Title: NMR Spectroscopy
1NMR Spectroscopy Dr Graeme Jones
Applications of Chemistry
2Basic Spectroscopy
Absorptionof radiation
Transmitted radiation
Spectrum of radiation
Excited Molecule
Radiation Source
Detector
Low energy Molecule
3Analysing Organic Molecules
4IR Spectroscopy
Absorptionof radiation
Transmitted radiation
Spectrum of radiation
Vibrating bonds
Infra-red Radiation Source
Infra-red Detector
bonds
5NMR Spectroscopy
Absorptionof radiation
Transmitted radiation
Spectrum of radiation
Excited Nuclei
Radiofrequency Radiation
magnet
magnet
Nuclei
Emitted Radiation
Detector
6A 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.
71H NMR of Ethanol
8Expansion
91H NMR of ethyl ethanoate
10Things 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
11IR and 1H NMR of Aspirin
12The 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.
13The Energy Barrier Between Spin States
14Boltzmann 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
15Resonance Frequency
- Relationship between Resonance Frequency
- and Field Strength in 1H NMR
16NMR activity of different Nuclei
17The 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.
18Practical 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.
20The 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.
21Measuring 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.
22By 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.
24Equivalence
- 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.
25Additional 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