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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Intermolecular Coupling in Liquid and Crystalline States of trans-N-Methylacetamide Investigated by Polarized Raman and FT-IR Spectroscopies
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Intermolecular Coupling in Liquid and Crystalline States of trans-N-Methylacetamide Investigated by Polarized Raman and FT-IR Spectroscopies

机译：极性拉曼光谱和FT-IR光谱研究反式-N-甲基乙酰胺在液晶状态下的分子间偶联

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摘要

The isotropic and anisotropic Raman spectra of neat N-methylacetamide (NMA) at different temperatures between -10 and 60 ℃ and NMA in acetonitrile were measured in order to spectroscopically compare and characterize the crystallized (T < 28 ℃) and liquid states. These plus infrared data were subjected to a self-consistent component band analysis. We found that the amide I band is composed of two subbands in the solid phase and three in the liquid phase. For the former, the subbands at 1633 and 1656 cm~(-1) arise from transition dipole coupling interactions associated with the A_g and B_(2g) species of the crystal unit cell. Depolarization ratio measurements suggest a departure from strict D_(2h) symmetry. The three subbands in the liquid phase reflect different aggregate structures. The lowest frequency band at 1634 cm~(-1) results from an NMA oligomer exhibiting a structure similar to that observed in the ordered crystal phase. The most intense subband shows a significant negative noncoincidence effect, its isotropic component appearing at 1650 cm~(-1) and its anisotropic part at 1655 cm~(-1). This subband is interpreted as resulting from locally ordered short oligomeric hydrogen-bonded structures. The third subband is at 1675 cm~(-1) and results from isolated non-hydrogen-bonded NMA molecules or from amide I modes of the terminal groups of the above oligomers. Amide III shows a small but detectable positive noncoincidence effect in the liquid phase (2 cm~(-1)), which is also assignable to transition dipole coupling between adjacent molecules in a locally ordered environment. The Raman bands arising from the symmetric bending modes of the two methyl groups are significantly affected by crystallization; the CCH_3 symmetric bending mode becomes depolarized and less intense while the NCH_3 symmetric bending mode gains intensity and becomes polarized. Ab initio calculations of torsional distortions of the CH_3 groups, caused by interactions between adjacent non-hydrogen-bonded NMA molecules in the crystal, qualitatively reproduce these effects.
机译：在-10至60℃的不同温度下测定了NMA的各向异性和各向异性的拉曼光谱，并测定了乙腈中的NMA的光谱，以光谱比较和表征结晶（T <28℃）和液态。这些加上红外的数据进行了自洽分量带分析。我们发现酰胺I谱带由固相的两个子带和液相的三个子带组成。对于前者，在1633和1656 cm〜（-1）处的子带来自与晶体晶胞的A_g和B_（2g）物种相关的跃迁偶极耦合相互作用。去极化比的测量表明偏离了严格的D_（2h）对称性。液相中的三个子带反映了不同的聚集体结构。 NMA低聚物产生的最低频段为1634 cm〜（-1），其结构类似于在有序晶相中观察到的结构。最强的子带表现出显着的负非重合效应，其各向同性分量出现在1650 cm〜（-1），各向异性部分出现在1655 cm〜（-1）。该子带被解释为由局部有序的短寡聚氢键结构产生。第三子带在1675cm-1（-1）处，由分离的非氢键结合的NMA分子或上述低聚物的末端基团的酰胺I模式产生。酰胺III在液相中（2 cm〜（-1））表现出很小但可检测到的正不一致现象，这也可归因于局部有序环境中相邻分子之间的跃迁偶极耦合。由两个甲基的对称弯曲模式产生的拉曼谱带受结晶的影响很大。 CCH_3对称弯曲模式变为去极化且强度较小，而NCH_3对称弯曲模式获得强度并变为极化。由晶体中相邻的无氢键合NMA分子之间的相互作用引起的CH_3基团的扭曲变形的从头算计算，从质上再现了这些效果。

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