basic information

1. Presented by,
Dr. Gopalkrushna H. Murhekar
Shri Dr. R. G. Rathod Arts and Science, College Murtizapur
Nuclear magnetic resonance
spectroscopy

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• Nuclear magnetic resonance spectroscopy is a powerful analytical technique
used to characterize organic molecules by identifying carbon-hydrogen
frameworks within molecules.
• Two common types of NMR spectroscopy are used to characterize organic
structure: 1H NMR is used to determine the type and number of H atoms in a
molecule; 13C NMR is used to determine the type of carbon atoms in the
molecule.
• The source of energy in NMR is radio waves which have long wavelengths, and
thus low energy and frequency.
• When low-energy radio waves interact with a molecule, they can change the
nuclear spins of some elements, including 1H and 13C.
Introduction

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• When a charged particle such as a proton spins on its axis, it
creates a magnetic field. Thus, the nucleus can be considered to
be a tiny bar magnet.

4. NMR History
• 1937 Rabi’s prediction and observation of nuclear magnetic resonance
• 1945 First NMR of solution (Bloch et al for H2O) and solids (Purcell et
al for parafin)!
• 1953 Over hauser NOE (nuclear Over hauser effect)
• 1966 Ernst, Anderson Fourier transform NMR
• 1975 Jeener, Ernst 2D NMR
• 1980 NMR protein structure by Wuthrich
• 1990 3D and 1H/15N/13C Triple resonance
• 1997 Ultra high field (~800 MHz) & TROSY(MW 100K)

5. Continuation of NMR History
Nobel prizes
1944 Physics Rabi (Columbia)
1991 Chemistry Ernst (ETH)
"for his resonance
method for recording
the magnetic
properties of atomic
nuclei"
1952 Physics Bloch (Stanford),
Purcell (Harvard)
"for their development
of new methods for
nuclear magnetic
precision measurements
and discoveries in
connection therewith"
"for his contributions to
the development of the
methodology of high
resolution nuclear
magnetic resonance
(NMR) spectroscopy"

6. 2002 Chemistry Wüthrich (ETH)
2003 Medicine Lauterbur (University of Illinois in
Urbana ), Mansfield (University of Nottingham)
"for his development of nuclear magnetic
resonance spectroscopy for determining the
three-dimensional structure of biological
macromolecules in solution"
"for their discoveries concerning
magnetic resonance imaging"

7. Origin of NMR
Nuclear magnetic resonance (NMR) spectroscopy is based on the measurement of
absorption of electromagnetic radiation in the radio-frequency region of roughly 4
to 900 MHz.

9. Schematic NMR Spectrometer

11. • The frequency at which a particular proton absorbs is determined by its electronic
environment.
• Protons in different environments absorb at slightly different frequencies, so they are
distinguishable by NMR.
• The size of the magnetic field generated by the electrons around a proton determines
where it absorbs.
• Only nuclei that contain odd mass numbers (such as 1H, 13C, 19F, and 31P) or odd
atomic numbers (such as 2H and 14N) give rise to NMR signals. E
Electronic environment

12. Example
Equivalent
Nonequivalent

13. In NMR spectroscopy, the standard is often tetramethylsilane,
Si(CH3)4, abbreviated TMS.
Tetramethyl silane (TMS) is used as reference because it is
soluble in most organic solvents, is inert, volatile, and has
12 equivalent 1H and 4 equivalent 13C. TMS signal is set to 0

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Chemical Shift
• Measured in parts per million.
• Ratio of shift downfield from TMS (Hz) to total
spectrometer frequency (Hz).
• Same value for 60, 100, or 300 MHz machine.
• Called the delta scale.

16. Chemical Shift-d
When an atom is placed in a magnetic field, its electrons
circulate about the direction of the applied magnetic
field. This circulation causes a small magnetic field at
the nucleus which opposes the externally applied field
B = B0 (1-s), So u = g B0 (1-s) / 2p
The magnetic field at the nucleus (the effective field)
is therefore generally less than the applied field by a
fraction :

17. Shielding and Deshielding
A nucleus is said to be shielded when
electrons around the nucleus circulates in a
magnetic field and create a secondary
induced magnetic field which opposes the
applied field .
Trends in chemical shift are explained based
on the degree of shielding or deshielding ,
e.g. of deshielding effect

19. Spin-spin coupling:
The coupling of the intrinsic angular momentum of different particles.
Such coupling between pairs of nuclear spins is an important feature of
nuclear magnetic resonance (NMR) spectroscopy as it can provide
detailed information about the structure and conformation of molecules.
Spin-spin coupling between nuclear spin and electronic spin is
responsible for hyperfine structure in atomic spectra.
Spin-Spin Coupling

20. The relative peak intensities for
multiplet peaks arising from J-
coupling of a 1H to N equivalent
1H can be determined using Pascal’s
triangle:
Pascal’s Triangle

21. J-CouplingJ-coupling:
also called indirect spin-spin coupling, is the coupling between two
nuclear spins due to the influence of bonding electrons on the
magnetic field running between the two nuclei. J-coupling provides
information about dihedral angles, which can be estimated using
the Karplus equation. It is an important observable effect in 1D
NMR spectroscopy.
The coupling constant, J (usually in frequency units, Hz) is a
measure of the interaction between a pair of nuclei

22. Factor Affecting Chemical Shift
Chemical shift depends on:
• Electronegativity of nearby atoms
• Hybridization of adjacent atoms
• diamagnetic effects
• paramagnetic effects
• solvent effect

23. Aromatic Effect
The magnetic field induced by circulation of p electrons in
an aromatic ring deshields the hydrogens on the ring and
shifts their signal to higher frequency

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