkan, Hasan Can; Durak, Özce; Haşlak, Zeynep Pınar; Ünal, Uğur; Keskin, Seda; Uzun, Alper; PhD Student; Faculty Member; Faculty Member; Department of Chemical and Biological Engineering; Department of Chemistry; Koç University Tüpraş Energy Center (KUTEM) / Koç Üniversitesi Tüpraş Enerji Merkezi (KÜTEM); Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); College of Engineering; College of Sciences; Graduate School of Sciences and Engineering; N/A; N/A; N/A; N/A; 42079; 40548; 59917Owing to the possibility of generating theoretically unlimited numbers of ionic liquid (IL)-metal-organic framework (MOF) combinations, experimental studies on IL/MOF composites for gas separation applications are mostly conducted on a trial-and-error basis. To address this problem, an integrated computational-experimental hierarchical approach is presented for selecting the best IL-MOF combination for a target gas separation application. For this purpose, UiO-66 and pyrrolidinium-based ILs are chosen as the parent MOF and IL family, respectively, and three powerful computational tools, Conductor-like Screening Model for Realistic Solvents calculations, density functional theory calculations, and grand canonical Monte Carlo simulations, are integrated to identify the most promising IL-UiO-66 combination as 1-n-butyl-1-methylpyrrolidinium dicyanamide/UiO-66, [BMPyrr][DCA]/UiO-66. Then, this composite is synthesized, characterized in deep detail, and tested for CO2/N-2, CO2/CH4, and CH4/N-2 separations. Results demonstrate that [BMPyrr][DCA]/UiO-66 offers an extraordinary gas separation performance, with practically infinite CO2 and CH4 selectivities over N-2 at 15 degrees C and at low pressures. The integrated hierarchical approach proposed in this work paves the way for the rational design and development of novel IL/MOF composites offering exceptional performance for any desired gas separation application.Publication Open Access Angular-momentum-driven chaos in small clusters(American Physical Society (APS), 1998) Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129The effects of the rotational motion on the chaotic behavior of triatomic Lennard-Jones clusters are studied. A set of initial momentum distributions with tunable parameters is chosen to correspond to various rigid-body rotations around symmetry axes of the cluster. By smoothly varying the direction of the initial kicks given to the cluster, periodic transitions between regular and chaotic regimes are obtained. A study of initial conditions leading up to such transitions shows that the major factor that determines the extent of the chaotic behavior is the initial partitioning of the kinetic energy between the rotational and vibrational motion. From the analysis of the time evolution of various properties it is concluded that the basic role of this initial partitioning is to control the energy transfer between the kinetic and the potential energy.Publication Metadata only Antibacterial silicone-urea/organoclay nanocomposites(Springer, 2009) Department of Chemistry; N/A; N/A; Department of Chemistry; Yılgör, Emel; Nugay, Işık Işıl; Bakan, Murat; Yılgör, İskender; Researcher; Undergraduate Student; Undergraduate Student; Faculty Member; Department of Chemistry; College of Sciences; College of Engineering; College of Engineering; College of Sciences; N/A; N/A; N/A; 24181Montmorillonite modified with distearyldimethyl ammonium chloride (C18-QAC) (Nanofil-15) (NF15) was incorporated into polydimethylsiloxane-urea (silicone-urea, PSU) copolymers. PSU was obtained by the reaction of equimolar amounts of aminopropyl terminated polydimethylsiloxane (PDMS) oligomer (= 3,200 g/mol) and bis(4-isocyanatohexyl) methane (HMDI). A series of PSU/NF15 nanocomposites were prepared by solution blending with organoclay loadings ranging from 0.80 to 9.60% by weight, corresponding to 0.30 to 3.60% C18-QAC. Colloidal dispersions of organophilic clay (NF15) in isopropanol were mixed with the PSU solution in isopropanol and were subjected to ultrasonic treatment. Composite films were obtained by solution casting. FTIR spectroscopy confirmed that the organoclay mainly interacted with the urea groups but not with PDMS. XRD analysis showed that nanocomposites containing up to 6.40% by weight of organoclay had fully exfoliated silicate layers in the polymer matrix, whereas 9.60% loading had an intercalated structure. Physicochemical properties of nanocomposites were determined. PSU/NF15 nanocomposites displayed excellent long-term antibacterial properties against E. coli.