Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
J Chem Phys. 2012 Dec 21;137(23):234501. doi: 10.1063/1.4770264.
Combination bands that involve CH- or OH-stretch vibrations appear in the near-infrared (NIR) region (4000-10 000 cm(-1)). Because they arise from anharmonic coupling between the component fundamentals, detailed analysis of the frequency and intensity of NIR combination bands allows one to elucidate the mechanisms behind the vibrational coupling in the condensed phase in terms of mechanical and electrical anharmonicities. Nevertheless, little has been studied, in particular experimentally, on the origin of the combination band intensity. Here, we show that NIR electroabsorption (EA) spectroscopy, which directly probes the effects of an externally applied electric field on a combination band, can shed new light on anharmonic vibrational coupling through determination of the direction of the transition moment for the combination band. We studied the combination band of the CH-stretch (ν(1)) and CH-bend (ν(4)) modes of liquid chloroform. The electric-field induced absorbance change of the ν(1) + ν(4) combination band caused by reorientation of the chloroform molecule was measured at various χ angles, where χ is the angle between the direction of the applied electric field and the polarization of the incident IR light. We were able to detect an absorbance change as small as 5 × 10(-8) for the combination band. Using the NIR EA spectra of the combination band together with those of the CH-stretch and bend fundamentals, the angle between the transition moment for the combination band and the permanent dipole moment was determined experimentally for the first time to be (79 ± 14)°. The present investigation indicates that the contribution of the CH-stretch mode to the mechanical anharmonicity is minor and that the CH-bend mode plays a dominant role in the mechanical part of the vibrational coupling between the two fundamentals. Furthermore, density functional theory calculations show that both the mechanical anharmonicity of the CH-bend mode and the electrical anharmonicity may contribute equally to the anharmonic coupling.
涉及 CH 或 OH 伸缩振动的组合带出现在近红外 (NIR) 区域 (4000-10000cm(-1))。由于它们是由组成基频之间的非谐耦合引起的，因此对 NIR 组合带的频率和强度进行详细分析，可以根据机械和电非谐性来阐明凝聚相中振动耦合的机制。然而，特别是在实验方面，对组合带强度的起源研究甚少。在这里，我们表明，近红外电吸收 (EA) 光谱学可以直接探测外加电场对组合带的影响，通过确定组合带跃迁矩的方向，可以为非谐振动耦合提供新的见解。我们研究了液态氯仿的 CH 伸缩 (ν(1)) 和 CH 弯曲 (ν(4)) 模式的组合带。在各种 χ 角下测量了由于氯仿分子取向而引起的 ν(1) + ν(4) 组合带的电致吸收变化，其中 χ 是外加电场的方向与入射红外光的偏振之间的角度。我们能够检测到组合带的吸收变化小至 5×10(-8)。利用组合带的近红外 EA 光谱以及 CH 伸缩和弯曲基频的光谱，首次实验确定了组合带跃迁矩与永久偶极矩之间的角度为 (79 ± 14)°。本研究表明，CH 伸缩模式对机械非谐性的贡献较小，而 CH 弯曲模式在两个基频之间的振动耦合的机械部分中起主导作用。此外，密度泛函理论计算表明，CH 弯曲模式的机械非谐性和电非谐性都可能对非谐耦合有同等贡献。
