Ljungberg Mathias P, Lyubartsev A P, Nilsson Anders, Pettersson Lars G M
FYSIKUM, Stockholm University, AlbaNova University Center, SE-106 91 Stockholm, Sweden.
J Chem Phys. 2009 Jul 21;131(3):034501. doi: 10.1063/1.3154144.
We analyze the validity of the commonly used electric-field (E-field) approximation to vibrational OH stretch Raman spectra of dilute HOD in D(2)O by computing the OH stretch frequency of all molecules in several different structure models, each containing around 2000 molecules. The calculations are done at the B3LYP level using clusters containing 32 molecules centered around the molecule for which the frequencies are calculated; the large cluster size is required due to significant nonlocal contributions influencing the computed frequencies. The vibrational frequencies are determined using a six-point potential optimized discrete variable representation. Raman and infrared intensities are furthermore computed to generate the spectra. We find that a quadratic fit of E-field versus frequency gives a reasonable representation of the calculated distribution of frequencies. However, the mapping depends significantly on the structural model and is thus not universal. Anharmonic couplings are calculated for several optimized clusters showing a general trend to compress the computed frequency distributions, which is in agreement with dynamical simulations (motional narrowing).
我们通过计算几种不同结构模型中所有分子的OH伸缩频率，来分析常用的电场（E场）近似对D₂O中稀HOD振动OH伸缩拉曼光谱的有效性，每个模型包含约2000个分子。计算在B3LYP水平上进行，使用以计算频率的分子为中心、包含32个分子的团簇；由于显著的非局部贡献影响计算频率，所以需要大的团簇尺寸。使用六点势优化离散变量表示法确定振动频率。此外，计算拉曼和红外强度以生成光谱。我们发现E场与频率的二次拟合能合理地表示计算出的频率分布。然而，这种映射很大程度上取决于结构模型，因此不具有普遍性。对几个优化团簇计算了非谐耦合，显示出压缩计算频率分布的总体趋势，这与动力学模拟（运动窄化）一致。
