Person: ALTUN, ZİKRİ
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ALTUN
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ZİKRİ
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Publication Open Access Stochastic enhancement of high-order harmonic generation(2012-01-01) ALTUN, ZİKRİ; YAVUZ, İLHAN; Yavuz İ., Bleda E. A., Altun Z.Publication Open Access Bonding Analysis of Compounds with Unusual Coordination of Carbon: Proposed Symmetric Systems with Six-Coordinate Carbon(MDPI, 2020-08-28) BLEDA, ERDİ ATA; Trindle, Carl; Altun, Zikri; Bleda, Erdi AtaThe possibility of carbon tetravalence in geometries other than tetrahedral and of carbon hypervalence has been taken seriously since the 1970s. Computational modeling and subsequent experimental validation have established the existence of molecules with carbon atoms with planar tetravalence and as many as six objects in carbon's coordination sphere. In this work, we develop insight into the nature of bonding to carbon in these unusual environs as provided by Bader's Atoms in Molecules (AIM) analysis of the electron density, along with the electron localization function (ELF) and the non-covalent index (NCI). We review several well-established systems (spiropentadiene dication, hexamethyl benzene dication, dimethanospiro[2.2]octaplane dication, and 1,8-dimethoxy-9-dimethoxyanthracene cation) and propose new D-2d-symmetric variants of a hexacoordinated species.Publication Metadata only Complexes with Tunable Intramolecular Ferrocene to Ti-IV Electronic Transitions: Models for Solid State Fe-II to Ti-IV Charge Transfer(AMER CHEMICAL SOC, 2016) ALTUN, ZİKRİ; Turlington, Michael D.; Pienkos, Jared A.; Carlton, Elizabeth S.; Wroblewski, Karlee N.; Myers, Alexis R.; Trindle, Carl O.; Altun, Zikri; Rack, Jeffrey J.; Wagenknecht, Paul S.Iron(II)-to-titanium(IV) metal-to-metal-charge transfer (MMCT) is important in the photosensitization of TiO2 by ferrocyanide, charge transfer in solid-state metal-oxide photocatalysts, and has been invoked to explain the blue color of sapphire, blue kyanite, and some lunar material. Herein, a series of complexes with alkynyl linkages between ferrocene (Fc) and Ti-IV has been prepared and characterized by UV-vis spectroscopy and electrochemistry. Complexes with two ferrocene substituents include Cp2Ti(C(2)Fc)(2), Cp*Ti-2(C(2)Fc)(2), and Cp2Ti(C(4)Fc)(2). Complexes with a single ferrocene utilize a titanocene with a trimethylsilyl derivatized Cp ring, Cp-TMS, and comprise the complexes (Cp2Ti)-Cp-TMS(C(2)Fc)(C2R), where R = C6H5, p-C6H4CF3, and CF3. The complexes are compared to Cp2Ti(C2Ph)(2), which lacks the second metal. Cyclic voltammetry for all complexes reveals a reversible Ti-IV/III reduction wave and an Fe-II/III oxidation that is irreversible for all complexes except (Cp2Ti)-Cp-TMS-(C(2)Fc)(C2CF3). All of the complexes with both Fc and Ti show an intense absorption (4000 M-1 cm(-1) < epsilon < 8000 M-1 cm(-1)) between 540 and 630 nm that is absent in complexes lacking a ferrocene donor. The energy of the absorption tracks with the difference between the Ti-IV/III and Fe-III/II reduction potentials, shifting to lower energy as the difference in potentials decreases. Reorganization energies, lambda, have been determined using band shape analysis (2600 cm(-1) < lambda < 5300 cm(-1)) and are in the range observed for other donor-acceptor complexes that have a ferrocene donor. Marcus-Hush-type analysis of the electrochemical and spectroscopic data are consistent with the assignment of the low-energy absorption as a MMCT band. TD-DFT analysis also supports this assignment. Solvatochromism is apparent for the MMCT band of all complexes, there being a bathochromic shift upon increasing polarizability of the solvent. The magnitude of the shift is dependent on both the electron density at Ti-IV and the identity of the linker between the titanocene and the Fc. Complexes with a MMCT are photochemically stable, whereas Cp2Ti(C2Ph)(2) rapidly decomposes upon photolysis.Publication Open Access Phosphorescence Tuning through Heavy Atom Placement in Unsymmetrical Difluoroboron beta-Diketonate Materials(WILEY-V C H VERLAG GMBH, 2018-02-06) ALTUN, ZİKRİ; Liu, Tiandong; Zhang, Guoqing; Evans, Ruffin E.; Trindle, Carl O.; Altun, Zikri; DeRosa, Christopher A.; Wang, Fang; Zhuang, Meng; Fraser, Cassandra L.Difluoroboron beta-diketonates (BF(2)bdks) show both fluorescence (F) and room-temperature phosphorescence (RTP) when confined to a rigid matrix, such as poly(lactic acid). These materials have been utilized as optical oxygen sensors (e.g., in tumors, wounds, and cells). Spectral features include charge transfer (CT) from the major aromatic donor to the dioxaborine acceptor. A series of naphthyl-phenyl dyes (BF(2)nbm) (1-6) were prepared to test heavy-atom placement effects. The BF(2)nbm dye (1) was substituted with Br on naphthyl (2), phenyl (3), or both rings (4) to tailor the fluorescence/phosphorescence ratio and RTP lifetimeimportant features for designing O-2 sensing dyes by means of the heavy atom effect. Computational studies identify the naphthyl ring as the major donor. Thus, Br substitution on the naphthyl ring produced greater effects on the optical properties, such as increased RTP intensity and decreased RTP lifetime compared to phenyl substitution. However, for electron-donating piperidyl-phenyl dyes (5), the phenyl aromatic is the major donor. As a result, Br substitution on the naphthyl ring (6) did not alter the optical properties significantly. Experimental data and computational modeling show the importance of Br position. The S-1 and T-1 states are described by two singly occupied MOs (SOMOs). When both of these SOMOs have substantial amplitude on the heavy atom, passage from S-1 to T-1 and emission from T-1 to S-0 are both favored. This shortens the excited-state lifetimes and enhances phosphorescence.Publication Metadata only Studies on spin state preferences in Fe(II) complexes(ELSEVIER, 2015) BLEDA, ERDİ ATA; Bleda, Erdi A.; Trindle, Carl; Altun, ZikriWe report modeling studies of high spin and low spin forms of the test set of problematic cases of pseudo-octahedral Fe(II) complexes defined by Ye and Neese. These include the most thoroughly studied spin crossover compound Fe(SCN)(2)(Phen)(2) and a more recently discovered Fe(II)-2,13-dimethyl-6,9-dio xa-3,12,18-triaza-bicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentene. Reliable estimates of the often small high-spin to low-spin energy gap require balanced treatment of electronic structure in very different bonding environments. Density functionals differ in their bias toward low spin or high spin configurations. For the test set defined by Ye and Neese, we describe the estimates of their energy gaps between high-spin and low-spin forms for 75 functionals; geometries are obtained by optimization with B3LYP with zero-order relativistic approximation (ZORA) in a triple-zeta quality basis. Fairly reliable results for high-spin to low-spin energy gaps are obtained by functionals IMP, HCTH, OPBE, and B3LYP*, and with the omega B97XD functional expressed in the cc-pVTZ basis, about half the size of the def2-QZVPP used by Ye and Neese. The best functionals are still biased systematically toward the high spin by about 20 kJ/mol. We apply these methods in a new study of a pseudo C-3v four coordinate crossover species reported by Scepaniak et al. Their four coordinate spin crossover complex of Fe with aryl-substituted scorpionate and N = P(Ph)(3) ligands is modeled by a truncated structure lacking aromatic substituents. These substituents appear to stabilize the lower spin. Reproducing the 80 K crossover temperature requires an empirical correction of about 12 kJ/mot in favor of the singlet for this truncated model system. (C) 2015 Elsevier B.V. All rights reserved.Publication Metadata only Photophysical and Analyte Sensing Properties of Cyclometalated Ir(III) Complexes(SPRINGER/PLENUM PUBLISHERS, 2012) ALTUN, ZİKRİ; Leavens, Bethany B. H.; Trindle, Carl O.; Sabat, Michal; Altun, Zikri; Demas, James N.; DeGraff, B. A.The synthesis of some heteroleptic, cyclometalated iridium(III) complexes is described. The utility of these [Ir(ppy)(2)(N-N)]Cl (ppy=2-phenylpyridine and N-N=substituted bipyridine, biquinoline, or phenanthroline) complexes as luminescence-based sensors is assessed. The emission intensity of an Ir(III) complex featuring the 3,3'-H(n)dcbpy ligand (H(n)dcbpy=dicarboxylic acid-2,2'-bipyridine; n=0,1,2 to indicate deprotonated, mono-and diprotonated species, respectively) is seen to increase in the presence of Pb(II). Insight into the structure and analyte-sensing capability is achieved by X-ray crystallography in conjunction with computational modeling. Complexes incorporating carboxylic acid-functionalized bipyridine and biquinoline as the polypyridyl ligand show pH sensitivity while similar phenanthroline complexes do not.Publication Metadata only Thermochemistry of N-Heterocyclic Carbenes with 5-, 4-, 3-, and 2-Membered Rings(WILEY, 2014) BLEDA, ERDİ ATA; Altun, Zikri; Bleda, Erdi A.; Trindle, Carl; Wang, JasonN-heterocyclic carbenes (NHCs) based on imidazole-2-ylidene (1) or the saturated imidazolidine-2-ylidene (2) scaffolds are long-lived singlet carbenes. Both benefit from inductive stabilization of the sigma lone pair on carbon by neighboring N atoms and delocalization of the N pi lone pairs into the nominally vacant p-pi atomic orbital at the carbene carbon. With thermochemical schemes G4 and CBS-QB3, we estimate the relative thermodynamic stabilization of smaller ring carbenes and acyclic species which may share the keys to NHC stability. These include four-membered ring systems incorporating the carbene center, two trivalent N centers, and either a boron or a phosphorus atom to complete the ring. Amino-substituted cyclopropenylidenes have been reported but three-membered rings containing the carbene center and two N atoms are not known. Our calculations suggest that amino-substituted cyclopropenylidenes are comparable in stability to the four-membered NHCs but that diazacyclopropanylidenes would be substantially less effectively stabilized. Concluding the series are acyclic carbenes with and without neighboring N atoms and a series of two-membered ring azapropadienenylidene cations of form :CNW with W = an electron-withdrawing agent. We have studied W = NO2, CH2(+), CF2(+), and (CN)(2)C(+). Although these systems display a degree of stabilization and carbene-like electronic structure, the stability of the NHCs is unsurpassed. (c) 2014 Wiley Periodicals, Inc.Publication Metadata only Enhancement of high-order harmonic generation in the presence of noise(IOP PUBLISHING LTD, 2011) YAVUZ, İLHAN; Yavuz, I.; Altun, Z.; Topcu, T.We report on our simulations of the generation of high-order harmonics from atoms driven by an intense femtosecond laser field in the presence of noise. We numerically solve the non-perturbative stochastic time-dependent Schrodinger equation and observe how varying noise levels affect the frequency components of the high harmonic spectrum. Our calculations show that when an optimum amount of noise is present in the driving laser field, roughly a factor of 45 net enhancement can be achieved in high-order harmonic yield, especially, around the cut-off region. We observe that, for a relatively weak noise, the enhancement mechanism is sensitive to the carrier-envelope phase. We also investigate the possibility of generating ultra-short intense attosecond pulses by combining the laser field and noise and observe that a roughly four orders of magnitude enhanced isolated attosecond burst can be generated.Publication Open Access Electron-Impact Ionization of Se16+ and Ser(17+) Ions(IOP PUBLISHING LTD, 2015-09-07) BLEDA, ERDİ ATA; Bleda, Erdi A.; Tikman, Yavuz; Altun, Zikri; Diaz, C; Rabadan, I; Garcia, G; Mendez, L; Martin, FElectron-impact ionization cross sections for Se16+ and Se17+ ions have been calculated using semirelativistic configuration-average distorted-wave (CADW) method for both the direct and indirect ionization contributions. The excitation-autoionization contributions originating from the excitations of 2s and 2p subshels were calculated using a semirelativistic level to level distorted-wave (LLDW) method. Direct ionization cross section calculations involving 2s,2p,3s, and 3p subshells were also calculated using the LANL collisional atomic code and comparisons are made.Publication Open Access Focal Point Evaluation of Energies for Tautomers and Isomers for 3-hydroxy-2-butenamide: Evaluation of Competing Internal Hydrogen Bonds of Types -OH horizontal ellipsis O=, -OH horizontal ellipsis N, -NH horizontal ellipsis O=, and CH horizontal ellipsis X (X=O and N)(MDPI, 2021-04-30) BLEDA, ERDİ ATA; Altun, Zikri; Bleda, Erdi Ata; Trindle, CarlThe title compound is a small molecule with many structural variations; it can illustrate a variety of internal hydrogen bonds, among other noncovalent interactions. Here we examine structures displaying hydrogen bonding between carbonyl oxygen and hydroxyl H; between carbonyl oxygen and amino H; hydroxyl H and amino N; hydroxyl O and amino H. We also consider H-bonding in its tautomer 2-oxopropanamide. By extrapolation algorithms applied to Hartree-Fock and correlation energies as estimated in HF, MP2, and CCSD calculations using the cc-pVNZ correlation-consistent basis sets (N = 2, 3, and 4) we obtain reliable relative energies of the isomeric forms. Assuming that such energy differences may be attributed to the presence of the various types of hydrogen bonding, we attempt to infer relative strengths of types of H-bonding. The Atoms in Molecules theory of Bader and the Local Vibrational Modes analysis of Cremer and Kraka are applied to this task. Hydrogen bonds are ranked by relative strength as measured by local stretching force constants, with the stronger =O horizontal ellipsis HO- > NH horizontal ellipsis O= > -OH horizontal ellipsis N well separated from a cluster > NH horizontal ellipsis O= approximate to >NH horizontal ellipsis OH approximate to CH horizontal ellipsis O= of comparable and intermediate strength. Weaker but still significant interactions are of type CH horizontal ellipsis N which is stronger than CH horizontal ellipsis OH.
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