2H-solid state NMR and DSC study of isobutyric acid in mesoporous silica materials

被引:43
作者
Vyalikh, A.
Emmler, Th.
Shenderovich, I.
Zeng, Y.
Findenegg, G. H.
Buntkowsky, G.
机构
[1] FSU Jena, Inst Chem Phys, D-07743 Jena, Germany
[2] Free Univ Berlin, Inst Chem, D-14195 Berlin, Germany
[3] Tech Univ Berlin, Stranski Lab Phys & Theoret Chem, D-10623 Berlin, Germany
[4] IFW Dresden, Leibniz Inst Solid State & Mat Res, D-01171 Dresden, Germany
关键词
D O I
10.1039/b617744a
中图分类号
O64 [物理化学(理论化学)、化学物理学];
学科分类号
070304 ; 081704 ;
摘要
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 con. ned systems previously found for benzene is not restricted to molecules with non-specifi. c interactions, but represents a more general phenomenon.
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页码:2249 / 2257
页数:9
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