2. 𝑐13NMR Spectroscopy
Carbon-13 (C13) nuclear magnetic resonance (most
commonly known as carbon-13 NMR or 13C NMR or sometimes
simply referred to as carbon NMR) is the application of nuclear
magnetic resonance (NMR) spectroscopy to carbon. It is
analogous to proton NMR
Allows the identification of carbon atoms in an organic
molecule just as proton NMR identifies hydrogen atoms. As
such 13C NMR is an important tool in chemical
structure elucidation in organic chemistry.
13C NMR detects only the 𝐶13isotope of carbon, whose natural
abundance is only 1.1%, because the main carbon isotope, 12C, is
not detectable by NMR since its nucleus has zero spin.
3. Sensitivity of 𝑪 𝟏𝟑
NMR
13C NMR has a number of complications that are not encountered in
proton NMR. 13C NMR is much less sensitive to carbon than 1H NMR
is to hydrogen since the major isotope of carbon, the 12C isotope,
has a spin quantum number of zero and so is not magnetically
active and therefore not detectable by NMR. Only the much less
common 13C isotope, present naturally at 1.1% natural abundance,
is magnetically active with a spin quantum number of 1/2 (like 1H)
and therefore detectable by NMR. Therefore, only the few 13C nuclei
present resonate in the magnetic field, although this can be
overcome by isotopic enrichment of e.g. protein samples.
In addition, the gyromagnetic ratio (6.728284 107 rad T−1 s−1) is
only 1/4 that of 1H, further reducing the sensitivity. The
overall receptivity of 13C is about 4 orders of magnitude lower
than 1H.
In a typical run on an organic compound, a 13C NMR may require
several hours to record the spectrum of a one-milligram sample,
compared to 15–30 minutes for 1H NMR, and that spectrum would
be of lower quality. The nuclear dipole is weaker, the difference in
energy between alpha and beta states is one-quarter that of proton
NMR, and the Boltzmann population difference is correspondingly
less.
5. The gyromagnetic ratio (also sometimes known as
the magnetogyric ratio in other disciplines) of a particle or
system is the ratio of its magnetic moment to its angular
momentum, and it is often denoted by the symbol γ, gamma.
Its SI unit is the radian per second per tesla (rad⋅s−1⋅T−1) or,
equivalently, the coulomb per kilogram (C⋅kg−1).
The term "gyromagnetic ratio" is often used as a synonym for
a different but closely related quantity, the g-factor. The g-factor,
unlike the gyromagnetic ratio, is dimensionless.
Gyromagnetic
ratio
6. The principle
behind NMR
The principle behind NMR is that many nuclei have spin and all
nuclei are electrically charged. If an external magnetic field is
applied, an energy transfer is possible between the base energy to a
higher energy level (generally a single energy gap).
The energy transfer takes place at a wavelength that corresponds
to radio frequencies and when the spin returns to its base level,
energy is emitted at the same frequency.
The signal that matches this transfer is measured in many ways
and processed in order to yield an NMR spectrum for the nucleus
7. Chemical shift
C13 has Huge chemical
shift values around 0-
200ppm as against
Proton NMR of only 0-
10ppm
The precise resonant frequency of the energy transition is dependent
on the effective magnetic field at the nucleus.This field is affected by
electron shielding which is in turn dependent on the chemical
environment. As a result, information about the nucleus' chemical
environment can be derived from its resonant frequency.
In general, the more electronegative the nucleus is, the higher the
resonant frequency. Other factors such as ring currents (anisotropy)
and bond strain affect the frequency shift. It is customary to adopt
tetramethylsilane (TMS) as the proton reference frequency.This is
because the precise resonant frequency shift of each nucleus depends
on the magnetic field used.The frequency is not easy to remember
(for example, the frequency of benzene might be 400.132869 MHz) so
it was decided to define chemical shift as follows to yield a more
convenient number such as 7.17 ppm.
δ = (ν-ν0)/ν0
The chemical shift, using this equation, is not dependent on the
magnetic field and it is convenient to express it in ppm where
(for proton)TMS is set to ν0 thereby giving it a chemical shift of zero.
For other nuclei, ν0 is defined as Ξ νTMS where Ξ (Greek letter Xsi) is
the frequency ratio of the nucleus (e. g., 25.145020% for 13C).
10. Because a C-13 nucleus behaves like a little magnet, it means
that it can also be aligned with an external magnetic field or
opposed to it.
Again, the alignment where it is opposed to the field is less
stable (at a higher energy). It is possible to make it flip from the
more stable alignment to the less stable one by supplying
exactly the right amount of energy.
11. Resonance
condition.
The energy needed to make this flip depends on the strength of
the external magnetic field used, but is usually in the range of
energies found in radio waves - at frequencies of about 25 - 100
MHz.If you have also looked at proton-NMR, the frequency is
about a quarter of that used to flip a hydrogen nucleus for a
given magnetic field strength.
It's possible to detect this interaction between the radio waves
of just the right frequency and the carbon-13 nucleus as it flips
from one orientation to the other as a peak on a graph. This
flipping of the carbon-13 nucleus from one magnetic alignment
to the other by the radio waves is known as the resonance
condition.
12. The importance
of the carbon's
environment
What we've said so far would apply to an isolated carbon-13
nucleus, but real carbon atoms in real bonds have other things
around them - especially electrons. The effect of the electrons is
to cut down the size of the external magnetic field felt by the
carbon-13 nucleus.
13. The
importance of
the carbon's
environment
Suppose you were using a radio frequency of 25 MHz, and you
adjusted the size of the magnetic field so that an isolated
carbon-13 atom was in the resonance condition.
If you replaced the isolated carbon with the more realistic case
of it being surrounded by bonding electrons, it wouldn't be
feeling the full effect of the external field any more and so would
stop resonating (flipping from one magnetic alignment to the
other). The resonance condition depends on having exactly the
right combination of external magnetic field and radio
frequency.
How would you bring it back into the resonance condition
again? You would have to increase the external magnetic field
slightly to compensate for the shielding effect of the electrons.
Now suppose that you attached the carbon to something more
electronegative. The electrons in the bond would be further
away from the carbon nucleus, and so would have less of a
lowering effect on the magnetic field around the carbon nucleus.
14. The
importance
of the
carbon's
environment
The external magnetic field needed to bring the carbon into
resonance will be smaller if it is attached to a more
electronegative element, because the C-13 nucleus feels more of
the field. Even small differences in the electronegativities of the
attached atoms will make a difference to the magnetic field
needed to achieve resonance
18. Thank you
Please refer thegiven Lecture
throughLinkProvided hereor
at Your Googleclassroom
https://www.youtube.com/watch?v=mmSNITb-
cNw&list=PL9m2Lkh6odgIEq94ERJIAaPg0z_1WeCrl&index=18
Lecture 19 By prof. Hanudatta Atreya
