Ch 235.42: Advanced Analytical Chemistry: NMR · 11/1/2012 · 1. Introduction and history of NMR (Dayrit) 14 1. 1921 - 45: Theories on atomic nuclei • In 1924, Wolfgang Pauli

Ch 235.42: Advanced Analytical

Chemistry: NMR

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:

Wavelength:

Frequency:

Classification:

Excitation:

200 - 800 nm

~1 x 1015 Hz

UV-visible

2.5 - 25 mm

4000-600 cm-1

Infrared

~ 1 m

~100 x 106 Hz (MHz)

Radiofrequency

Electronic

energy level Chemical

bond Atomic

nucleus

Higher frequency

Higher energy

Lower frequency

Lower energy

E = h n

1. Introduction and history of NMR (Dayrit) 7

Excitation: Electron Chemical

bond

Atomic

nucleus

• atomic and

molecular

electronic energy

levels

• electron spin

transition

(fluorescence)

What are the spectroscopic methods based on?

• bond energies

• molecular symmetry • type of nucleus (e.g.,

1H, 13C, 31P)

• chemical environment

• inter-nuclear interaction

• molecular symmetry

• dynamic motion


UV-visible spectroscopy:

208 nm: unsaturated acid or ester

OR

O

NMR spectroscopy ():

H3C

C C

C O CH2 CH3

O

H

H

178

145123

60

19

15

1.35 (t)

1.94 (dd)

4.14 (q)

5.80 (dq)

6.90 (dq)

Comparison of Methods of Molecular Spectroscopy .

H3C

O CH2 CH3

O

H

H

trans-ethyl crotonate

IR spectroscopy (n): 3050 cm-1 : C=C-H 2980-2910 cm-1 : alkyl groups: CH3, CH2

1710 cm-1 : unsaturated acid or ester . 1660 cm-1 : C=C


NMR is able to give much more structural

information than UV-visible and IR.

For example, cholesterol:

HO

CH3

CH3

HH3C

CH3

CH3

• UV-visible, lmax: ring olefin • IR, n: stretch, bend,

rocking: -OH, -CH3, -CH2-,

CH-, and fingerprint pattern


• NMR: 1H: 46 hydrogen atoms 13C: 27 carbon atoms.

CH2

CH

CH2

C

C

CH2 CH

CH

CH2

CH

CH

C

CH2

CH2

CH

CH2

CH2

CH3

CH3CH

H3C CH2

CH2

CH2

CH

CH3

CH3

HO

CH3CH3

HH3C

CH3

CH3

HO

H

H

H

H

H

Chemical shift, ,

and integration

1H-1H splitting

and NOE

Relative

configuration:

1H NMR

13C NMR


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


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. 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.



• 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.



• 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.


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



• 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.



• 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.



• 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.



• 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.


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.


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.



• 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.



• 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.


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.


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.

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)



• 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.

P

O

O

O

HO

N

NN

N

NH2

OP

OPO-

O

O- O-

O

O-

OH

HOP

NC

NCH2

O

OH

NH

CH3

CO2H

H



• 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 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 Peter Mansfield


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.


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.


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.


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


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

Documents

Ch 235.42: Advanced Analytical Chemistry: NMR · 11/1/2012 · 1. Introduction and history of NMR (Dayrit) 14 1. 1921 - 45: Theories on atomic nuclei • In 1924, Wolfgang Pauli