Nuclear Magnetic Resonance Spectroscopy:

1
Nuclear Magnetic Resonance Spectroscopy Based
on measurement of absorption of
electromagnetic radiation in the radio frequency
region. Atomic nuclei, rather than outer
electrons, are involved in the absorption
process. To cause nuclei to develop the energy
states required for absorption to occur, it is
necessary to expose the analyte to an intense
magnetic field of several thousand
gauss. Certain atomic nuclei have properties of
spin and magnetic moment and, as a consequence,
exposure to a magnetic field will lead to
splitting of their energy levels.
2
Nuclear Properties Assume nuclei rotate about
their axis. Thus, they have spin. angular
momentum associated with particle spin is a
half- integral multiple of h/2p, where h is
Plancks constant. The maximum spin component
for a particular nucleus is its spin quantum
number I. A nucleus has (2I 1) discrete
states. The angular momentum for these states are
integral values from I to I. In the absence of
an external field, these states have identical
energy. The magnetic dipole, m, resulting from
nuclear spin is oriented along the axis of the
spin and has a value characteristic for each type
of nucleus.
3
Energy Levels in a Magnetic Field In an
external magnetic field, a particle possessing a
magnetic Moment tends to become oriented such
that its magnetic Dipole (hence, its spin axis)
is parallel to the field. In the absence of a
magnetic field, the energies of the
magnetic quantum states are identical.
Consequently, a large assemblage of protons will
contain an identical number of nuclei with m
1/2 and m -1/2. When placed in a field, the
nuclei tend to orient themselves so that the
lower energy state (m 1/2) predominates.
4
Potential energy of precessing particle is
directly proportional to the applied field (B0)
and indirectly proportional to the angular
momentum (mz).
Source Skoog, Holler, and Nieman, Principles of
Instrumental Analysis, 5th edition, Saunders
College Publishing.
5
In order for a dipole to flip, there must be a
magnetic field at right angles to the fixed field
and one with a circular component that can move
in phase with the precessing dipole.
Source Skoog, Holler, and Nieman, Principles of
Instrumental Analysis, 5th edition, Saunders
College Publishing.
6
(Left) Altering macroscopic nuclear magnetization
away from its equilibrium position parallel to
the applied magnetic field. (Right) The
resulting precession that induces an
electrical signal in the receiver coil.
Source E. D. Becker, A Brief History of Nuclear
Magnetic Resonance, Anal. Chem. 65, 295A-302A
(1993).
7
Source in NMR systems is coil of an RF
oscillator. Radiation is plane polarized. If
located 90 deg. to fixed magnetic field,
sample volume is in proper plane for absorption
by nuclei.
Source Skoog, Holler, and Nieman, Principles of
Instrumental Analysis, 5th edition, Saunders
College Publishing.
8
Common Nuclei with Nuclear Magnetic Properties
Capable of Yielding Spectra 1H 7Li 13
C 14N 17O 19F 23Na 25Mg 27
Al 29Si 31P 33S 109Ag
9
Resolution in NMR
Source Skoog, Holler, and Nieman, Principles of
Instrumental Analysis, 5th edition, Saunders
College Publishing.
10
Chemical Shift Chemical shift arises from
circulation of electrons surrounding the nucleus
under the influence of the applied magnetic
field. Movement of electrons creates a small
magnetic field that normally opposes the applied
field. So, the nucleus is exposed to an
effective field that is usually smaller than the
external field. The magnitude of the field
developed internally is directly proportional to
the applied external field.
11
Spectral Characteristics
Distance between fine-structure peaks
unchanged Chemical shift is proportional to field
strength
Source Skoog, Holler, and Nieman, Principles of
Instrumental Analysis, 5th edition, Saunders
College Publishing.
12
Diamagnetic Shielding Under the influence of
the magnetic field, electrons bonding the proton
tend to precess around the nucleus in a
plane perpendicular to the magnetic field.
This develops a secondary field that opposes the
primary field. So nucleus experiences resultant
field that is smaller (I.e., shielded from full
effect of primary field). As a consequence,
external field must be increased to cause
nuclear resonance. The frequency of the
precession and the magnitude of the secondary
field is a direct function of the external field.
Source Skoog, Holler, and Nieman, Principles of
Instrumental Analysis, 5th edition, Saunders
College Publishing.
13
Effects of multiple bonds on the chemical shift
are explained by taking into account the
anisotropic magnetic properties.
Source Skoog, Holler, and Nieman, Principles of
Instrumental Analysis, 5th edition, Saunders
College Publishing.
14
Spin-Spin Splitting The splitting of chemical
shift peaks is explained by assuming that the
effective field around one nucleus is further
enhanced or reduced by local fields generated by
the hydrogen nuclei bonded to an adjacent
atom. A small interaction (coupling) between
groups of protons exists. That is, the spins of
one set of nuclei influence the resonance
behavior of another. J coupling constant The
areas of peaks in the multiplet are approximately
equal to an integral ratio to each other.
15
Example of Spin-Spin Splitting in Iodopropane
Source Skoog, Holler, and Nieman, Principles of
Instrumental Analysis, 5th edition, Saunders
College Publishing.
16
Example of Double Resonance Techniques Spin-Spin
Decoupling
Source Skoog, Holler, and Nieman, Principles of
Instrumental Analysis, 5th edition, Saunders
College Publishing.
17
Schematic of FT-NMR
Source Skoog, Holler, and Nieman, Principles of
Instrumental Analysis, 5th edition, Saunders
College Publishing.
18
Absorption and Integration Curves
Peak areas important to allow estimation of
relative number of absorbing nuclei in each
chemical environment.
Source Skoog, Holler, and Nieman, Principles of
Instrumental Analysis, 5th edition, Saunders
College Publishing.
19
Types of Proton Decoupling in 13C NMR
Broad-band Decoupling
Off-Resonance Decoupling
Source Skoog, Holler, and Nieman, Principles of
Instrumental Analysis, 5th edition, Saunders
College Publishing.
20
Chemical Shifts for 13C
Source Skoog, Holler, and Nieman, Principles of
Instrumental Analysis, 5th edition, Saunders
College Publishing.
21
Comparison of NMR of PHF2
1H at 60 MHz
19F at 94.1 MHz
31P at 40.4 MHx
Source Skoog, Holler, and Nieman, Principles of
Instrumental Analysis, 5th edition, Saunders
College Publishing.
22
Example of Two-Dimensional NMR
Source Skoog, Holler, and Nieman, Principles of
Instrumental Analysis, 5th edition, Saunders
College Publishing.
23
Principles of NMR Imaging
Source R. A. Komoroski, Nonmedical Applications
of NMR Imaging, Anal. Chem. 65, 1068A-1077A
(1993).
24
Nonmedical Applications of NMRI Polymeric
Materials Manufacturing defects voids,
occluded solvents or particles, cracks,
channels Phase structure mixing of blends,
composites, fillers, plasticizers,
foams Reactions polymerization, thermoset
curing, adhesives, crosslinking Diffusion
swelling agents, solvents, gases Environmental
effects aging, wear, mechanical or other
stress, oxidation heat
distribution Inorganic Materials Ceramics
cracks, voids, binder distribution Oil well
cores physical defects, fluid quantification,
distribution and mixing, diffusion,
flow Metals hydrogen distribution, current
flow Crystals defects Chemistry and Chemical
Engineering Reactions in Solution spatial
heterogeneity of reactants, products, or
conditions, oscillations,
kinetics Solid-State Reactions anisotropy of
reaction, kinetics Chromatography column
packing, band spread, elution Flow flow
dynamics, turbulence, multiphase Bioreactors
design Agriculture and Food Wood Harvesting
knots, defects, diseases Plant Biology water
flow and diffusion, diseases Soil water
content and distribution Grain water
distribution and transport Agricultural Products
damage, ripening, diseases, insects Processed
Foods baking, cooking, storage, spoilage
Source R. A. Komoroski, Nonmedical Applications
of NMR Imaging, Anal. Chem. 65, 1068A-1077A
(1993).