Department of Chemistry, Tulane University, New Orleans, Louisiana 70118, United States.
J Phys Chem B. 2011 May 12;115(18):5347-53. doi: 10.1021/jp1094189. Epub 2010 Dec 28.
The off-diagonal anharmonicity for a pair of vibrational modes, determined as a shift of their combination level, Δ(12), can be linked to the molecular structure via modeling. The anharmonicity, Δ(12), also determines the amplitude and shape of the cross-peak between modes 1 and 2 measured using 2DIR spectroscopy. For large anharmonicities, the anharmonicity value can be readily obtained from the shape of the cross peak. In practice, however, the anharmonicities are often small (≪1 cm(-1)). In this case, the amplitude of the cross peak rather than its shape is sensitive to the anharmonicity value, and determination of the anharmonicity requires absolute cross-peak measurements. We proposed and tested a new approach of determining anharmonicities, which is based on sensitivity of high-frequency vibrational modes to temperature. The approach permits calibrating the cross-peak amplitude in terms of the effective anharmonicity resulting from the thermal excitation of lower-frequency vibrational modes. It relies on a series of relative 2DIR measurements. While the sensitivity of the method depends on various specific parameters of the molecular system, such as transition dipoles and temperature sensitivity of the high-frequency modes involved, we have estimated that the anharmonicities as small as 0.02 cm(-1) can be determined for the cross peaks between -N(3) and C═O stretching modes using this approach.
对于一对振动模式的非对角非谐性，可以通过建模将其确定为它们的组合能级的位移 Δ(12)。非谐性 Δ(12) 还决定了使用 2DIR 光谱测量的模式 1 和 2 之间的交叉峰的幅度和形状。对于大的非谐性，非谐性值可以很容易地从交叉峰的形状中获得。然而，在实践中，非谐性通常很小（≪1 cm(-1)）。在这种情况下，交叉峰的幅度而不是其形状对非谐性值敏感，并且需要进行绝对交叉峰测量来确定非谐性。我们提出并测试了一种新的确定非谐性的方法，该方法基于高频振动模式对温度的敏感性。该方法允许根据由低频振动模式的热激发引起的有效非谐性来校准交叉峰的幅度。它依赖于一系列相对的 2DIR 测量。虽然该方法的灵敏度取决于分子系统的各种特定参数，例如涉及的高频模式的跃迁偶极子和温度灵敏度，但我们已经估计，使用这种方法可以确定 -N(3)和 C═O 伸缩模式之间的交叉峰的非谐性小至 0.02 cm(-1)。
