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NMR Spectroscopy

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Title: NMR Spectroscopy


1
NMR Spectroscopy Dr Graeme Jones
Applications of Chemistry
2
Basic Spectroscopy
Absorptionof radiation
Transmitted radiation
Spectrum of radiation
Excited Molecule
Radiation Source
Detector
Low energy Molecule
3
Analysing Organic Molecules
4
IR Spectroscopy
Absorptionof radiation
Transmitted radiation
Spectrum of radiation
Vibrating bonds
Infra-red Radiation Source
Infra-red Detector
bonds
5
NMR Spectroscopy
Absorptionof radiation
Transmitted radiation
Spectrum of radiation
Excited Nuclei
Radiofrequency Radiation
magnet
magnet
Nuclei
Emitted Radiation
Detector
6
A simple analogy
  • The bar magnet is the nucleus.
  • The magnetic field is the applied magnetic field.
  • The force is radio frequency radiation.
  • The ALIGNED and OPPOSED orientations are the LOW
    and HIGH ENERGY SPIN STATES of the nucleus.

7
1H NMR of Ethanol
8
Expansion
9
1H NMR of ethyl ethanoate
10
Things to note about NMR
  • NMR named after the part of the molecule that is
    excited and the magnet, not the radiation used as
    in IR.
  • NMR requires the molecule to be placed in a
    magnetic field in order to create the low energy
    and high energy states
  • Two ways of taking the spectrum
  • Keep the frequency constant and change the
    magnetic field strength (the old way)
  • Keep the magnetic field strength constant and
    change the frequency (the modern way)
  • In NMR only small amounts of radiation at
    specific frequencies are absorbed and in modern
    NMR it is the emitted radiation not the
    transmitted radiation that is detected

11
IR and 1H NMR of Aspirin
12
The Fundamentals of NMR
  • All nuclei carry a charge and in some nuclei this
    charge spins around an axis generating a magnetic
    dipole along the axis of the nucleus.
  • 1H has a spin I ½. There are two allowed spin
    states ½ and ½. In the absence of a magnetic
    field the two spin states are degenerate and are
    equally populated.
  • In a magentic field the low energy spin state is
    aligned with the magnetic field and the high
    energy opposed to it.

13
The Energy Barrier Between Spin States
14
Boltzmann Distribution
  • The difference between Na and Nb is very small.
    The excess nuclei are those which allow us to
    detect resonance. Therefore the greater the
    difference between the two populations the more
    sensitive the NMR machine. Increasing the
    magnetic field strength (Bo) of the magnet and
    hence the resonance frequency increases the
    number of excess nuclei

15
Resonance Frequency
  • Relationship between Resonance Frequency
  • and Field Strength in 1H NMR

16
NMR activity of different Nuclei
17
The Spectroscopy
  • The local magnetic field can either reinforce the
    applied magnetic field, deshield the hydrogen
    nucleus, and hence a higher frequency will be
    required to bring it into resonance.
  • The local magnetic field can oppose the applied
    magnetic field, shield the hydrogen nucleus from
    the strength of the magnetic field and hence a
    lower frequency will be required to bring it into
    resonance.
  • Thus slightly different amounts of energy are
    needed to excite each individual hydrogen nucleus
    to its own higher energy level.
  • In summary, different electron densities create
    different magnetic environments around each
    hydrogen atom and therefore a series of signals
    are seen across a spectrum.

18
Practical Considerations of NMR
The sample is dissolved in a deuterated solvent
(CDCl3, CD2Cl2, CD3OD, C6D6, D2O), placed in an
NMR tube and positioned inside the magnet field.
The sample is then spun using a stream of
compressed air and the machine locked onto the
solvent and the tuned, in a process called
shimming.. In a Fourier Transform (FT) NMR
machine the sample pulsed with a broad band of
radio frequency radiation which excites all the
hydrogen nuclei into the higher energy level. As
each hydrogen nucleus relaxes to the ground state
it emits the same radio frequency as it absorbed.
The relaxation occurs over a period of time and
has a half-life. The cycle of pulse followed by
a period of relaxation is called a pulse
sequence.
19
  • The emission contains a function of time and
    appears as a cosine wave that decays. This is
    called a free induction decay (FID).
  • The emitted radiation is detected and the data
    stored. In FT NMR machines the pulse sequence is
    repeated many times and each FID is added to the
    last. The sum of the FID's is called an
    acquisition which is said to be in the time
    domain. To obtain a spectrum the FID has to
    undergo a Fourier Transformation (FT) which is a
    complex mathematical process which concerts it
    into a spectrum that is in the frequency domain.

20
The Advantages of FT NMR
  • Because the strength of the magnetic field
    remains constant high field super conducting
    magnets can be used in FT NMR machines - 250 to
    1000 MHz. This gives greater resolution to the
    spectrum (see resonance frequency diagram -
    larger magnets mean wider spectrum).
  • Since the whole frequency spectrum scanned at
    once and the FID's can be added together the
    signal to noise ratio on the final spectrum can
    easily be improved by taking many acquisitions.
  • Small sample sizes are required 5 mg (MW 500) of
    compound is usually sufficient to accumulate a 1H
    NMR spectrum in less than 5 minutes. Smaller
    quantities will require longer acquisition times.
    Most importantly 13C NMR can be carried out on
    10 - 20 mg samples and spectrums acquired in
    hours rather than days.

21
Measuring Chemical Shift
  • When recording a 1H NMR spectra in a NMR machine
    with a magnet of a Bo field strength of 4.7 Tesla
    the hydrogens in tetramethylsilane (TMS) resonate
    at 200 MHz (200 000 000Hz). Clearly it would be
    a nightmare if you had to record these massive
    numbers when notating NMR spectra and hence a
    relative resonance scale has been introduced,
    referenced against the resonance frequency of
    TMS. This is called Chemical Shift and is given
    the symbol d. It is calculated using the
    equation below.

22
By convention the NMR spectrum is recorded with
TMS on the right hand side of the page and the
peaks are presented on a parts per million (ppm)
chemical shift scale (d) with the TMS methyl
hydrogens set at 0. All peaks to the left have
positive values of d, generally less that 10 ppm.
23
  • One important outcome of using chemical shift is
    that protons resonate at the same chemical shift
    regardless of the magnetic field strength of the
    NMR machine. However as the magnetic field
    strength of Bo is increased the number of Hz per
    ppm increases (see table). The effect of this is
    that at higher field strength there is an
    improvement in resolution and individual peaks
    become more separated from each other and more
    distinct.

24
Equivalence
  • Hydrogen nuclei that are in the same chemical and
    magnetic environment are said to be equivalent.
    Equivalent nuclei will resonate at the same
    frequency and have the same coupling constants to
    adjacent nuclei.

25
Additional Factors Affecting Chemical Shift
  • Proton Exchange - It is very difficult to predict
    the chemical shift of protons attached to
    heteroatoms (O-H, N-H, S-H). This is because
    they are often involved in hydrogen bonding which
    has the effect deshielding the proton and in
    exchange processes which tend to broaden the
    signal.
  • A method of identifying the position of
    exchangeable protons such as O-H, N-H and S-H is
    by a trying a D2O shake. The spectrum of the
    compound is acquired in CDCl3 and then a small
    drop of D2O is added to the solution, the tube
    shaken and the spectrum is reacquired. This
    replaces the exchangeable protons with deuteriums
    which cannot be detected in the proton NMR.
    Therefore the peak disappears from the spectrum
    and hence its chemical shift can be identified in
    the original spectrum. This is a useful
    technique for alcohols, amines and carboxylic
    acids.
  • Solvent - Changing the solvent has a dramatic,
    yet unpredictable effect on the chemical shift of
    signals. This is useful if important peaks
    overlap in a CDCl3 spectrum then another solvent,
    D6 benzene, D3 acetonitrile etc can be used
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