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Sample Text
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¡ã) that are, in turn, joined via centrosymmetric methyl CH---O (C---O: 3.553 A˚ , 164.0¡ã) contacts of acetyl groups, thus forming 1D chains [24,25]. Lest data assigned the observed higher frequency band in 1800¨C400 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¡ã (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 ´ß»¯·à×Ó on 2011-3-24 at 19:24 ]

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