The role of superheated water on shielding and mediating hydrogen bonding in N,N′-1,2-ethanediyl-bis(6-hydroxy-hexanamide) crystallization
The role of superheated water on shielding and mediating hydrogen bonding in N,N′-1,2-ethanediyl-bis(6-hydroxy-hexanamide) crystallization
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DOI：
10.1021/cg8002405
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发表时间：
2008-09-01
影响因子：
3.8
通讯作者：
Rastogi, Sanjay
中科院分区：
文献类型：
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作者：
Harings, Jules A. W.;van Asselen, Otto;Rastogi, Sanjay
通过使用低分子量N，N '-1，2-乙二基-双（6-羟基-己酰胺）模型晶体来研究过热水对酰胺结晶中氢键的作用，所述低分子量N，N '-1，2-乙二基-双（6-羟基-己酰胺）模型晶体通过货车德瓦尔斯力形成氢键片层堆叠。热力学、结构和构象研究表明，尽管低温转变起源于从协同构型翻转到构象羟基氢键的可逆变化，但脂族链段中的构象无序/有序与氢键部分之间的氢键效率之间的微妙平衡证实了N，N '-1，2-乙二基-双（6-羟基-己酰胺）作为代表聚酰胺中结晶域的模型化合物。从水的过热状态结晶导致形成结晶亚稳相，其中酰胺部分附近的水分子屏蔽链间氢键。由于这种屏蔽，水分子在结晶过程中消除了平面酰胺基序的构象限制，并且反式亚甲基构象沿着整个分子与强分子间羟基氢键共存。通过退火或连续的温度循环，亚稳态晶体倾向于不可逆地转变成结晶学上更稳定的晶体。在转化过程中，水分子从酰胺部分迁移。由于高度有序的羟基端基之间的高效氢键降低了热触发旋转的笨拙构象的影响，在亚甲基链段中的曲轴类型的运动的影响较小，需要更多的能量来追求固态结晶转变。总之，物理结合的水分子附近的酰胺基序屏蔽分子间氢键和介导的形成稳定的羟基氢键。
The role of superheated water on hydrogen bonding in amide crystallization was examined by the use of low molecular weight N,N '-1,2-ethanediyl-bis(6-hydroxy-hexanamide) model crystals that form hydrogen bonded sheets stacked by van der Waals forces. Thermodynamic, structural and conformational studies reveal that despite a low temperature transition, which originates in a reversible change from cooperative configurational flip-flop to conformational hydroxylic hydrogen bonding, a delicate balance between conformational disorder/order in the aliphatic segments and hydrogen bonding efficiency between the hydrogen bonded moieties validates the role of N,N '-1,2-ethanediyl-bis(6-hydroxy-hexanamide) as a model compound representing the crystalline domains in polyamides. Crystallization from the superheated state of water results in the formation of a thermodynamically metastable phase, where water molecules in the vicinity of the amide moieties shield the interchain hydrogen bonding. Because of this shielding water molecules erase the conformational limitations of the planar amide motifs during crystallization, and trans methylene conformations along the entire molecules coexist with strong intermolecular hydroxylic hydrogen bonding. By annealing or sequential temperature cycles the metastable crystals tend to transform irreversibly into thermodynamically more stable crystals. During the transformation the water molecules migrate from the amide moieties. Since the highly efficient hydrogen bonding between highly ordered hydroxylic end groups decreases the effect of thermally triggered rotating gauche conformers, less effect of the crankshaft type of motion in the methylene segments requires more energy to pursue a solid state crystalline transition. Conclusively, physically bound water molecules near the amide motifs shield intermolecular hydrogen bonding and mediate the formation of stabilizing hydroxylic hydrogen bonds.