Struktura a nieelastyczne rozpraszanie neutronów przez kryształy molekularne z wiązaniami wodorowymi Structure and inelastic neutron scattering by the molecular crystals with the hydrogen bonds
The molecular crystals, which are built of organic or organic-inorganic molecules,
are characterized by the weak intermolecular interactions. From the viewpoint
of the potential applications in electronics or optoelectronics the electric properties
of the crystals are essential. In turn these properties are related to their crystal structure
as well as the dynamics of the molecules in the solid state. The existence of the
hydrogen bonds in the crystal structures, conventional and unconventional, is crucial
from the viewpoint of the electric properties of the crystals. The dynamics of methyl
groups present in the structure can be a measure of the molecular interactions in the
crystals. In this work the dynamical properties, first of all taking into account the
research results concerning the neutron scattering, will be discussed. The neutron
technique is very effective as regards the methyl group dynamics investigations. The
relationship between a formation of the conventional and unconventional hydrogen
bonds and a tunneling of the methyl groups at low temperature will be discussed.
The method of the interpretation of the INS spectra will be described taking into
account the theoretical model, the parameters of which are fitted to the experimental
data. The examples will regard the following molecular crystals: p-N,N’-1,10-tetraacethyldiaminodurene
(TADD) (Figs. 2 and 4), 2,3,5,6-tetramethylpyrazine with
chloranilic acid (TMP·CLA) (Figs. 5, 6 and 7), 2,3,5,6- tetramethylpyrazine with
bromanilic acid (TMP·BRA) (Figs. 5 and 6) and the crystal of 3,4,7,8-tetramethylphenantroline
(Me4phen) and its complex with picric acid (Me4phen·PIC) (Figs. 8
and 9). In this paper we have shown that the surrounding of the methyl group and
its interactions with the adjacent molecules has a stronger effect than the changes in
the electronic charge density in the molecule.
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