3 research outputs found
Studies on the local structure of the F ∕ OH site in topaz by magic angle spinning nuclear magnetic resonance and Raman spectroscopy
The mutual influence of F and OH groups in neighboring
sites in topaz (Al2SiO4(F,OH)2) was investigated using magic
angle spinning nuclear magnetic resonance (MAS NMR) and Raman spectroscopy.
The splitting of 19F and 1H NMR signals, as well as the OH Raman band,
provides evidence for hydrogen bond formation within the crystal structure.
Depending on whether a given OHÂ group has another OHÂ group or fluoride as
its neighbor, two different hydrogen bond constellations may form: either
OH⋯O⋯HO or F⋯H⋯O. The proton accepting oxygen was determined to be part of the
SiO4 tetrahedron using 29Si MAS NMR. Comparison of the MAS NMR
data between an OH-bearing and an OH-free topaz sample confirms that the
19F signal at −130 ppm stems from F− ions that take part in
H⋯F bonds with a distance of
∼ 2.4 Å, whereas the main signal at −135 ppm belongs to
fluoride ions with no immediate OH group neighbors. The Raman OH sub-band at
3644 cm−1 stems from OH groups neighboring other OH groups, whereas the
sub-band at 3650 cm−1 stems from OH groups with fluoride neighbors,
which are affected by H⋯F bridging. The
integrated intensities of these two sub-bands do not conform to the expected
ratios based on probabilistic calculations from the total OH concentration.
This can be explained by (1)Â a difference in the polarizability of the OH bond
between the different hydrogen bond constellations or (2)Â partial order
or unmixing of F and OH, or a combination of both. This has implications for
the quantitative interpretation of Raman data on OH bonds in general and
their potential use as a probe for structural (dis-)order. No indication of
tetrahedrally coordinated Al was found with 27Al MAS NMR, suggesting
that the investigated samples likely have nearly ideal Al/Si ratios, making
them potentially useful as high-density electron microprobe reference
materials for Al and Si, as well as for F
Melting of Low Molecular Weight Compounds in Confinement Observed by ²H-Solid State NMR: Biphenyl, a Case Study
The ²H-NMR solid echo spectra of biphenyl molecules as guests in the mesopores of neat
and silylated SBA-15 have been measured as a function of temperature. At low temperatures
typical ²H-Pake patterns with parameters of (Qzz = 132 kHz, corresponding to Qcc = 176 kHz) and
(η = 0.04) are observed. All samples exhibit a strong reduction of the melting point from the bulk
value of 342.4 K to values between 222 K and 229 K, depending on both the pore diameter and the
surface state and a glass like behavior of the biphenyl molecules in the melting regime. Employing
the Roessler two-phase model of the modeling of glass-transitions by ²H-solid state NMR the
distribution of activation energies for the rotational motions has been determined. At temperatures
closely below the glass-transition temperature deviations from a static Pake pattern of an aromatic
deuteron are observed, which indicate a pre-melting motion of biphenyl, which could be caused
by Câ‚‚-ring flips of the phenyl rings