Transcript rangus-prezentacija

NMR spectroscopy in solids:
A comparison to NMR
spectroscopy in liquids
Mojca Rangus
Mentor: Prof. Dr. Janez Seliger
Comentor: Dr. Gregor Mali
Introduction
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NMR specrtometer
Basics of NMR
Types of interaction (concentrate on: spin ½
nuclei, diamagnetic compounds)
Solution-state spectra
Solid-state spectra
Methods for improving solid-state NMR spectra
Magnetization and magnetism
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Nuclear magnetic moment
When magnetic field
is applied,
starts to
precess around it’s direction with Larmor
frequency
We describe the movement of the magnetic
moment with equation
It is conventent to go from lab. frame to
rotating frame, which rotates around
direction with the same frequency that
precesses:
NMR
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We observe the total magnetization of the sample
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Magnetization is fliped in xy plane with the aid of rotating
radiofrequency (rf) field
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Projection of the magnetization on the xy plane is then recorded
The nuclei with nonzero spin (nonzero magnetic moment) can be
observed
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NMR periodic table
Single pulse
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The same coils are used for excitation and recording
After a pulse a delay is needed before the start of the recording
From recorded signal (FID) a spectrum is obtained with Fourier
transormation
This means that the magnetic moments are alredy scattered in xy
plane
Spin echo
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With a special pulse sequence
the magnetic moments are
gathered before the recording
starts
An efficient method to avoid dead
time problem
Types of interaction
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Zeeman interaction
Chemical shift
Direct dipole coupling
Indirect dipole coupling or J-coupling
Quadrupolar interaction
Zeeman interaction
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It can be described with a Hamiltonian
or in ternsor form
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In the magnetic field the two spin states
have different energies
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It is far the strongest interaction and all
other types of interaction can be
considered as corrections
Order of the magnitude:
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Chemical shift
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Indirect interaction of the nuclear spins with
the external magnetic field through the
surrounding electrons
If the electronic environment of nuclei differ,
the local mag. fields differ and therefore the
resonance frequencies are different
Contains information about electronic states
Chemical shifts also depend on the
orientation of the molecule in the magnetic
field
Direct dipole coupling
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Two neighbouring nuclei are coupled through their magnetic dipole
moments
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Useful for molecule structure studies and
provides a good way to estimate distances
between nuclei and hence the geometrical
form of the molecule
J-coupling
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Nuclear spins are coupled with the help of the molecular electrons
It is exclusively intramolecular
The mechanism responsible for the multiplet structure
It can be viewed only in solution-state NMR spectra where the
spectral lines are narrow enough to observe the interaction
Electric quadrupole coupling
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Nucleus with the electric quadrupole moment intarects strongly
with the electric field gradients generated by surrounding
electron clouds
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Quadrupole interaction is totaly averaged
in liquids, but in solids is the strongest after
Zeeman
In solids we often need to take into account
second order contributions
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Solution-state NMR spectrum
Single crystal spectra
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All interactions are orientation dependent
Therefore it is possible to conduct NMR
experiments in similar way as X-ray
diffraction
From single crystal spectra it is possible to
reconstruct the interaction tensors and from
there the electronic and geometric
characteristics of the compound
Powder spectra
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All orientations of the molecules are
presented equally
Resonance lines become extremely
broad
Anisotropic nature of the interactions
comes fully into account
Magic angle spinning
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Spinning the sample under the magic angle
considerably narrows the resonance lines
static
MAS
solution
Decoupling
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In the mechanism of
decoupling a strong rf field is
applied so that magnetic
moments are flipped
randomely back and forth to
narrow the anisotropic
broadeneng of the resonance
lines
static
static with low
power decoupling
static with high
power decoupling
decoupling + MAS
solution-state
spectrum
Cross polarization (CP)
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Cross polarization (CP) is one of
the most important techniques in
solid-state NMR
Polarization from abundant spins
is transferred to dilute ones via
the direct diploe coupling
Summary
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Basic principles of NMR
Viewed the most important type of interactions
that are encountered in a compound
Similarities and differences of solution-state and
solid-state spectra
The most important techniques used to improve
powder NMR spectra