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催化粪子

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IR-LD(Solid-state linear-dichroic infrared) spectroscopic characterization of Aspirin form I

Difference IR-LD spectrum of form I (Fig. 1(1)) illustrated a signiﬁcant level of macro-orientation of solid guest molecule in nematic liquid crystal mesophase. The broad absorption peak between 3200 and 2300 cm-1 belongs to VOH stretching vibration of COOH group in the Aspirin molecule and indicated the strong intermolecular interactions, determined by single crystal X-ray diffraction [24,25]. According to[24,25], form I is characterized P21/C space group and consists of centrosymmetric carboxylic acid dimmer moieties (O---O: 2.635 A˚ , 177.7°) that are, in turn, joined via centrosymmetric methyl CH---O (C---O: 3.553 A˚ , 164.0°) contacts of acetyl groups, thus forming 1D chains [24,25]. Lest data assigned the observed higher frequency band in 1800–400 cm-1 region (Fig. 2(1)) at 1752 cm-1 to VC=O stretching mode of acetoxy group and the maximum at 1693 cm-1 to VC=O of carboxylic one as a result of different intermolecular hydrogen bondlengths. Both maxima are observed as doublets, which could be explained with crystal ﬁeld splitting effect. The other intensive peak at 1606 cm-1 corresponds to 8a skeletal in-plane vibration of aromatic ring [26]. The adequate experimentally IR-band assignment of the IR-bands in spectral region lower than 1400 cm-1 is possible through next IR-LD spectral analysis. Elimination of 1752 cm-1 and 1693 cm-1 peaks in different dichroic ratio (Fig. 3(1)) is a result of near to perpendicular orientation of both VC=O transition moments in the frame of Aspirin molecule, closing a dihedral angle of 83.6° (Scheme 2A) [24,25]. Elimination of 1606 cm-1 peak provoked the disappearance of peaks at 1307 cm-1and 917 cm-1 (Fig. 3(2)) belonging to same symmetry class (A1) and assigned as 3 and 1 modes of o-disubstituted benzene ring (Scheme 2B) [26]. Simultaneous disappearance of 836 cm-1 and 755 cm-1 (Fig. 4(1)) assigned their out-of-plane origin (B1 symmetry class) and belonged to 11-rCH and 4-rArbending vibrations (Scheme2B) [24]. Reduction of peaks at 668 cm-1and 599 cm-1 in same dichroic ratio (Fig. 4(2)) assigned their origin as 12(X) and 6a(X), both in-plane bending modes [26].

[ Last edited by 催化粪子 on 2011-3-24 at 19:24 ]

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1楼 2011-03-24 19:22:20

已阅回复此楼关注TA 给TA发消息送TA红花 TA的回帖

zxs1989

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催化粪子(金币+108, 翻译EPI+1): 感谢！！ 2011-03-25 10:21:39

阿司匹林形态I的 IR-LD (固态线二色红外) 波谱表征
形态I的差示IR-LD光谱（图 1（1））说明了固态客体分子在向列液晶中间相中的宏观取向显著存在。3200-2300 cm-1 的宽吸收峰由阿司匹林分子中羧基的OH伸缩振动（νOH）导致，它表明了阿司匹林分子间存在强的相互作用，这种相互作用已用单晶X射线衍射测定[24,25]。根据文献[24,25]，形态I以P21/C空间群为特点，并由中心对称的羧酸二聚体(O---O: 2.635 A˚ , 177.7°)组成，这些二聚体依次通过中心对称的甲基-乙酰基CH---O (C---O: 3.553 A˚ , 164.0°)相互作用而形成一维的链状结构[24,25]。在1800–400 cm-1区，1752 cm-1处的高频吸收峰被归因于乙酰氧基的C=O伸缩振动，而在1693 cm-1处的最高吸收峰则被认为是羧基的C=O伸缩振动峰，两峰的位置不同是由分子间氢键键长不同所致。此两峰均是双峰，这可用晶体场裂分效应解释。在1606 cm-1处的另一强峰对应于苯环的8a面内骨架振动[26].波数在1400cm-1以下的IR吸收峰也许可以通过以下的IR-LD波谱分析合理地确定归属。不同双色比条件下1752和1693cm-1处两吸收峰的消失（图3（1））是由于在阿司匹林骨架中两种VC=O跃迁矩近似垂直的取向，其二面角接近83.6° (结构式 2A) [24,25].1606cm-1处吸收峰的消失伴随着属于同一对称类的1307cm-1和917cm-1处吸收峰的消失，这是由于邻位二取代苯环的振动模式3和1所致（结构式 2B）[26]。836cm-1和755cm-1处两峰的同时消失显示它们同属于面外振动峰（B1 对称类），分别由11-γCH和4-γAr弯曲振动引起（结构式 2B）[24]。在相同双色比条件下，668 cm-1和 599 cm-1处两峰的消失（图4（2））则证实了它们对应于12(X) 和 6a(X)的面内弯曲振动模式。

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