Yamashita Takefumi, Takatsuka Kazuo
Department of Basic Science, Graduate School of Arts and Sciences, University of Tokyo, Komaba, Tokyo 153-8902, Japan.
J Chem Phys. 2007 Feb 21;126(7):074304. doi: 10.1063/1.2434778.
The infrared spectrum of phenol-water cationic cluster, [PhOH.H2O]+, taken by Sawamura et al. [J. Phys. Chem. 100, 8131 (1996)] is puzzling in that the peak due to the stretching mode of the phenolic OH (3657 cm-1 for a neutral monomer and 3524 cm-1 for PhOH.H2O) seemingly disappears and instead an extremely broad tail extending down to 2900 cm-1 is observed. The present authors theoretically ascribe this anomalous spectrum to an inhomogeneous broadening of the OH stretching peak caused by the hydrogen bond, the strength of which has been greatly enhanced by ionization of the phenyl ring. Indeed they estimate that the peak position is at 2300 cm-1 and the spectral width can become as wide as 1000 cm-1 at the cluster energy of 32 kcal/mol. This surprisingly wide broadening can be generic in hydrogen-bond systems, which in turn is useful to study the nature of the hydrogen-bond assisted dynamics in various systems such as those in DNA and proteins. To study the present system quantitatively, the authors have developed an ab initio mixed quantum-classical method, in which the nuclear motions on an adiabatic ab initio potential surface are treated such that only the OH stretching motion is described quantum mechanically, while all the other remaining modes are treated classically with on-the-fly scheme. This method includes the implementation of many numerical methodologies, which enables it to deal with a relatively large molecular system. With this theoretical method, the authors analyze the present anomalous broadening in a great detail. In particular, they suggest that one can extract direct information about the hydrogen-bond dynamics with respect to the clear correlation between the vibrational excitation energy of the OH stretching and intermolecular distance by means of a time-resolved infrared spectroscopy: Reflecting the slow and wide-range variation of the intermolecular distance of the relevant hydrogen bond, the time-resolved spectrum is predicted to vary (shift) largely covering the wide range of frequency domain. Thus, it is found that the short-time average along a selected trajectory sensitively reflects the change of the intermolecular distance. The authors also study the effect of internal energy on the hydrogen bonding and the OH spectrum.
泽村等人[《物理化学杂志》100, 8131 (1996)]测得的苯酚 - 水阳离子簇[PhOH·H₂O]⁺的红外光谱令人费解，因为酚羟基拉伸模式产生的峰（中性单体为3657 cm⁻¹，PhOH·H₂O为3524 cm⁻¹）看似消失了，取而代之的是观察到一个向下延伸至2900 cm⁻¹的极宽的尾部。本文作者从理论上将这种异常光谱归因于氢键导致的OH拉伸峰的非均匀展宽，苯环的电离极大地增强了氢键的强度。实际上，他们估计在簇能量为32 kcal/mol时，峰位置在2300 cm⁻¹，光谱宽度可达1000 cm⁻¹。这种惊人的宽展宽在氢键系统中可能是普遍存在的，这反过来又有助于研究各种系统（如DNA和蛋白质中的系统）中氢键辅助动力学的本质。为了定量研究当前系统，作者开发了一种从头算混合量子 - 经典方法，其中在绝热从头算势能面上处理核运动，使得只有OH拉伸运动用量子力学描述，而所有其他剩余模式用即时方案进行经典处理。该方法包括许多数值方法的实现，这使其能够处理相对较大的分子系统。利用这种理论方法，作者对当前的异常展宽进行了详细分析。特别是，他们认为通过时间分辨红外光谱，可以提取关于OH拉伸振动激发能与分子间距离之间明确相关性的氢键动力学的直接信息：反映相关氢键分子间距离的缓慢和大范围变化，时间分辨光谱预计会在很大的频率域范围内变化（移动）。因此，发现沿着选定轨迹的短时间平均值敏感地反映了分子间距离的变化。作者还研究了内能对氢键和OH光谱的影响。
