Oscillations of Molecules in Melamine Crystal Lattices with Hydrogen Bonds

Video in TIB AV-Portal: Oscillations of Molecules in Melamine Crystal Lattices with Hydrogen Bonds

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Oscillations of Molecules in Melamine Crystal Lattices with Hydrogen Bonds
Alternative Title
Schwingungen von Molekülen im Kristallgitter - Melamin mit Wasserstoffbrücken
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C 1212
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Technical Metadata

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Film, 16 mm, LT, 68 m ; F, 6 1/2 min

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Subject Area
Translatory and torsional vibrations of the molecules. Using melamine for an example, the elastic properties of the intermolecular hydrogen bonds are demonstrated. Using the thermally induced vibrations at 60 and 300 K it is shown, that they are the mechanical storage means of the specific heat of the melamine crystal.
Keywords molecular structure oscillations / in molecules specific heat hydrogen bonding
Raman scattering Molecule VSEPR theory Raman scattering Melamine Translation <Genetik> Quartz Raman spectroscopy
Molecule Quantum chemistry Quartz
Deformity Schwingungsspektroskopie
Raman scattering Translation <Genetik> Translation <Genetik> Raman spectroscopy
Sense District
Molecule Sense District Chain (unit)
Hydrogen bond
Translation <Genetik>
Raman scattering Translation <Genetik>
Molecule Motion (physics)
Heat capacity Quartz
Molecule Exciter (effect)
Motion (physics)
Auftauen Human body temperature
Gasverflüssigung Nitrosamine
The Raman and infrared spectra of molecules in the crystalline state show bands between 50 and 300 wave numbers, which disappear when the crystal dissolves or melts. These bands are caused by vibrations of the molecules about their positions in the lattice: Translational vibrations along the axes of the molecules and torsional vibrations (also called librations) about the axes of the molecules. Their frequencies are low, since the masses of the molecules are large, and the intermolecular elastic forces are small. The broad band in the Raman spectrum of melamine at 126 wave number is due to a libration. Here
you see a lattice plane in a melamine crystal. Four of the molecules are coloured. We shall now show vibrations, which are excited by one quantum of energy. We will first look at the vibrations
of two of these molecules.
The vibrational amplitudes are magnified
by a factor of ten.
The libration, a torsional vibration
about the horizontal y-axis, is
clearly visible. An internal molecular vibration, the out-of-plane deformation vibration
of the NH2-group, is coupled
with this lattice vibration. We
will now see the vibration
of all the molecules in
one plane. The strongest band in the Raman spectrum is however due to a translation vibration. Here is the translation
vibration in the direction of
the horizontal y-axis. The molecules
are moving an an antiparallel
sense. Now we can see
that chains of molecules are
moving in an antiparallel sense.
It is evident that the
hydrogen bonds, characterized by broken
lines, are stretched considerably. Translation
and libration can occur simultaneously,
as for example in this lattice vibration. During the translational vibration the centre of gravity of the moelcule moves in the direction of the vertical
axis. A libration about the
horizontal axis occurs simultaneously. We
shall now see the same
motions for all the molecules
in one layer of the
lattice. The lattice vibrations can
also be thermally excited: in
this manner the specific heat
of the crystals is stored.
Here you see the true
vibrational amplitudes of the thermally
excited lattice vibrations. At 60 Kelvin, the molecules move only slightly. These are the true
amplitudes at 300 Kelvin, that
is at room temperature. All
the different lattice vibrations are thermally excited. The mean amplitude
of the translational motion is
0,8 Ångström, or 80 pm,
that of the librations about four degrees. One can imagine,
that when the temperature is raised still further, the lattice
will collapse at the melting
point, that is for melamine at about 630 Kelvin.


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