Ultrafast Proton Transfer to Solvent: Molecularity and Intermediates from Solvation- and Diffusion-Controlled Regimes
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- 作者：J. L. Pé ; rez-Lustres ; F. Rodriguez-Prieto ; M. Mosquera ; T. A. Senyushkina ; N. P. Ernsting ; S. A. Kovalenko
- 刊名：Journal of the American Chemical Society
- 出版年：2007
- 出版时间：May 2, 2007
- 年：2007
- 卷：129
- 期：17
- 页码：5408 - 5418
- 全文大小：825K
- 年卷期：v.129,no.17(May 2, 2007)
- ISSN：1520-5126
文摘
Photoinduced proton transfer (PT) from cations 6-hydroxyquinolinium (6HQc) and 6-hydroxy-1-methylquinolinium (6MQc) to water and alcohols, and solvation of the zwitterionic conjugate base1-methylquinolinium-6-olate (6MQz) were studied with stationary and transient absorption spectroscopyand by quantum chemical calculations. Transient emission spectra from 6MQz in acetonitrile and proticsolvents shift dynamically to the red without changing their shape and intensity. The shift matches thesolvation correlation function C(t) either measured with known solvatochromic probes coumarin 343 andcoumarin 153 or derived from infrared/dielectric-loss data on neat solvents. This indicates that 6MQz monitorsthe solvation dynamics and that no intramolecular electron transfer occurs on a subpicosecond or longertime scale. The PT dynamics S(t) from 6HQc and 6MQc closely follows C(t), being initially 2-3 timesslower. This allows for the conclusion that PT is controlled by solvation, with a barrier of 2 kJ/mol. In water,a pre-condition of this ultrafast reaction seems to be hydrogen-bonding between the negatively chargedoxygen and two water molecules, resulting in a complex 6HQc:H2O:H2O. The complex is stable due to ahigh (47 kJ/mol) bonding energy between 6HQc and a water molecule. In acetonitrile, the reaction equilibriumis strongly shifted to the cation. There an intermediate PT state was detected, which may be ascribed tothe cationic form 6HQc:H2O due to residual water impurities. In water-acetonitrile mixtures, the ultrafastsolvent-controlled PT is followed by a diffusion-controlled reaction; the measured rate kD 
p.gif"> 1010 s-1 M-1 ischaracteristic for simple bimolecular diffusion. The dependence of the short-time PT signal on waterconcentration can be fitted with a Poisson distribution of water molecules around the cation. Altogether,the short-time and long-time behaviors provide strong evidence that diffusion of only one water moleculeis sufficient to detach the proton. Subsequent solvent stabilization of the products completes the PT reaction.
p.gif"> 1010 s-1 M-1 ischaracteristic for simple bimolecular diffusion. The dependence of the short-time PT signal on waterconcentration can be fitted with a Poisson distribution of water molecules around the cation. Altogether,the short-time and long-time behaviors provide strong evidence that diffusion of only one water moleculeis sufficient to detach the proton. Subsequent solvent stabilization of the products completes the PT reaction.