Lin Y-S, Ramesh S G, Shorb J M, Sibert E L, Skinner J L
Theoretical Chemistry Institute and Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706, USA.
J Phys Chem B. 2008 Jan 17;112(2):390-8. doi: 10.1021/jp075682s. Epub 2007 Nov 29.
The population lifetimes of the bend fundamental of dilute water in liquid chloroform (8.5 ps) and d-chloroform (28.5 ps) display an interesting solvent isotope effect. As the lowest excited vibrational state of the molecule, the water bend fundamental relaxes directly to the ground state with about 1600 cm-1 of energy released to the other degrees of freedom. The strong solvent isotope effect along with the large energy gap indicates the participation of solvent vibrational modes in this vibrational energy relaxation process. We calculate the vibrational energy relaxation rates of the water bend in chloroform and d-chloroform using the Landau-Teller formula with a new potential model developed and parametrized self-consistently to describe the chloroform-water interaction. The computed values are in reasonable agreement with the experimental results, and the trend for the isotope effect is correct. It is found that energy transfer to the solvent vibrations does indeed play an important role. Nevertheless, no single dominant solvent accepting mode can be identified; the relaxation appears to involve both the bend and the C-Cl stretches, and frequency changes of all of these modes upon deuteration contribute to the observed solvent isotope effect.
液态氯仿（8.5皮秒）和氘代氯仿（28.5皮秒）中稀水溶液弯曲基态的分子寿命呈现出有趣的溶剂同位素效应。作为分子的最低激发振动态，水的弯曲基态直接弛豫到基态，约1600厘米-1的能量释放到其他自由度。强烈的溶剂同位素效应以及较大的能隙表明溶剂振动模式参与了这个振动能量弛豫过程。我们使用Landau-Teller公式和一个新的势模型来计算氯仿和氘代氯仿中水弯曲振动的能量弛豫速率，该势模型是经过自洽开发和参数化以描述氯仿-水相互作用的。计算值与实验结果合理吻合，同位素效应的趋势也是正确的。结果发现，向溶剂振动的能量转移确实起着重要作用。然而，无法确定单一的主导溶剂接受模式；弛豫似乎涉及弯曲振动和C-Cl伸缩振动，并且氘代后所有这些模式的频率变化都对观察到的溶剂同位素效应有贡献。
