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    Anchor Effect on Pedal Motion Observed in Crystal Phase of an Azobenzene Derivative
    (Springer/Plenum Publishers, 2011) Karabiyik, Hasan; Petek, Hande; Iskeleli, Nazan Ocak; Albayrak, Cigdem; Agar, Erbil
    Some molecules having a molecular skeleton similar to that of stilbenes and azobenzenes show orientational disorder in the crystals due to pedal motion. Heretofore, the orientational disorder through pedal motion has been observed for the compounds containing only two aromatic rings in the absence of bulky substituent groups. Here we report that the pedal motion can be detected even in the presence of a bulky substituent group to which orientational disorder becomes invisible as a result of anchor effect arising from phenoxyphtalonitrile group. X-ray crystallographic analysis of the compound, C23H18N4O, reveals the existence of partially overlapped two pedal conformers. The compound crystallizes in the monoclinic space group P2(1)/c with a = 12.9429(11) a<<, b = 8.5075(5) a<<, c = 21.063(2) a<< and beta = 123.155(6)A degrees. Major pedal conformer is stabilized by weak C-H center dot center dot center dot O type hydrogen bond and C-H center dot center dot center dot pi type edge-to-face interactions in solid state. Quantum chemical calculations at B3LYP/6-311G+(d,p) level suggest that the stabilization of the compound decreases with increasing deviation from the planar geometry of trans-azobenzene fragment. Molecular and crystal structure of 4-[2-Methyl-4-(4-ethylphenyldiazenyl)]phenoxyphtalonitrile, C23H18N4O, indicate the existence of partially overlapped two pedal conformers in which orientational disorder becomes invisible as a result of anchor effect arising from phenoxyphtalonitrile group.
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    Crystallographic and conformational analysis of two novel trans-azo benzene compounds
    (Springer/Plenum Publishers, 2009) Karabiyik, Hasan; Petek, Hande; Iskeleli, Nazan Ocak; Albayrak, Cigdem
    The molecular and crystal structure of (E)-2-Acetyl-4-(2-bromophenyldiazenyl)phenol (1) and (E)-2-Methyl-4-(o-tolyldiazenyl)phenol (2) were characterized and determined by single crystal X-ray diffraction method besides spectroscopic means. The periodic organization of 1 is stabilized by C-H center dot center dot center dot O type weak H-bond and Br center dot center dot center dot O type weak halogen bonding and thus, a two dimensional puckered network is established almost parallel to 10((1) over bar) the plane. Molecules of 2 are linked into C(7) chains generated by translation along the [1 0 1] direction with the aid of O-H center dot center dot center dot N type H-bonds, and these chains are strengthened by C-H center dot center dot center dot pi interactions involving o-tolylphenol ring. Quantum chemical studies at B3LYP/6-311 ++G(d,p) level reveal that potential barrier of the compounds around Ar-N torsions is of double minimum character unless it is defected by the presence of o-substituent groups in the vicinity of the azo bridge. The results from crystallographic and quantum chemical studies suggest that azo benzene compounds may adapt non-planar geometry apart from the most stable planar conformation, which is located on the secondary minima of double potential barrier regarding rotational motion around Ar-N bonds.
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    Structural and aromatic aspects for tautomerism of (Z)-6-((4-bromophenylamino)methylene)-2,3-dihydroxycyclohexa-2,4-dienone
    (Springer/Plenum Publishers, 2009) Karabiyik, Hasan; Petek, Hande; Iskeleli, Nazan Ocak; Albayrak, Cigdem
    The molecular and crystal structure of (Z)-6-((4-bromophenylamino)methylene)-2,3-dihydroxycyclohexa- 2,4-dienone were determined by single crystal X-ray diffraction and spectroscopic methods. Molecules of the compound can be regarded as a resonance hybrid of cis-keto tautomer and zwitterionic form. Pairs of molecules of the compound generate pseudocyclic centrosymmetric R-2(2)(10) supramolecular synthons with the aid of O-H center dot center dot center dot O type intermolecular H-bonds. Stacking of R-2(2)(10) synthons along b-axis is stabilized by pi center dot center dot center dot pi interactions. Changes in both covalent topology and molecular geometry of the compound accompanying proton transfer were monitored by a relaxed PES scan with respect to hydroxyl bond length used as redundant internal coordinate. Quantum chemical studies at 6-311 + G(d,p) level reveal that bond lengths which are indicative to tautomerization process cannot reach their expected values even if proton transfer occurs in gas phase and pseudo-aromatic chelate ring formation has primary effect on the stabilization of NH tautomer. Resonance-assisted intramolecular H-bond affects the electronic state of its neighboring aromatic fragments.
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    Öğe
    The proton transfer process observed in the structure analysis and DFT calculations of (E)-2-ethoxy-6-[(2-methoxyphenylimino)methyl]phenol
    (Springer/Plenum Publishers, 2010) Petek, Hande; Albayrak, Cigdem; Odabasoglu, Mustafa; Senel, Ismet; Buyukgungor, Orhan
    The crystal and molecular structures of an o-hydroxy Schiff base derivative, (E)-2-ethoxy-6-[(2-methoxyphenylimino)methyl]phenol, have been determined by single crystal X-ray diffraction analyses at 296 and 100 K. The results from temperature-dependent structural analysis regarding the tautomeric equilibrium of the compound were interpreted with the aid of quantum chemical calculations. To clarify the tautomerization process and its effects on the molecular geometry, the gas-phase geometry optimizations of two possible tautomers of the title molecule, its OH and NH form, were achieved using DFT calculations with B3LYP method by means of 6-31 + G(d,p) basis set. In order to describe the potential barrier belonging to the phenolic proton transfer, nonadiabatic Potential Energy Surface (PES) scan was performed based on the optimized geometry of the OH tautomeric form by varying the redundant internal coordinate, O-H bond distance. The Harmonic Oscillator Model of Aromaticity (HOMA) indices were calculated in every step of the scan process so as to express the deformation in the aromaticities of principal molecular moieties of the compound. The results show that there is a dynamic equilibrium between the aromaticity level of phenol and chelate ring and furthermore pi-electron coupling affecting overall molecule of the title compound. Charge transfer from phenol ring to pseudo-aromatic chelate ring increases with increasing temperature, whereas pi-electron transfer from chelate ring to anisole ring is decreased as temperature increases. The most strength intramolecular H-bonds are observed for conformers close to transition state.