Hydrogen-bond assisted enormous broadening of infrared spectra of phenol-water cationic cluster: an ab initio mixed quantum-classical study.
Hydrogen-bond assisted enormous broadening of infrared spectra of phenol-water cationic cluster: an ab initio mixed quantum-classical study.
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DOI：
10.1063/1.2434778
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发表时间：
2007-02
期刊：
影响因子：
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通讯作者：
Takefumi Yamashita;K. Takatsuka
中科院分区：
文献类型：
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作者：
Takefumi Yamashita;K. Takatsuka
Sawamura等人拍摄的苯酚-水阳离子簇合物[PhOH.H2O]+的红外光谱。作者声明：[J.Phys.化学。100,8131(1996)]令人费解的是，由酚羟基的伸缩模式引起的峰(中性单体3657 cm-1，PhOH.H2O 3524 cm-1)似乎消失了，相反，观察到一条向下延伸到2900 cm-1的极宽的尾巴。本文作者从理论上将这种反常光谱归因于氢键引起的OH伸缩峰的不均匀展宽，苯环的电离大大增强了氢键的强度。事实上，他们估计峰值位置在2300厘米-1，当团簇能量为32千卡/摩尔时，光谱宽度可达1000厘米-1。这种令人惊讶的宽广可能在氢键系统中是通用的，这反过来又有助于研究各种系统中氢键辅助动力学的性质，如DNA和蛋白质中的那些系统。为了定量地研究本体系，作者发展了一种从头算混合量子经典方法，其中绝热从头算势面上的核运动被处理成只用量子力学描述OH的伸缩运动，而其余所有剩余的模都用On-the-Fly格式经典地处理。这种方法包括许多数值方法的实现，这使得它能够处理相对较大的分子系统。利用这种理论方法，作者对现今的反常展宽现象进行了详细的分析。特别是，他们提出，人们可以通过时间分辨红外光谱来提取关于氢键动力学的直接信息，这是关于氢键伸缩的振动激发能与分子间距离之间的明确关联：反映了相关氢键分子间距离的缓慢而广泛的变化，时间分辨光谱预测在很大的频域范围内变化(移动)很大。由此发现，沿选定轨道的短时平均值灵敏地反映了分子间距离的变化。研究了内能对氢键和氢谱的影响。
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.