Department of Chemistry, School of Science and Technology, and Research Center for Environment Friendly Polymers, Kwansei Gakuin University, Gakuen 2-1, Sanda, Hyogo 669-1337, Japan.
J Phys Chem B. 2013 Feb 21;117(7):2180-7. doi: 10.1021/jp309704k. Epub 2013 Feb 7.
Low-frequency vibrational bands observed in the Raman and terahertz (THz) spectra in the region of 50-150 cm(-1) of crystalline powder poly-(R)-3-hydroxybutyrate (PHB) were assigned based on comparisons of the Raman and THz spectra, polarization directions of THz absorption spectra, and their congruities to quantum mechanically (QM) calculated spectra. This combination, Raman and THz spectroscopies and the QM simulations, has been rarely adopted in spite of its potential of reliable assignments of the vibrational bands. The QM simulation of a spectrum has already been popular in vibrational spectroscopies, but for low-frequency bands of polymers it is still a difficult task due to its large scales of systems and a fact that interactions among polymer chains should be considered in the calculation. In this study, the spectral calculations with the aid of the Cartesian-coordinate tensor transfer (CCT) method were applied successfully to the crystalline PHB, which include the explicit consideration of an intermolecular interaction among helical polymer chains. The agreements between the calculations and the experiments are good in both the Raman and THz spectra in terms of spectral shapes, frequencies, and intensities. A Raman active band at 79 cm(-1) was assigned to the intermolecular vibrational mode of the out-of-plane C═O + CH(3) vibration. A polarization state of the corresponding far-infrared absorption band at ∼82 cm(-1), perpendicular to the helix-elongation direction of PHB, was reproduced only under the explicit correction, which indicates that this polarized band originates from the interaction among the polymer chains. The calculation explored that the polarization direction of this band was along the a axis, which is consistent with the direction in which weak intermolecular hydrogen bonds are suggested between the C═O and CH(3) groups of two parallel polymer chains. The results obtained here have confirmed sensitivity of the low-frequency vibrational bands to the weak hydrogen bonds among the polymer chains.
在结晶粉末聚（R）-3-羟基丁酸（PHB）的拉曼和太赫兹（THz）光谱中，在 50-150 cm(-1) 区域观察到低频振动带，根据拉曼和太赫兹光谱、THz 吸收光谱的偏振方向以及它们与量子力学（QM）计算光谱的一致性进行了归属。尽管这种组合（拉曼和太赫兹光谱学以及 QM 模拟）具有可靠分配振动带的潜力，但它很少被采用。尽管在振动光谱学中 QM 模拟已经很流行，但对于聚合物的低频带，由于系统规模较大以及在计算中应考虑聚合物链之间的相互作用这一事实，它仍然是一项具有挑战性的任务。在这项研究中，借助笛卡尔坐标张量转移（CCT）方法的光谱计算成功应用于结晶 PHB，其中包括对螺旋聚合物链之间的分子间相互作用的明确考虑。在拉曼和太赫兹光谱中，计算与实验在光谱形状、频率和强度方面都具有良好的一致性。79 cm(-1)处的拉曼活性带被分配给非平面 C═O + CH(3)振动的分子间振动模式。在明确校正下，仅在聚合物链的螺旋伸长方向垂直的情况下，模拟了相应的远红外吸收带（约 82 cm(-1)) 的极化状态，这表明该极化带源于聚合物链之间的相互作用。计算结果表明该带的偏振方向沿着 a 轴，这与两个平行聚合物链的 C═O 和 CH(3)基团之间建议的弱分子间氢键的方向一致。这里获得的结果证实了低频振动带对聚合物链之间的弱氢键的敏感性。
