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NMR Spectroscopy

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Lillian Dawit

on 11 February 2014

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Transcript of NMR Spectroscopy

NMR Spectroscopy
Nobel Prize Winners
Nobel Prize Winners (cont.)
2002 Chemistry Wüthrich (ETH)








2003 Medicine Lauterbur (University of Illinois in Urbana ), Mansfield (University of Nottingham)

Ethanol
References
WHAT IS IT?
Commonly referred to as NMR, is a technique which exploits the magnetic properties of certain nuclei with a static magnetic field. This magnetic field makes the possible spin-states of the nucleus differ in energy, and using NMR techniques the spins can be made to create observable transitions between the spin states.
The first 40 MHz NMR spectrometer (Varian, 1953)
1. Number of Different Hydrogens

To check: replace the H atoms with X groups one at a time
If they produce the same compound, they are of the same type

EXAMPLE: Ethyl Acetate (C4H8O2)
How many different types of H hydrogens are in this molecule?

1. Number of Diff. Hs (continued)

ANS = 3
Thus, there are 3 peaks

THEORY
In a magnetic field, there are now two energy states for a proton: a lower energy state with the nucleus aligned in the same direction as B0, and a higher energy state in which the nucleus aligned against B0.
NMR Timeline
1926
Pauli’s Prediction of Nuclear Spin
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
Overhauser NOE (nuclear Overhauser 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)
VARIABLES
1. Applied magnetic field B0
Measured in tesla (T)

2. Frequency ν of radiation used for resonance
Measured in hertz (Hz), or megahertz (MHz)
By: Gaurav & Lily
ADVANTAGES
Non-Invasive
Non-Destructive
Biologically Harmless
Method is Nonspecific
Measurements can be made of several compounds simultaneously
Intracellular pH can be monitored
Only method for atomic resolution structure determination of biomacromolecules in aqueous solutions

NMR Experts
Compared to mass spec, larger amounts of sample are needed
Lack of sensitivity
Prolonged measurements may be required
NMR magnets are very expensive and very specific
Require purpose-built accommodation
The bore (section of the machine) is what is the major limiting piece

NMR Experts
DISADVANTAGES
Uses for NMR Spectroscopy
Primarily focused on studying the brain
The determination of the structure of unknown organic compounds
Unfolded proteins, solutions, nucleic acids, etc.
MRIs (magnetic resonance imaging)
Drug screening and design
Direct detection of hydrogen bonding
Helps determine composition and condition of foods & ingredients
Used to conduct diagnostic tests (identify markers for diabetes & cardiovascular disease)
In academic and government research labs

CLINICAL APPLICATIONS/USES

1944 Physics Rabi (Columbia)







1991 Chemistry Ernst (ETH)








1952 Physics Bloch (Stanford),
Purcell (Harvard)

2. Integration
Although there are 3 different types of hydrogens, how many of each type are there?

Symbol = integral symbol (∫) or a line above the peaks

Area under NMR resonance is proportional to # of hydrogens represented by the resonance
Number usually is given (calculated), but you can measure
http://users.wfu.edu/ylwong/chem/nmr/h1/images/integration.swf


4. SPLITTING
Info on how many hydrogen neighbors there are for a particular hydrogen group of equivalent hydrogens
Resonance is split into N+1 peaks, where N= # of hydrogens on adjacent atom

There are:
Singlets
Doublets
Triplets
Quartets
Multiplets

Downfield → left, farther away from zero/TMS
Upfield → right, closer to zero/TMS

Generally, the closer the hydrogen is to an electron withdrawing group (atom of high electronegativity), the more downfield it is
Resonates at higher frequency
Generally, the greater the electron density → the more shielded the proton
Peaks are more upfield (right)

Typical NMR Spectrum
READING NMRs
THEORY (cont.)
When an external energy source (hν) that matches the energy difference (ΔE) between these two states is applied, energy is absorbed, causing the nucleus to “spin flip” from one orientation to another.
H NMR tells you 4 things:

1. Number of different types of hydrogens present in the molecule

2. Relative # of the different types of hydrogens

3. Electronic environment of the different types of hydrogens

4. Number of hydrogen “neighbors” a hydrogen has

Typical H NMR Spectrum
"for his resonance method for recording the magnetic properties of atomic nuclei"
"for his contributions to the development of the methodology of high resolution nuclear magnetic resonance (NMR) spectroscopy"
"for their development of new methods for nuclear magnetic precision measurements and discoveries in connection therewith"
"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"
The Spin Flip
3. Chemical shift


X-axis of Spectrum
The position of where a particular hydrogen atom resonates relative to TMS
Expressed in parts per million (ppm)
Equation: difference between the resonance frequency of the nucleus and a standard, relative to the standard
AKA:

3. Chemical shift (cont.)


Chemical Shift Standard:
TMS = Tetramethyl Silane
Referred to as the standard → the “reference substance” from the equation
Why use TMS?
Very Soluble
Volatile
Convenient Structure
In the Spectrum, TMS = 0
Signal is set as the “zero”

4. SPLITTING
n
0 singlet 1
1 doublet 1 1
2 triplet 1 2 1
3 quartet 1 3 3 1
4 quintet 1 4 6 4 1
5 sextet 1 5 10 10 5 1
6 septet 1 6 15 20 15 6 1
7 octet 1 7 21 35 35 21 7 1
8 nonet 1 8 28 56 70 56 28 8 1



]

1. Clinical applications of nuclear magnetic resonance spectroscopy: a review.
Journal of the Royal Society of Medicine. 1984 September; 77(9)774
2. http://users.wfu.edu/ylwong/chem/nmr/h1/index.html
3. http://www.agilent.com/labs/features/2013_101_nmr.html
4.http://boneandjoint.org.uk/highwire/filestream/13029/field_highwire_article_pdf/0/533.full-text.pdf
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