Symmetric Stretching Vibration of CH4 in Clathrate Hydrate Structures
Symmetric Stretching Vibration of CH4 in Clathrate Hydrate Structures
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DOI：
10.1002/cphc.201000519
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发表时间：
2010-10-04
期刊：
影响因子：
2.9
通讯作者：
Nagao, Jiro
中科院分区：
文献类型：
--
作者：
Ohno, Hiroshi;Kida, Masato;Nagao, Jiro
笼形水合物（也称为气体水合物）是冰状结晶固体，当氢键水笼结构在适当条件（压力，温度和浓度）下包封小分子时形成。[1]对于气态客体如天然气组分，水合物在适度低温和高压下形成。[1]天然气水合物结晶为立方结构I（sI）、立方结构II（sII）或六方结构H（sH）。[1]sI的晶胞由46个水分子组成，它们形成2个具有12个五边形面的五边形十二面体（512）和6个具有12个五边形面和2个六边形面的四面体（51262）。[2]sII的晶胞由136个水分子组成，形成16个512笼和8个具有12个五边形面和4个六边形面的六边形（51264）。[3]sH的晶胞由34个水分子组成，形成3512个笼，2个不规则十二面体，3个正方形面，6个五边形面和3个六边形面（435663）和一个二十面体，12个五边形面和8个六边形面（51268）。[4]天然气的主要成分是甲烷，约占天然气平均成分的90%。纯甲烷形成sI水合物; sII和sH水合物由甲烷和较大烃分子的气体混合物形成。拉曼光谱已被用于研究包括笼形水合物相的分子的振动状态。甲烷的对称伸缩模是拉曼活性的。在水合物笼中包体化时发生红移。[5]关于sI和sII水合物，小腔（512）中的CH 4的拉曼频率容易与大腔（51262或51264）中的那些区分开。因此，CH 4拉曼谱带已被用来识别水合物相，也调查笼填充。[5，6]然而，sH水合物中甲烷的拉曼特征仍然令人困惑。在大分子客体物质（LMGS）存在下，客体甲烷可以占据小（512）和中等（435663）笼的sH水合物。认为512和435663椎间融合器的CH 4所经受的椎间融合器力场明显不同。由于这个原因，观察到不同的甲烷频带的小型和中型笼分子，在这种情况下，为sI和sII水合物。然而，先前对具有LMGS的sH样品的拉曼研究仅显示在约2913 cm-1处的一个峰[5，7]，除了一份报告[8]，即在约2901 cm-1处存在另外的峰。使用多种分析技术，包括粉末X射线衍射（PXRD）和拉曼和NMR光谱，Susilo等人。[7]进行了系统的表征sH水合物样品合成与几个LMGS和驱动力。他们认为，在2901 cm ↑ [2]-1附近观察到的峰值来自与sH相混合的sI水合物。他们的结论是，甲烷在sH小型和中型腔中的拉曼谱线太接近而无法分辨。与客体甲烷的拉曼测量相关的另一个问题是振动频率与笼形水合物结构的函数关系。对于sI水合物，报告的512个洞穴中甲烷的峰值位置为2914-2916 cm ± 1; 51262个洞穴中的峰值位置为2904-2905 cm ± 1。[5，6，9，10]对于sII样品，以前的工作表明，小（512）和大（51264）笼分子的甲烷振动分别为2913-2915和2903-2904 cm-1。[5，9-11]这些观察结果表明，尽管sI 51262和sII 51264的笼形几何形状不同，但它们中甲烷的拉曼频率几乎相等，而512笼的甲烷带位置略微取决于水合物结构。不过...
Clathrate hydrates (also called gas hydrates) are ice-like crystalline solids that form when hydrogen-bonded water cage structures enclathrate small molecules under appropriate conditions (pressure, temperature, and concentration).[1] For gaseous guests such as natural gas components, hydrates form at moderately low temperatures and high pressures.[1] Natural gas hydrates crystallize into a cubic structure I (sI), a cubic structure II (sII), or a hexagonal structure H (sH).[1] A unit cell of sI consists of 46 water molecules that form 2 pentagonal dodecahedra with 12 pentagonal faces (512) and 6 tetrakaidecahedra with 12 pentagonal faces and 2 hexagonal faces (51262).[2] A unit cell of sII comprises 136 water molecules that form 16 512 cages and 8 hexakaidecahedra with 12 pentagonal faces and 4 hexagonal faces (51264).[3] A unit cell of sH comprises 34 water molecules that form 3 512 cages, 2 irregular dodecahedra with 3 square faces, 6 pentagonal faces and 3 hexagonal faces (435663) and a single icosahedron with 12 pentagonal faces and 8 hexagonal faces (51268).[4] The main constituent of natural gases is methane, which accounts for approximately 90% of the average gas composition. Pure methane forms sI hydrate; sII and sH hydrates are formed from gas mixtures of methane and larger hydrocarbon molecules. Raman spectroscopy has been used to study vibrational states of molecules comprising clathrate hydrate phases. The symmetric stretching mode of methane is Raman active. It is red-shifted when enclathrated in hydrate cages.[5] Regarding sI and sII hydrates, Raman frequencies of CH4 in small cavities (512) are readily distinguishable from those in large cavities (51262 or 51264). Therefore, CH4 Raman bands have been used to identify hydrate phases and also to investigate cage fillings.[5, 6] Nevertheless, Raman features of methane in sH hydrates remain puzzling. In the presence of large molecule guest substances (LMGSs), guest methane can occupy small (512) and medium (435663) cages of sH hydrates. It is considered that the cage force field that is experienced by CH4 is markedly different for the 512 and 435663 cages. For that reason, distinct methane bands are observed for the small and medium cage molecules, as in the case for sI and sII hydrates. However, previous Raman studies of sH samples with LMGSs showed only one peak at approximately 2913 cmÀ1,[5, 7] except for one report [8] that an additional peak exists at around 2901 cmÀ1. Using multiple analytical techniques including powder X-ray diffraction (PXRD) and Raman and NMR spectroscopy, Susilo et al.[7] performed systematic characterization of sH hydrate samples synthesized with several LMGSs and driving forces. They argued that the observed peak at around 2901 cmÀ1 is from sI hydrates mixed with sH phases. They concluded that Raman lines for methane in the sH small and medium cavities are too close to be resolved. Another question related to Raman measurements of guest methane is the vibrational frequency as a function of clathrate hydrate structures. For sI hydrates, the reported peak positions of methane in 512 cavities are 2914–2916 cmÀ1; those in 51262 cavities are 2904–2905 cmÀ1.[5, 6, 9, 10] For sII samples, previous works showed that methane vibrations of the small (512) and large (51264) cage molecules are, respectively, 2913–2915 and 2903–2904 cmÀ1.[5, 9–11] These observations imply that Raman frequencies of methane in the sI 51262 and sII 51264 are nearly equal despite their different cage geometries, whereas methane band positions for 512 cages are slightly dependent on hydrate structures. Nevertheless …