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  • Hydroxo-Bridged Dicopper(II,III) and -(III,III) Complexes: Models for Putative Intermediates in Oxidation Catalysis JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Halvagar, M. R., Solntsev, P. V., Lim, H., Hedman, B., Hodgson, K. O., Solomon, E. I., Cramer, C. J., Tolman, W. B. 2014; 136 (20): 7269-7272


    A macrocyclic ligand (L(4-)) comprising two pyridine(dicarboxamide) donors was used to target reactive copper species relevant to proposed intermediates in catalytic hydrocarbon oxidations by particulate methane monooxygenase and heterogeneous zeolite systems. Treatment of LH4 with base and Cu(OAc)2·H2O yielded (Me4N)2[L2Cu4(μ4-O)] (1) or (Me4N)[LCu2(μ-OH)] (2), depending on conditions. Complex 2 was found to undergo two reversible 1-electron oxidations via cyclic voltammetry and low-temperature chemical reactions. On the basis of spectroscopy and theory, the oxidation products were identified as novel hydroxo-bridged mixed-valent Cu(II)Cu(III) and symmetric Cu(III)2 species, respectively, that provide the first precedence for such moieties as oxidation catalysis intermediates.

    View details for DOI 10.1021/ja503629r

    View details for Web of Science ID 000336416600021

    View details for PubMedID 24821432

  • Excited-state proton-relay dynamics of 7-hydroxyquinoline controlled by solvent reorganization in room temperature ionic liquids PHYSICAL CHEMISTRY CHEMICAL PHYSICS Lim, H., Jeong, H., Park, S., Lee, J. Y., Jang, D. 2012; 14 (1): 218-224


    The excited-state triple proton relay of 7-hydroxyquinoline (7HQ) along a hydrogen-bonded methanol chain in room temperature ionic liquids (RTILs) has been investigated using picosecond time-resolved fluorescence spectroscopy. The rate constant of the proton relay in a methanol-added RTIL is found to be slower by an order of magnitude than that in bulk methanol and to have unity in its kinetic isotope effect. These suggest that the excited-state tautomerization dynamics of 7HQ in methanol-added RTILs is mainly controlled by the solvent reorganization dynamics to form a cyclically hydrogen-bonded complex of 7HQ·(CH(3)OH)(2) upon absorption of a photon due to high viscosity values of RTILs. Because the cyclic complex of 7HQ·(CH(3)OH)(2) at the ground state is unstable in RTILs, the collision-induced slow formation of the cyclic complex should take place upon excitation prior to undergoing subsequent intrinsic proton transfer rapidly.

    View details for DOI 10.1039/c1cp22329a

    View details for Web of Science ID 000297593800025

    View details for PubMedID 22073404

  • Excited-State Double Proton Transfer of 7-Azaindole Dimers in a Low-Temperature Organic Glass PHOTOCHEMISTRY AND PHOTOBIOLOGY Lim, H., Park, S., Jang, D. 2011; 87 (4): 766-771


    The excited-state double proton transfer of model DNA base pairs, 7-azaindole (7AI) dimers, is explored in a low-temperature organic glass of n-dodecane using picosecond time-resolved fluorescence spectroscopy. Reaction mechanisms are found to depend on the conformations of 7AI dimers at the moment of excitation; whereas planar conformers tautomerize rapidly (<10 ps), twisted conformers undergo double proton transfer to form tautomeric dimers on the time scale of 250 ps at 8 K. The proton transfer is found to consist of two orthogonal steps: precursor-configurational optimization and intrinsic proton transfer via tunneling. The rate is almost isotope independent at cryogenic temperatures because configurational optimization is the rate-determining step of the overall proton transfer. This optimization is assisted by lattice vibrations below 150 K or by librational motions above 150 K.

    View details for DOI 10.1111/j.1751-1097.2011.00923.x

    View details for Web of Science ID 000292864200004

    View details for PubMedID 21413991

  • Excited-State Double Proton Transfer Dynamics of Model DNA Base Pairs: 7-Hydroxyquinoline Dimers JOURNAL OF PHYSICAL CHEMISTRY A Lim, H., Park, S., Jang, D. 2010; 114 (43): 11432-11435


    The excited-state double proton transfer of model DNA base pairs, 7-hydroxyquinoline dimers, in benzene has been investigated using picosecond time-resolved fluorescence spectroscopy. Upon excitation, whereas singly hydrogen-bonded noncyclic dimers do not go through tautomerization within the relaxation time of 1400 ps, doubly hydrogen-bonded cyclic dimers undergo excited-state double proton transfer on the time scale of 25 ps to form tautomeric dimers, which subsequently undergo a conformational change in 180 ps to produce singly hydrogen-bonded tautomers. The rate constant of the double proton transfer reaction is temperature-independent, showing a large kinetic isotope effect of 5.2, suggesting that the rate is governed mostly by tunneling.

    View details for DOI 10.1021/jp106301q

    View details for Web of Science ID 000283471900009

    View details for PubMedID 20939620