Siwick B J, Cox M J, Bakker H J
FOM Institute AMOLF, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands.
J Phys Chem B. 2008 Jan 17;112(2):378-89. doi: 10.1021/jp075663i. Epub 2007 Dec 8.
We study the mechanism of proton transfer (PT) in the aqueous acid-base reaction between the photoacid 8-hydroxy-1,3,6-pyrenetrisulfonic acid (HPTS) and acetate by probing the vibrational resonances of HPTS, acetate, and the hydrated proton with femtosecond mid-infrared laser pulses. We find that PT takes place in a distribution of hydrogen-bound reaction complexes that differ in the number of water molecules separating the acid and the base. The number of intervening water molecules ranges from 0 to 5, which, together with a strongly distance-dependent PT rate, explains the observed highly nonexponential reaction kinetics. The kinetic isotope effect for the reaction is determined to be 1.5, indicating that tunneling does not play a significant role in the transfer of the proton. Rather, the transfer mechanism is best described in terms of the adiabatic PT picture as it has been formulated by Hynes and co-workers [Staib, A.; Borgis, D.; Hynes, J. T. J. Chem. Phys. 1995, 102, 2487. Ando, K.; Hynes, J. T. J. Phys. Chem. B 1997, 101, 10464.], where solvent fluctuations play an essential role in forming the correct hydrogen-bond configuration and solvent polarization to facilitate PT.
我们通过用飞秒中红外激光脉冲探测8-羟基-1,3,6-芘三磺酸（HPTS）、乙酸根以及水合质子的振动共振，研究了光酸HPTS与乙酸根在水相酸碱反应中的质子转移（PT）机制。我们发现质子转移发生在一系列氢键连接的反应复合物中，这些复合物中酸和碱之间的水分子数量不同。中间水分子的数量范围为0到5，这与强烈依赖距离的质子转移速率一起，解释了所观察到的高度非指数反应动力学。该反应的动力学同位素效应确定为1.5，表明隧穿在质子转移中不起重要作用。相反，转移机制最好用Hynes及其同事提出的绝热质子转移图景来描述[Staib, A.; Borgis, D.; Hynes, J. T. J. Chem. Phys. 1995, 102, 2487. Ando, K.; Hynes, J. T.; J. Phys. Chem. B 1997, 101, 10464.]，其中溶剂涨落在形成正确的氢键构型和溶剂极化以促进质子转移方面起着至关重要的作用。
