Download 1 Intro and history of NMR

Ch 235.42: Advanced Analytical
Chemistry: NMR
Rene Angelo S. Macahig, PhD
2nd Sem AY 2012-2013
COURSE OBJECTIVES
1. Understand the theoretical basis of NMR;
2. Use of NMR for organic compounds and to
observe other nuclei, such as 31P or 19F
3. Understand common 1D and 2D NMR pulse
sequences
4. Basic principles of NMR instrumentation;
5. Operate the NMR instrument properly and develop
sufficient skill to be an NMR operator; and
6. Interpret NMR spectra of organic compounds.
Spectroscopy:
The study of molecular structure and
dynamics through the absorption,
emission and scattering of light.
(www.science.marshall.edu)
(apwww.smu.ca/~ishort/Astro/spectroscopy.gif)
Spectroscopy timeline:
1. Introduction and History of NMR
• NMR belongs to the family of spectroscopic methods:
E=hn
Wavelength:
Frequency:
Classification:
200 - 800 nm
~1 x 1015 Hz
UV-visible
2.5 - 25 mm
4000-600 cm-1
Infrared
Higher frequency
Higher energy
Excitation:
Electronic
energy level
~1m
~100 x 106 Hz (MHz)
Radiofrequency
Lower frequency
Lower energy
Chemical
bond
Atomic
nucleus
What are the spectroscopic methods based on?
Excitation:
• atomic and
molecular
electronic energy
levels
• electron spin
transition
(fluorescence)
Electron
Chemical
bond
• bond energies
• molecular symmetry
Atomic
nucleus
• type of nucleus (e.g.,
1H, 13C, 31P)
• chemical environment
• inter-nuclear interaction
• molecular symmetry
• dynamic motion
1. Introduction and history of NMR (Dayrit)
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Comparison of Methods of Molecular Spectroscopy .
O
H
H3C
O CH2 CH3
trans-ethyl crotonate
H
UV-visible spectroscopy:
208 nm: unsaturated acid or ester
O
NMR spectroscopy 
( ):
O
178
C O
6.90 (dq)
OR
H
19
145 123
C
H3 C
IR spectroscopy (n):
3050 cm-1
: C=C-H
-1
2980-2910 cm : alkyl groups: CH3, CH2
1710 cm-1
: unsaturated acid or ester .
1660 cm-1
: C=C
1.94 (dd)
1. Introduction and history of NMR (Dayrit)
C
60
15
CH2 CH3
4.14 (q)
H
5.80 (dq)
8
1.35 (t)
NMR is able to give much more structural
information than UV-visible and IR.
H
H3C
For example, cholesterol:
CH3
CH3
CH3
CH3
HO
• UV-visible, lmax: ring olefin
• IR, n: stretch, bend,
rocking: -OH, -CH3, -CH2-,
CH-, and fingerprint pattern
1. Introduction and history of NMR (Dayrit)
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1H
NMR
• NMR:
1H: 46 hydrogen atoms
13C: 27 carbon atoms.
H3C
CH3
CH2
CH2
C
CH2
CH
CH2
CH
C
HO
13C
NMR
CH2
CH
C
CH
CH
CH2
Chemical shift, ,
and integration
CH2
CH2
CH3
CH
CH3
CH
CH3
CH2
CH2
CH2
CH
Relative
configuration:
HC
CH3 3
CH3
H
CH3
H
CH3
H
HO
H
H
H
1. Introduction and history of NMR (Dayrit)
1H-1H
splitting
and NOE
10
How does NMR compare with the other
spectroscopic methods?
• Highest information content  Highest complexity
• Theory
• Sensitivity vs. Resolution
• NMR vs. X-ray crystallography
• Diversity of application
• High cost of technology
1. Introduction and history of NMR (Dayrit)
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The History of NMR
can be divided into the following distinct periods:
• 1921 - 45: theories on atomic nuclei (Physics)
• 1946 - 55: first NMR experiments (Physics)
• 1956 - 65: early structural analysis (Organic Chemistry)
• 1966 - 75: first quantum leap in technology and methodology
(Chemistry & Electronics)
• 1976 - 85: the rise of biological NMR (Biochemistry)
• 1986 - present: second quantum leap in technology and
methodology (Medicine, Materials Science)
1. Introduction and history of NMR (Dayrit)
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1. 1921 - 45: Theories on atomic nuclei
• In 1921, Otto Stern and Walther Gerlach performed an
experiment wherein they passed a beam of hydrogen
molecules through a magnetic field and discovered that
nuclei, like electrons, are split into two streams. This
indicated that nuclei of atoms have quantized magnetic
moments.
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1. 1921 - 45: Theories on atomic nuclei
• In 1924, Wolfgang Pauli generalized this observation
with the suggestion that sub-atomic particles (electrons
and nuclei) have angular momentum, P. The nuclear
angular momentum is quantized according to the spin
quantum number, I. Because these nuclei also possess
charge, the result is a nuclear magnetic moment, m.
•In 1939, Isidor Rabi and co-workers sent a beam of H2
through a homogeneous magnetic field in the presence
of a RF field. They observed a deflection of the
molecular beam indicating that the hydrogen nuclei
absorbed the RF energy. This experiment provided the
first observation of the NMR phenomenon: the
absorption of RF energy by nuclei in a magnetic field.
1. Introduction and history of NMR (Dayrit)
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1. 1921 - 45: Theories on atomic nuclei
• Stern received the Nobel prize in Physics in 1943 his
discovery of the magnetic moment of the proton and for the
development of the molecular beam method
• Rabi received the Nobel prize in Physics in 1944 for
developing the resonance method of recording the magnetic
properties of atomic nuclei.
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2. 1946 - 55: Early NMR experiments
•
Early NMR experimental work was done by physicists. In
1946, two groups of physicists, one led by Felix Bloch at
Stanford University and the other by Edward Purcell at Harvard
University, rushed to apply these theories to the detection of the
proton magnetic absorption signal in substances in the liquid or
solid phase. Both groups succeeded within a few weeks of each
other and simultaneously published their results in the same
issue of Physical Reviews, vol. 69 (1946).
Felix Bloch
Edward Purcell
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2. 1946 - 55: Early NMR experiments
• In 1952, Bloch and Purcell were jointly awarded the Nobel
prize in Physics and the technique was subsequently christened
Nuclear Magnetic Resonance spectroscopy.
• The initial interest in NMR was limited to the physical study
of the nuclear parameters of the nuclei of various elements in
the periodic table and nuclear absorption phenomena. Topics
included the determination of the magnetogyric ratio, , and
nuclear spin quantum number, I, of the various nuclei.
• In another paper in 1946, Bloch suggested that there were two
methods of observing the NMR phenomenon: first was by
scanning through the appropriate range using a continuous
wave (CW) while the second was through the pulse technique.
1. Introduction and history of NMR (Dayrit)
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This new property aroused intense interest among physicists because it
seemed to offer new insight into nuclear structure. Then (horror of
horrors) evidence began to emerge that these fundamental "constants"
were not really constant at all, but appeared to depend on the nature of
the sample used for the measurement. This disturbing observation is
vividly illustrated by the case of ammonium nitrate, investigated by Proctor
and Yu in an attempt to derive an accurate value for the magnetic moment of
14N. To their consternation they observed two resonance lines. Owing to
an understandable reluctance to accept that this might be a mere chemical
effect, every effort was made to seek a less disturbing explanation. To this
end they placed an order for a sample of 15N-enriched ammonium nitrate in
the (vain) hope that the unwanted second resonance line was actually from
the 15N isotope… About the same time, Dickinson found similar
"discrepancies" in the 19F resonance frequencies of several fluorine
compounds… and Thomas observed further instances where the proton
resonance frequencies depended on the sample used. Something was
seriously wrong. How could all the excitement about a fundamental, highprecision nuclear property of such intrinsic importance be so cruelly dashed
by a mere chemical shift? Most physicists gave up in disgust.
Freeman, 2012
Many setbacks in science turn out to have a silver lining. A few far-sighted
scientists began to wonder if this new magnetic resonance discovery
could be applied to chemistry, by concentrating attention on the
chemical effects so despised by the physicists. Among them, the worldfamous chemist Linus Pauling suggested to Rex Richards at Oxford that
NMR might be a promising new avenue of research in chemistry. The
justification for such optimism was rather thin at that time, but at least NMR
was an open field, and Rex is a courageous researcher. However in that
era, a typical chemistry laboratory would be mainly devoted to "wet
chemistry" and it was really no place for two-ton magnets. To introduce
any large physical machine into that environment was a real battle,
involving banishment to some dreary basement room that no one else
wanted to use. On the positive side for an NMR pioneer there was a good
supply of war-surplus radar equipment that could be usefully
cannibalized, and it was always possible to ask those now disillusioned
physicists for technical advice.
Freeman, 2012
2. 1946 - 55: Early NMR experiments
• The concept of the chemical shift was developed to account for
the variations in shielding provided by the local electron density
surrounding the nucleus. This is the basis for the use of NMR as
a tool for structural analysis.
• In 1951, the first NMR spectrum – a 1H spectrum of ethanol –
was observed.
40 MHz 1H NMR of ethanol.
This spectrum was
photographed from an
oscilloscope which was used
as the recorder.
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2. 1946 - 55: Early NMR experiments
• Although the CW and pulse techniques were proposed at the
same time, their development and applications diverged
during these early stages of NMR. CW NMR was applied
extensively in organic chemistry. Much of this rapid
development was also due to the success of Russell Varian
who introduced the first commercial CW NMR in 1953.
The most successful commercial CW
high-resolution NMR instrument was
the Varian A-60 which was introduced in
1961. The A-60 was a 60MHz 1H NMR
spectrometer which was able to give
highly reproducible results.
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2. 1946 - 55: Early NMR experiments
• In 1953, Albert Overhauser predicted
that nuclear spin polarization can be
transferred from one spin population
to another via cross-relaxation. This
theory which is now known as the
nuclear Overhauser effect is a very
important tool for the determination
of molecular structure and dynamics.
Receiving the US Presidential
Medal of Honor, 1994.
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3. 1956 - 65: Early applications in organic chemistry
•
In 1957, Paul Lauterbur recorded the first 13C NMR
spectrum at natural abundance (1.1%). However, because of
its lower inherent sensitivity, the study of 13C lagged behind.
•
In 1959, Martin Karplus developed empirical correlations
between vicinal angles of protons and their spin-spin
coupling constants.
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4. 1966 - 75: First quantum leap in NMR methodology
The main NMR breakthroughs of this decade can be
summarized in the following:
1. Application of Fourier transform (FT) to pulse NMR;
2. Development of commercial high-field superconducting
magnets;
3. Advances in electronics and computers; and
4. These developments in turn made possible the development
of 2-D NMR.
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4. 1966 - 75: First quantum leap in NMR methodology
• In 1966, Ernst and Anderson proposed the use of Fourier
transform mathematics to process the free induction decay
obtained from a pulse experiment. This suggestion also became
implementable with the entry of practical computing and
improved electronics. The convergence of these techniques and
technologies gave birth to practical pulse FT-NMR.
The pulse FT-NMR method is used in
virtually all of its modern applications.
For this and related achievements,
Richard Ernst was awarded the Nobel
prize in Chemistry in 1991.
1. Introduction and history of NMR (Dayrit)
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4. 1966 - 75: First quantum leap in NMR methodology
•
The workhorse NMR equipment were based on the iron
magnet. Unfortunately, the upper limit of iron magnets is
about 2.35 tesla (100-MHz 1H NMR). The development of
superconducting magnets enabled the development of
magnetic fields in commercial instruments of 6.35T, 8.46T
and 9.40T (270, 360, 400 MHz).
•
The implementation of pulse FT-NMR was made possible
by the rapid rise in computer and electronics technology
because such techniques require the rapid and accurate
accumulation of data and their processing. In addition,
experiments requiring many hours of accumulation became
possible with improved electronics and computer
technology.
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4. 1966 - 75: First quantum leap in NMR
methodology
• In 1971, Jean Jeener suggested the use of
two independent time domains as a means
of obtaining new NMR information. This
resulted in the birth of 2-dimensional
NMR. 2-D NMR revolutionized the practice
of NMR.
• In 1978, N. Watanabe at JEOL reported the coupling of a
liquid chromatograph to an NMR using a stopped flow
technique. An on-line system was developed a year later..This
was the first report of a “hyphenated” NMR technique.
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5. 1976 - 85: The rise of biological NMR
• 1984. The NMR signals of
biopolymers (proteins and nucleic
acids) are usually bunched up in small
regions of the NMR spectrum leading
to severe overlap of resonances.
Despite these difficulties, by 1984 the
structure of a 57-residue protein was
achieved. Although this is small by
protein standards, it established the
viability of NMR as an alternative to
X-ray crystallography for the
determination of biopolymer structure
particularly in solution.
1. Introduction and history of NMR (Dayrit)
The first protein
structure determined by
NMR. All heavy-atom
presentation of the NMR
structure of the
proteinase inhibitor IIA
from bull seminal plasma
(BUSI IIA) (from:
Wuthrich 2001)
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5. 1976 - 85: The rise of biological NMR
•
In vivo NMR was developed. Metabolic studies on live
animals were carried using the 31P signals of intracellular
constituents such as ATP, ADP, phosphocreatine, and other
biological phosphates. By using compounds enriched with
13C or labeled with 19F, it became possible to carry out
studies on drug metabolism or enzymatic disorders.
NH2
N
O-
O-
P
O
O
O-
O
O-
P
P
O
O
N
O
O
HO
N
H
CH3
N
N
OH
N
HO
P
C
O
NH
CH2
CO2H
OH
1. Introduction and history of NMR (Dayrit)
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5. 1976 - 85: The rise of biological NMR
•
Since the late 1970s, Magnetic Resonance Imaging (MRI)
has developed into a separate and more lucrative branch.
MRI is based on the analysis of water in biological
organisms. These signals are processed to show an image.
MRI has become a major non-invasive diagnostic tool in
medicine. In 2003, the pioneers of MRI, Paul Lauterbur and
Peter Mansfield were awarded the Nobel prize in
physiology or medicine.
Paul Lauterbur
1. Introduction and history of NMR (Dayrit)
Peter Mansfield
30
6. 1986 - present: second quantum in
technology and methodology
•
The rapid increase in computing
capabilities and magnetic field strengths
(now up to 1 GHz) has extended the
sizes of proteins that can be studied.
•
NMR has also been applied to the
field of materials science following
developments in solid state NMR; the
availability of broad band transmitters
means that virtually any element in the
periodic table can be studied.
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6. 1986 - present: second quantum in technology and methodology
• The use of NMR in routine analytical chemistry is
expanding. NMR methods have been developed for analysis in
the food, fats and oils, paint, and polymer industries. For
example, it is used to determine moisture, fat, and solid
content of products, ranging from chocolate, seed, vegetable
oil, plant, among others.
• Although NMR has been largely used as a qualitative
technique, the quantitative applications of NMR are now being
developed.
Low-resolution NMR for
analysis of solid fat content.
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6. 1986 - present: second quantum in technology and methodology
•
NMR is one of the principal methods, along with Mass
Spectrometry, in the field of Proteomics.
The RIKEN “NMR Park”
Yokohama, Japan.
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7. Current applications in NMR
• Organic structure determination
• Inorganic systems: metal or ligand analysis (e.g.,
31P, 15N, 13C, 17O)
• Medical applications for diagnostics:
• functional MRI (fMRI)
• Image processing
• Structural Biology
• Proteins, protein dynamics protein-drug interaction,
membrane proteins
• Metalloproteins
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7. Current applications in NMR
• Materials Science
• Solids NMR
• Sensitivity for multi-nuclear detection
• High temperature, high pressure cells
• Polymers:
• Polymer structure
• Natural polymers
• Metabolomics
• Small molecules in urine
• Quantitative NMR
• Industrial applications
• NMR vs. X-ray
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