Vyalikh A, Emmler Th, Shenderovich I, Zeng Y, Findenegg G H, Buntkowsky G
Freie Universität Berlin, Institut für Chemie, Takustrasse 3, 14195 Berlin, Germany.
Phys Chem Chem Phys. 2007 Jun 14;9(18):2249-57. doi: 10.1039/b617744a. Epub 2007 Mar 15.
Solid state deuterium NMR has been used to study the molecular motion of d(6)-isobutyric acid (d(6)-iBA) in the pure (unconfined) state and confined in the cylindrical pores of two periodic mesoporous silica materials (MCM-41, pore size 3.3 nm and SBA-15, pore size 8 nm), and in a controlled pore glass (CPG-10-75, pore size ca. 10 nm). The line shape analysis of the spectra at different temperatures revealed three rotational states of the iBA molecules: liquid (fast anisotropic reorientation of the molecule), solid I (rotation of the methyl group) and solid II (no rotational motion on the time scale of the experiment). Transition temperatures between these states were determined from the temperature dependence of the fraction of molecules in these states. Whereas the solid I-solid II transition temperature is not affected by confinement, a significant lowering of the liquid-solid I transition temperature in the pores relative to the bulk acid was found for the three matrix materials, exhibiting an unusual dependence on pore size and pore morphology. Complementary DSC measurements on the same systems show that the rotational melting (solid I-liquid) of d(6)-iBA in the pores occurs at a temperature 20-45 K below the thermodynamic melting point. This finding indicated that the decoupling of rotational and translational degrees of freedom in phase transitions in confined systems previously found for benzene is not restricted to molecules with non-specific interactions, but represents a more general phenomenon.
固态氘核磁共振已被用于研究处于纯态（无限制）以及限制在两种周期性介孔二氧化硅材料（MCM - 41，孔径3.3纳米和SBA - 15，孔径8纳米）的圆柱形孔中，还有在可控孔径玻璃（CPG - 10 - 75，孔径约10纳米）中的d(6)-异丁酸（d(6)-iBA）的分子运动。对不同温度下光谱的线形分析揭示了iBA分子的三种旋转状态：液态（分子快速各向异性重排）、固态I（甲基旋转）和固态II（在实验时间尺度上无旋转运动）。这些状态之间的转变温度由这些状态下分子分数的温度依赖性确定。虽然固态I - 固态II转变温度不受限制影响，但对于三种基质材料，发现孔中液态 - 固态I转变温度相对于本体酸有显著降低，且表现出对孔径和孔形态的异常依赖性。对相同体系的补充差示扫描量热法测量表明，孔中d(6)-iBA的旋转熔化（固态I - 液态）发生在比热力学熔点低20 - 45 K的温度下。这一发现表明，先前在苯中发现的受限体系相变中旋转和平动自由度的解耦并不局限于具有非特异性相互作用的分子，而是一种更普遍的现象。
