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Issue 18, 2007
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2H-solid state NMR and DSC study of isobutyric acid in mesoporous silica materials

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Abstract

Solid state deuterium NMR has been used to study the molecular motion of d6-isobutyric acid (d6-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 d6-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.

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

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Publication details

The article was received on 05 Dec 2006, accepted on 28 Feb 2007 and first published on 15 Mar 2007


Article type: Paper
DOI: 10.1039/B617744A
Citation: Phys. Chem. Chem. Phys., 2007,9, 2249-2257
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    2H-solid state NMR and DSC study of isobutyric acid in mesoporous silica materials

    A. Vyalikh, Th. Emmler, I. Shenderovich, Y. Zeng, G. H. Findenegg and G. Buntkowsky, Phys. Chem. Chem. Phys., 2007, 9, 2249
    DOI: 10.1039/B617744A

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