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 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 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.Publication Open Access Picosecond to Nanosecond Manipulation of Excited-State Lifetimes in Complexes with an Fe-II to Ti(IV )Metal-to-Metal Charge Transfer: The Role of Ferrocene Centered Excited States(AMER CHEMICAL SOC, 2019-11-18) ALTUN, ZİKRİ; Livshits, Maksim Y.; Turlington, Michael D.; Trindle, Carl O.; Wang, Lei; Altun, Zikri; Wagenknecht, Paul S.; Rack, Jeffrey J.Time-resolved transient absorption spectroscopy and computational analysis of D-pi-A complexes comprising Fe-II donors and Ti-IV acceptors with the general formula (Cp2Ti)-Cp-R(C(2)Fc)(2) (where RCp = Cp*, Cp, and Cp-MeOOC) and (Cp2Ti)-Cp-TMS(C(2)Fc)(C2R) (where R = Ph or CF3) are reported. The transient absorption spectra are consistent with an Fe-III/Ti-III metal-to-metal charge-transfer (MMCT) excited state for all complexes. Thus, excited-state decay is assigned to back-electron transfer (BET), the lifetime of which ranges from 18.8 to 41 ps. Though spectroscopic analysis suggests BET should fall into the Marcus inverted regime, the observed kinetics are not consistent with this assertion. TDDFT calculations reveal that the singlet metal-to-metal charge-transfer ((MMCT)-M-1) excited state for the Fe-II/Ti-IV complexes is not purely MMCT in nature but is contaminated with the higher-energy (1)Fc (d-d) state. For the diferrocenyl complexes, (Cp2Ti)-Cp-R(C(2)Fc)(2), the ratio of MMCT to Fc centered character ranges from 57:43 for the Cp* complex to 85:15 for the Cp-MeOOC complex. For the diferrocenyl and monoferrocenyl complexes investigated herein, the excited-state lifetimes decrease with increased (1)Fc character. The effect of Cu-I coordination was also analyzed by time-resolved transient absorption spectroscopy and reveals the elongation of the excited-state lifetime by 3 orders of magnitude to 63 ns. The transient spectra and TDDFT analysis suggest that the long-lived excited state in Cp2Ti(C(2)Fc)(2).CuX (where X is Cl or Br) is a triplet iron species with an electron arrangement of Ti-IV-Fe-3(II)-Cu-I.Publication Open Access Dielectronic Recombination in He-like, Li-like and Be-like Iodine and KLM Resonances(IOP PUBLISHING LTD, 2012-11-05) YUMAK YAHŞİ, AYŞE; Bleda, Erdi A.; Yumak, Ayse; Yavuz, Ilhan; Altun, Zikri; Williams, ID; VanDerHart, HW; McCann, JF; Crothers, DSFDR rate coefficients for He-like, Li-like and Be-like Iodine ions and KLM resonances are calculated using MCBP approximation.Publication Open Access Production of Carbamic Acid Dimer from Ammonia-Carbon Dioxide Ices: Matching Observed and Computed IR Spectra(MDPI, 2019-04-23) BLEDA, ERDİ ATA; Altun, Zikri; Bleda, Erdi; Trindle, CarlThe production of complex molecules in ammonia-carbon dioxide ices is presumed to pass through species of formula H3N:CO2 with further addition of ammonia and carbon dioxide. One possible landmark, carbamic acid, H2NCOOH, has been implicated among the products of warming and irradiation of such ices. Experimental study of the IR spectra of residues has suggested the presence of related species, including weakly bound 1:1 and 2:1 complexes of ammonia with carbon dioxide, zwitterionic carbamic acid, ammonium carbamate, and the dimer of carbamic acid. We computed the energetics and vibrational spectra of these species as well as the complex between ammonia and carbamic acid for gas and condensed phases. By means of a new spectrum-matching scoring between computed and observed vibrational spectra, we infer species that are most probably present. The leading candidates are ammonium carbamate, the carbamic acid-ammonia complex, and the carbamic acid dimer.Publication Open Access High-order harmonic generation from confined Rydberg atoms(IOP PUBLISHING LTD, 2015-09-07) BLEDA, ERDİ ATA; Orazymbetov, Kudiyar; Bleda, Erdi A.; Altun, Zikri; Topcu, Turker; Diaz, C; Rabadan, I; Garcia, G; Mendez, L; Martin, FWe report results from our simulations of High Harmonic Generation (HHG) from a confined atom in a Rydberg state. We find that for the n = 2 excited state of H the cut-off of the harmonic spectrum is substantially extended compared to that for a free atom at the expense of the harmonic yield. This effect is dependent on the radius of the confining shell for a given n. We also observe that the confined spectrum exhibits cusps similar to those seen in the HHG spectra from ground state atoms in the presence of Cooper minima.Publication Open Access Dielectronic Recombination in He-like, Li-like and Be-like Yttrium and KLM Resonances(IOP PUBLISHING LTD, 2012-11-05) YUMAK YAHŞİ, AYŞE; Yumak, Ayse; Karakas, Vedat; Altun, Zikri; Williams, ID; VanDerHart, HW; McCann, JF; Crothers, DSFDR rate coefficients for He-like, Li-like and Be-like Yttrium ions and KLM resonances are calculated using MCBP approximation.