NMR SPECRTOSCOPY THEORY OF NMR PRINCIPLE OF NMR CHEMICALSHIFT FT NMR COSY NOESY

1. NMR SPECTROSCOPY
Prepared by-
Sudhanshu Mishra
M.PHARM
Pharmaceutics

2. Contents
 Introduction
 Fundamental principle of NMR
 Interpretation
Chemical shift
Number of signals
Spin Spin coupling: Spliting of
signals
Coupling constant
Integrals

3. Introduction
 Nuclear Magnetic Resonance (NMR) is a spectroscopy
technique which is based on the absorption of the
electromagnetic radiation in the radio frequency
region 4 to 900 MHz by nuclei of the atoms.
 Nuclear magnetic resonance spectroscopy(NMR) is a
powerful analytical technique used to characterize
organic molecules by identifying carbon-hydrogen
frameworks within molecules.
 It is a research technique that exploits the magnetic
properties of certain atomic nuclei.
 It determines the physical and chemical properties
of atoms or the molecules in which they are
contained.

4.  Proton Nuclear Magnetic resonance spectroscopy is one of the most
powerful tools for elucidating the number of hydrogen or proton in the
compound.
 It is used to study wide variety of nuclei:

5. Principle of 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.

6.  The signal that matches this transfer is measured in many ways and processed
in order to yield an NMR spectrum for the nucleus concerned.

7. Theory of NMR
 The hydrogen nucleus or protons can be regarded as a
spinning positively charged unit and so it will generate a
tiny magnetic field Ho along its spinning axis.
 Now if this nucleus is placed in an external magnetic
field H0, it will naturally line up either parallel A or
antiparallel B to the direction of external field. The A
will be more stable, being of lower energy.

8. The energy difference E between two states will be
absorbed or emitted as the nucleus flips from one orientation
to the other.
Then
E = hv
where v = a radiation frequency and h = Planck’s constant
If correct frequency is applied to the sample containing
hydrogen nuclie and sample is placed in the external field
HQ, then low energy nuclie A will absorb AE = hv, and flips to
B. Thus on flipping back down, they remit hv as a radiation
signal which is picked up by the instrument.

10. Effect of magnetic field
 A nucleus is in resonance when it absorbs RF radiation
and “spin flips” to a higher energy state.
 Thus, two variables characterize NMR: an applied
magnetic field B0, the strength of which is measured in
tesla (T), and the frequency n of radiation used for
resonance, measured in hertz (Hz), or megahertz
(MHz).

11.  The frequency needed for resonance and the applied magnetic field
strength are proportionally related:
V α B0
When energy in the form of Radiofrequency
is applied and when,
Applied frequency = Precessional frequency
absorption of energy occurs and a NMR
signal is recorded .
• The nuclei are said to be in resonance, and
the energy they emit when flipping from
the high to the low energy state can be

12. Types of solvent used
 A substance free from proton should be used as a
solvent i.e which does not give absorption of its own in
NMR spectrum. Moreover , the solvent should be
capable of dissolving at least 10% of the substance
under investigation.
 Following solvents are commonly used in NMR
spectroscopy
 Carbon tetrachloride (CCl4)
 Carbon Disulphide (CS4)
 Deuterochloroform (CDCl3)
 Hexachloroacetone (CCl3)2CO

13. NMR SpectrumA Spectrum of Absorption of Radiation Vs. Applied Magnetic Strength is called as NMR
Spectrum.
 The number of signals shows how many different kinds of protons are present.
 The intensity of the signal shows the number of protons of each kinds.
 The location of the signals shows how shielded or deshielded the proton is.
 Signal splitting shows the number of protons on adjacent atoms.

14. CHEMICAL SHIFT
 The variations of nuclear magnetic resonance frequencies of the same kind
of nucleus, due to variations in the electron distribution.
Chemical Shift = Absorption Frequency relative to
TMS (Hz)
Spectrometer Frequency (MHz)

15.  The relative energy of resonance of a particular nucleus resulting from its local
environment is called chemical shift.
 NMR spectra show applied field strength increasing from left to right, Left part
is downfield, the right is upfield.
 Nuclei that absorb on upfield side are strongly shielded where nuclei that absorb
on downfield side is weakly shielded.
 Chart calibrated versus a reference point, set as 0, tetramethylsilane [TMS].

17.  Shielding:
The higher the electron density around the
nucleus, the higher the opposing magnetic field
to B0 from the electrons, the greater the
shielding. Because the proton experiences lower
external magnetic field, it needs a lower
frequency to achieve resonance, and therefore,
the chemical shift shifts upfield (lower ppms).

18.  Deshielding:
If the electron density around a nucleus decreases, the
opposing magnetic field becomes small and therefore, the
nucleus feels more the external magnetic field B0, and
therefore it is said to be deshielded. Because the proton
experiences higher external magnetic field, it needs a
higher frequency to achieve resonance, and therefore, the
chemical shift shifts downfield (higher ppms) .

19. Factor influencing chemical
shift
Both 1H and 13C Chemical shifts are related to the
following major factors:
Depends on Hydrogen bonding
Depends on adjacent group
Depends on carbon group attached
Depends on hybridization
Depends on anisotropy

20.  Hydrogen Bonding:
Molecules having hydrogen bonding have higher
chemical shift and absorb radiation at low field.
That is due to the decrease of electronic density
around the nucleus
 Adjacent Group:
For protons on carbon attached to an
electronegative atom or group X( Cl , F ,Br ,I), the
chemical shift increases with the electro negativity
of X. This is due to the inductive effect on the
shielding of the protons and is apparent in the
methyl halides.

21. Carbon group attached:

22. Anisotropy
Anisotropy refers to the property of the molecule
where a part of the molecule opposes the applied
field and the other part reinforces the applied
field. Chemical shifts are dependent on the
orientation of neighbouring bonds in particular the
π bonds. Examples of nucleus showing chemical
shifts due to π bonds are aromatics, alkenes and
alkynes. Such anisotropic shifts are useful in
characterizing the presence of aromatics or other
conjugated structures in molecules.

23. Hybridization
 In an sp2 C-H bond, the carbon atom has more s
character (33% s), which effectively renders it more
electronegative than an sp3 carbon (25% s).
 If the sp2 carbon atom holds its electrons more
tightly, this results in less shielding for the H nucleus
than in an sp3 bond.
 On the basis of hybridization, acetylenic proton to
have a chemical shift greater than that of vinyl
proton. But chemical shift of acetylenic proton is
less than that of vinyl proton.
 Finally sp2 > sp > sp3 .(Order of chemical shift)

24. Spin- Spin Coupling
 Spin-spin coupling is the interaction
between the spin magnetic moments
of different electrons and/or nuclei.
 In NMR spectroscopy it gives rise to
multiplet patterns, and cross-peaks in
two-dimensional NMR spectra.
Between electron and nuclear spins
this is termed the nuclear hyperfine
interaction. Between electron spins it
gives rise to relaxation effects and
splitting of the spectrum

25. FT NMR
 The Fourier Transformation is the basic
mathematical calculation necessary to convert the
data in time domain(interferogram) to frequency
domain(NMR Spectrum).
i.e, time domain - Intensity v/s Time.
Frequency domain - Intensity v/s Frequency.

26. Advantages of FT-NMR
 Dramatic increase in the sensitivity of NMR
measurements.
 Has widespread applications esp. for 13C NMR, 31P NMR
and 19F NMR giving high signal to noise ratio facilitating
rapid scanning.
 Can be obtained with less than 5 mg of the compound.
 The signals stand out clearly with almost no electronic
background noise.
 Used in engineering, industrial quality control and
medicine.
 MRI is most prominent FT NMR applications.

27. RELAXATION PROCESS
 Relaxation process involve some non radiative transition
by which a nucleus in an upper transition state return to
the lower spin state. Three kinds of relaxation process
are:
 Spin –Spin relaxation
 Spin – lattice relaxation
 Quadrapole relaxation

28.  Spin – Spin relaxation:
It is due to the mutual exchange of the spin by two
precessing nuclei which are in close proxemity to each
other. It involve the transfer of energy from one
nucleus to the other, there is no net loss of energy.
 Spin – lattice relaxation:
It involve the transfer of energy from the nucleus in
its higher energy state to the molecular lattice. The
energy is transfered to the component of the lattice as
the additional translational, vibrational and rotational
energy.
 Quadrapole Relaxation:
It is a prominent relaxation process for nuclie having
I > ½. The nuclie 14N, 17O, 11B etc.

29. 2 DIMENSIONAL NMR
 Two-dimensional nuclear magnetic resonance
spectroscopy (2D NMR) is a set of nuclear magnetic
resonance spectroscopy (NMR) methods which give data
plotted in a space defined by two frequency axes rather
than one.
 Types of 2D NMR include correlation spectroscopy
(COSY), J-spectroscopy, exchange spectroscopy (EXSY),
and nuclearOverhauser effect spectroscopy (NOESY).

30. Correlation Spectroscopy
(COSY)
 It is used to identify spins which are coupled to each
other. It consists of a single RF pulse (p1) followed by
the specific evolution time (t1) followed by a second
followed by a measurement pulse (p2) period (t2).
 The two-dimensional spectrum that results from the
COSY experiment shows the frequencies for a single
isotope, most commonly hydrogen (1H) along both axes.

31.  COSY spectra show two types of peaks:
A. Diagonal peaks
B. cross peaks

32. Nuclear Over- Hauser effect
Spectroscopy (NOESY)
 The spectrum obtained is similar to COSY, with diagonal
peaks and cross peaks, however the cross peaks connect
resonances from nuclei that are spatially close rather
than those that are throughbond coupled to each other.
NOESY spectra also contain extra axial peaks which do
not provide extra information and can be eliminated
through a different experiment by reversing the phase
of the first pulse.

33. THANK YOU