Nondestructive Testing

1. NMR Spectroscopy
Darshit Desai
19MMM003

2. INTRODUCTION
• Nuclear magnetic resonance (NMR) spectroscopy was discovered
shortly after the Second World War, and since then its applications to
chemistry have been continuously expanding.
• It was natural then that NMR took an important part in
undergraduate chemistry education, being taught within various
courses: physical chemistry, organic, inorganic and analytical
chemistry.
• In recent years, the applications of NMR have been extended to
biology and medicine

3. • This technique for determining the structure of organic compounds.
• All the spectroscopic methods, it is the only one for which a
complete analysis and interpretation of the entire spectrum is
normally expected.
• Although larger amounts of sample are needed than for mass
spectroscopy, NMR is non-destructive, and with modern instruments
good data may be obtained from samples weighing less than a
milligram.
• To be successful in using NMR as an analytical tool, it is necessary to
understand the physical principles on which the methods are based.

4. Characteristic Spin (I)
• The nuclei of many elemental isotopes have a characteristic spin (I).
• Some nuclei have integral spins (e.g. I = 1, 2, 3 ....)
• Some have fractional spins (e.g. I = 1/2, 3/2, 5/2 ....)
• A few have no spin, I = 0 (e.g. 12C, 16O, 32S, ....)
• Isotopes of particular interest and use to organic chemists
are 1H, 13C, 19F and 31P, all of which have I = ½
• Our discussion of NMR will be limited to these and other I = 1/2
nuclei.

5. • Odd mass nuclei (i.e. those having an odd number of nucleons) have
fractional spins. Examples are I = 1/2 ( 1H, 13C, 19F ), I = 3/2 ( 11B ) & I =
5/2 ( 17O ).
• Even mass nuclei composed of odd numbers of protons and neutrons
have integral spins. Examples are I = 1 ( 2H, 14N ).
• Even mass nuclei composed of even numbers of protons and
neutrons have zero spin ( I = 0 ). Examples are 12C, and 16O.
• Spin 1/2 nuclei have a spherical charge distribution, and their NMR
behavior is the easiest to understand.

6. Some of nuclei spin characteristics

7. Features of NMR Phenomenon
1. A spinning charge generates a magnetic field, as shown by the
animation . The resulting spin-magnet has a magnetic moment (μ)
proportional to the spin.
2. In the presence of an external magnetic field (B0), two spin states
exist, +1/2 and -1/2.
• The magnetic moment of the lower energy +1/2 state is aligned with
the external field, but that of the higher energy -1/2 spin state is
opposed to the external field.

8. 3. The difference in energy between the two spin states is dependent
on the external magnetic field strength, and is always very small.
• The diagram illustrates that the two spin states have the same energy
when the external field is zero, but diverge as the field increases. At a
field equal to Bx a formula for the energy difference is given
(remember I = 1/2 and μ is the magnetic moment of the nucleus in
the field).

9. • Strong magnetic fields are necessary for NMR spectroscopy.
• The international unit for magnetic flux is the tesla (T). The earth's
magnetic field is not constant, but is approximately 10-4 T at ground
level.
• Modern NMR spectrometers use powerful magnets having fields of 1
to 20 T. Even with these high fields, the energy difference between
the two spin states is less than 0.1 cal/mole.
• For NMR purposes, this small energy difference (ΔE) is usually given
as a frequency in units of MHz (106 Hz), ranging from 20 to 900 Mz,
depending on the magnetic field strength and the specific nucleus
being studied.

10. • Irradiation of a sample with radio frequency (rf) energy corresponding
exactly to the spin state separation of a specific set of nuclei will
cause excitation of those nuclei in the +1/2 state to the higher -1/2
spin state. This electromagnetic radiation falls in the radio and
television broadcast spectrum.
• NMR spectroscopy is therefore the energetically mildest probe used
to examine the structure of molecules.
• The nucleus of a hydrogen atom (the proton) has a magnetic moment
μ = 2.7927, and has been studied more than any other nucleus. The
previous diagram may be changed to display energy differences for
the proton spin states (as frequencies) by mouse clicking anywhere
within it.

11. 4. For spin 1/2 nuclei the energy difference between the two spin
states at a given magnetic field strength will be proportional to their
magnetic moments.
• For the four common nuclei noted above, the magnetic moments
are: 1H μ = 2.7927, 19F μ = 2.6273, 31P μ = 1.1305 & 13C μ = 0.7022.
• These moments are in nuclear magnetons, which are
5.05078*10-27 JT-1.
• The diagram gives the approximate frequencies that correspond to
the spin state energy separations for each of these nuclei in an
external magnetic field of 2.35 T.

12. EXPERIMENT SETUP

13. • The block diagram of the experiment setup is shown in fig
It consist of
1. A source of radio frequency radiation.
2. A receiver coil.
3. A D.C. magnetic field.
4. A sweep generator for varying the magnetic field.
5. A data acquisition system.

14. EXPRIMENT PROCEDURE
• Sample is placed between the poles of a powerful magnet, made of superconductor.
• The radio frequency (r.f) field is generated in the coil connected to the r.f oscillator.
• The detector coil is placed at right angles to both the direction of the magnetic field
and the transmitter coil.
• The magnet is provided with the sweep coils which are used to vary the magnetic
field.
• In general, the r.f. frequency is kept fixed. The magnetic field is varied until the
resonance condition is reached. The nuclear magnetic moment transition induces an
EMF in the detector coil, which is amplified and then recorded as resonance
absorption.
• Homogeneity or the magnetic field is achieved by spinning the sample tube with an
appropriate frequency. In this way, all the nuclei experience an average magnetic
field.

15. • The sensitivity is achieved by cooling the sample because,
• Where the electron population in the M1=+1/2 state n+1/2 and that in
M1=-1/2 state n-1/2

16. Proton NMR Spectroscopy
• Proton nuclear magnetic resonance (proton NMR, hydrogen-1 NMR,
or 1H NMR) is the application of nuclear magnetic resonance in NMR
spectroscopy with respect to hydrogen-1 nuclei within
the molecule of a substance, in order to determine the structure of its
molecules.
• In samples where natural hydrogen (H) is used, practically all the
hydrogen consists of the isotope 1H (hydrogen-1; i.e. having
a proton for a nucleus

17. • Proton NMR spectra of most organic compounds are characterized
by chemical shifts in the range +14 to -4 ppm and by spin-spin
coupling between protons.
• The integration curve for each proton reflects the abundance of the
individual protons.
• Simple molecules have simple spectra.
• The spectrum of ethyl chloride consists of a triplet at 1.5 ppm and a
quartet at 3.5 ppm in a 3:2 ratio.
• The spectrum of benzene consists of a single peak at 7.2 ppm due to
the diamagnetic ring current.

18. Solution in Proton NMR Spectroscopy
• Simple NMR spectra are recorded in solution, and solvent protons
must not be allowed to interfere.
• Deuterated (deuterium = 2H, often symbolized as D) solvents
especially for use in NMR are preferred,
1. deuterated water -- D2O,
2. deuterated acetone -- (CD3)2CO,
3. deuterated methanol -- CD3OD,
4. deuterated dimethyl sulfoxide -- (CD3)2SO,
5. deuterated chloroform -- CDCl3.

19. • However, a solvent without hydrogen, such as
1. carbon tetrachloride -- CCl4
2. carbon disulfide -- CS2

20. DEUTERATED SOLVENTS
• Historically, deuterated solvents were supplied with a small amount
(typically 0.1%) of tetramethylsilane (TMS) as an internal standard for
calibrating the chemical shifts of each analyte proton.
• TMS is a tetrahedral molecule, with all protons being chemically
equivalent, giving one single signal, used to define a chemical shift = 0
ppm.
• It is volatile, making sample recovery easy as well.
• Modern spectrometers are able to reference spectra based on the
residual proton in the solvent (e.g. the CHCl3, 0.01% in 99.99% CDCl3).
• Deuterated solvents are now commonly supplied without TMS.

21. CHEMICAL SHIFTS
• Chemical shift values, symbolized by δ, are not precise, but typical -
they are to be therefore regarded mainly as a reference.
• Deviations are in ±0.2 ppm range, sometimes more.
• The exact value of chemical shift depends on molecular structure and
the solvent, temperature, magnetic field in which the spectrum is
being recorded and other neighboring functional groups.

22. • Hydrogen nuclei are sensitive to the hybridization of the atom to
which the hydrogen atom is attached and to electronic effects.
• Nuclei tend to be deshielded by groups which withdraw electron
density.
• Deshielded nuclei resonate at higher δ values, whereas shielded
nuclei resonate at lower δ values

23. Proton NMR Spectroscopy
• Proton NMR Spectroscopy is important and well-established
application of nuclear magnetic resonance NMR this method.
• The NMR spectrometer must be tuned to a specific nucleus, in this
case the proton. The actual procedure for obtaining the spectrum
varies, but the simplest is referred to as the continuous wave (CW)
method.

24. CW-Spectrometer

25. CW-Spectrometer
• A solution of the sample in a uniform 5 mm glass tube is oriented between
the poles of a powerful magnet, and is spun to average any magnetic field
variations, as well as tube imperfections.
• Radio frequency radiation of appropriate energy is broadcast into the
sample from an antenna coil (colored red).
• A receiver coil surrounds the sample tube, and emission of absorbed rf
energy is monitored by dedicated electronic devices and a computer.
• An NMR spectrum is acquired by varying or sweeping the magnetic field
over a small range while observing the rf signal from the sample.
• An equally effective technique is to vary the frequency of the rf radiation
while holding the external field constant.

26. IMPORTANT REQUIREMENTS AND FOR SPIN 1/2 NUCLEI
1. Nuclei having the same chemical shift (called isochronous) do not exhibit
spin-splitting. They may actually be spin-coupled, but the splitting cannot
be observed directly.
2. Nuclei separated by three or fewer bonds will usually be spin-coupled
and will show mutual spin-splitting of the resonance signals , provided
they have different chemical shifts. Longer-range coupling may be
observed in molecules having rigid configurations of atoms.
3. The magnitude of the observed spin-splitting depends on many factors
and is given by the coupling constant J (units of Hz). J is the same for
both partners in a spin-splitting interaction and is independent of the
external magnetic field strength.

27. 4. The splitting pattern of a given nucleus (or set of equivalent nuclei)
can be predicted by the n+1 rule, where n is the number of
neighboring spin-coupled nuclei with the same Js.

28. Carbon NMR 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 spectroscopy (1H)
• It allows the identification of carbon atoms in an organic molecule
just as proton NMR identifies hydrogen atoms.
• 13C NMR is an important tool in chemical structure elucidation
in organic chemistry.

29. • 13C NMR detects only the 13C isotope 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.
• Unfortunately, when significant portions of a molecule lack C-H
bonds, no information from Proton NMR Spectroscopy. Examples
include polychlorinated compounds such as chlordane, polycarbonyl
compounds such as croconic acid, and compounds incorporating
triple bonds (structures below, orange colored carbons).

30. • Even when numerous C-H groups are present, an unambiguous
interpretation of a proton NMR spectrum may not be possible.
• The following diagram depicts three pairs of isomers (A & B) which
display similar proton NMR spectra.
• Although a careful determination of chemical shifts should permit the
first pair of compounds (blue box) to be distinguished.
• The second and third cases (red & green boxes) might be difficult to
identify by proton NMR alone

31. CHEMICAL SHIFTS
• In nuclear magnetic resonance (NMR) spectroscopy, the chemical shift is
the resonant frequency of a nucleus relative to a standard in a magnetic
field.
• Often the position and number of chemical shifts are diagnostic of the
structure of a molecule.
• Chemical shifts are also used to describe signals in other forms of
spectroscopy such as photoemission spectroscopy.
• 13C chemical shifts follow the same principles as those of 1H, although the
typical range of chemical shifts is much larger than for 1H (by a factor of
about 20).
• The chemical shift reference standard for 13C is the carbons
in tetramethylsilane (TMS), whose chemical shift is considered to be 0.0
ppm

33. • Some atomic nuclei possess a magnetic moment (nuclear spin) ,
which gives rise to different energy levels and resonance frequencies
in a magnetic field.
• The total magnetic field experienced by a nucleus includes local
magnetic fields induced by currents of electrons in the molecular
orbitals (note that electrons have a magnetic moment themselves).
• The electron distribution of the same type of nucleus
(e.g. 1H, 13C, 15N) usually varies according to the local geometry
(binding partners, bond lengths, angles between bonds, and so on),
and with it the local magnetic field at each nucleus.

34. • This is reflected in the spin energy levels (and resonance frequencies).
• The variations of nuclear magnetic resonance frequencies of the
same kind of nucleus, due to variations in the electron distribution, is
called the chemical shift.
• The size of the chemical shift is given with respect to a reference
frequency or reference sample (see also chemical shift referencing),
usually a molecule with a barely distorted electron distribution.

35. 1H NMR Spectroscopy vs 13C NMR
Spectroscopy
1H NMR Spectroscopy
1. 1H NMR a useful spectrum
can be obtained very quickly
(5 minutes)
2. 1H NMR a uses with a few
milligrams of material to
obtain a useful spectrum
13C NMR Spectroscopy
1. 13C NMR normally the
minimum scan time would be
longer (~20-30 minutes) and a
2. 13C NMR concentrated sample
would be needed (~30 mg/0.6
mL) to obtain a useful
spectrum

36. 3. In 1H NMR more information
can be obtained: integration,
multiplicity, coupling.
3. However by running DEPT
type 13C NMR experiments
different carbon environments
can be identified.

37. AREAS OF APPLICATION
1. Foodstuff
• One industry that is widely examining NMR methods for monitoring quality is
food processing.
• The non-hazardous nature of the technique is an obvious advantage in this
area.
• Instruments for on-line monitoring of moisture (e.g. in biscuits) and fats are
being developed.
• Velocity profiles of foods such as chocolate and tomato juice (the latter being
a good example of a suspension of particles. in a background fluid) can be
tracked

38. 2. On Site Inspection
• There are many straightforward applications of NMR in field situations that
demand the development of portable or on-site pieces of specific NMR kit.
• For example in trying to examine mail and luggage for explosives and other
illegal substances their characteristic resonances can be detected as
signatures without the need to open every package and such systems are in
regular operation.
• Portable kits are also available for monitoring on site the water content of
concrete bridges and telegraph poles to try and predict their catastrophic
failure. NMR systems for examining moisture contents of soils, literally in the
field, have been constructed, mounted on a tractor.

39. REFERENCES
• https://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy
/nmr/nmr1.htm
• https://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy
/nmr/nmr2.htm#nmr11
• https://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy
/nmr/nmr2.htm#nmrsol
• https://www.youtube.com/watch?v=ywR6aLpfjl0
• https://en.wikipedia.org/wiki/Carbon-
13_nuclear_magnetic_resonance

40. • https://www.researchgate.net/post/What_is_the_basic_difference_b
etween_13C-NMR_and_1H-
NMR_in_order_of_their_principle_and_functions
• https://doi.org/10.1080/10589759408956406