Hanna Gabriel, Geva Eitan
Department of Chemistry and FOCUS Center, University of Michigan, Ann Arbor, Michigan 48109-1055, USA.
J Phys Chem B. 2008 Dec 11;112(49):15793-800. doi: 10.1021/jp8072816.
Isotope effects on rate processes and spectra are often used to elucidate the nature of the interactions underlying molecular structure and dynamics. In this paper, we present a computational study of the effect of substituting hydrogen by deuterium in a solvated hydrogen-bonded complex on the rates of the various processes involved in the vibrational relaxation of the hydrogen/deuterium stretch and on the corresponding 1D and 2D infrared spectra. The vibrational relaxation is simulated via the mixed quantum-classical Liouville method, where the proton/deuteron is treated quantum mechanically while the other particles are treated in a classical-like manner. We find that the vibrational relaxation pathway and the rates of the various steps in it are similar for the deuterium and hydrogen stretches. However, we also find that isotope substitution modifies the 1D and 2D spectra of the stretch in a qualitative manner. The differences between the spectra are explained in terms of the narrowing and broadening of the fundamental and overtone transition frequency ranges, respectively, and the smaller transition dipole moments in the case of the deuterium stretch. Our results demonstrate that isotope substitution may have a rather dramatic effect on the infrared spectra of a vibrational mode strongly coupled to its environment even though the rate and pathway of the underlying vibrational relaxation may not be overly sensitive to it.
同位素对速率过程和光谱的影响常被用于阐明分子结构和动力学背后相互作用的本质。在本文中，我们进行了一项计算研究，考察在溶剂化氢键复合物中用氘取代氢对氢/氘伸缩振动弛豫所涉及的各种过程的速率以及相应的一维和二维红外光谱的影响。振动弛豫通过混合量子 - 经典刘维尔方法进行模拟，其中质子/氘核采用量子力学处理，而其他粒子则采用类似经典的方式处理。我们发现，对于氘和氢的伸缩振动，振动弛豫途径及其各个步骤的速率相似。然而，我们还发现同位素取代以定性方式改变了伸缩振动的一维和二维光谱。光谱之间的差异分别从基频和泛音跃迁频率范围的变窄和变宽，以及氘伸缩振动情况下较小的跃迁偶极矩方面进行了解释。我们的结果表明，即使潜在振动弛豫的速率和途径可能对其不太敏感，但同位素取代可能会对与环境强烈耦合的振动模式的红外光谱产生相当显著的影响。
