Kim Jung-Hwan, Kim Young-Kwang, Kang Heon
Department of Chemistry, Seoul National University, Gwanak-ro 599, Seoul 151-747, Republic of Korea.
J Chem Phys. 2009 Jul 28;131(4):044705. doi: 10.1063/1.3187544.
The effect of hydroxide ions on proton transfer and H/D isotopic exchange of water molecules was examined at the surface of amorphous ice films at temperatures of 92-140 K. Excess hydroxide ions were provided onto a D(2)O-ice film by the hydrolysis of Na atoms, and H(2)O was adsorbed onto the surface for a submonolayer coverage. The H/D isotopic exchange between H(2)O and D(2)O molecules on the ice film surface was monitored as a function of reaction time and temperature by using the techniques of reactive ion scattering and low-energy sputtering. The result was compared with that obtained on a hydroxide-free ice film. At a temperature of 92 K, proton transfer occurred from water mostly to adjacent hydroxide ions. The proton transfer distance and the H/D exchange reaction rate increased with increase in temperature above 105 K. The H/D exchange reaction propagated to several water molecules on the surface at 100-120 K. Kinetic measurement in this temperature range deduced the Arrhenius activation energy for the reaction, E(a) = 9.6+/-2.0 kJ mol(-1). The study shows that hydroxide ions promote the H/D exchange reaction on the ice surface compared with that on a hydroxide-free ice surface, but the promotion effect is moderate and the H/D exchange occurs on a substantial energy barrier. It is suggested that the stabilization of hydroxide ions at the ice surface produces an energy barrier for the proton transfer.
在92 - 140 K的温度下，研究了氢氧根离子对非晶态冰膜表面水分子质子转移和H/D同位素交换的影响。通过钠原子水解将过量的氢氧根离子提供到D₂O冰膜上，并且将H₂O吸附到表面以形成亚单层覆盖。利用反应离子散射和低能溅射技术，监测冰膜表面H₂O和D₂O分子之间的H/D同位素交换随反应时间和温度的变化。将结果与在无氢氧根离子的冰膜上获得的结果进行比较。在92 K的温度下，质子主要从水转移到相邻的氢氧根离子。质子转移距离和H/D交换反应速率随着温度高于105 K而增加。在100 - 120 K时，H/D交换反应扩散到表面上的几个水分子。在此温度范围内的动力学测量推导出该反应的阿仑尼乌斯活化能，E(a) = 9.6±2.0 kJ mol⁻¹。研究表明，与无氢氧根离子的冰表面相比，氢氧根离子促进了冰表面的H/D交换反应，但促进作用适中，且H/D交换发生在相当大的能垒上。有人认为，冰表面氢氧根离子的稳定化产生了质子转移的能垒。
