NMR
Nuclear magnetic resonance, or NMR as it is abbreviated by scientists, is a phenomenon
which occurs when the nuclei of certain atoms are immersed in a static magnetic field
and exposed to a second oscillating magnetic field. Some nuclei experience this
phenomenon, and others do not, dependent upon whether they possess a property called
spin. You will learn about spin and about the role of the magnetic fields in Chapter 2, but
first let's review where the nucleus is.
Most of the matter you can examine with NMR is composed of molecules. Molecules are
composed of atoms. Here are a few water molecules. Each water molecule has one
oxygen and two hydrogen atoms. If we zoom into one of the hydrogens past the
electron cloud we see a nucleus composed of a single proton. The proton possesses a
property called spin which:
1. can be thought of as a small magnetic field, and
2. will cause the nucleus to produce an NMR signal.
Not all nuclei possess the property called spin. A list of these nuclei will be presented in
Chapter 3 on spin physics.
Chapter 13: Spectroscopy
Nuclear Magnetic Resonance (NMR) Spectroscopy
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Basic principles of NMR
Chemical shift scale
Shielding in H-NMR
Table of H-NMR chemical shifts
H-NMR spectra I
Coupling in H-NMR
H-NMR spectra II
Interpreting H-NMR
Interpreting C-NMR
C-NMR spectra
Basics:
Nuclei with an odd mass or odd atomic number have "nuclear spin" (in a similar fashion
to the spin of electrons). This includes 1H and 13C (but not 12C). The spins of nuclei are
sufficiently different that NMR experiments can be sensitive for only one particular
isotope of one particular element. The NMR behaviour of 1H and 13C nuclei has been
exploited by organic chemist since they provide valuable information that can be used to
deduce the structure of organic compounds. These will be the focus of our attention.
Since a nucleus is a charged particle in motion, it will develop a magnetic field. 1H and
13
C have nuclear spins of 1/2 and so they behave in a similar fashion to a simple, tiny bar
magnet. In the absence of a magnetic field, these are randomly oriented but when a field
is applied they line up parallel to the applied field, either spin aligned or spin opposed.
The more highly populated state is the lower energy spin state spin aligned situation.
Two schematic representations of these arrangements are shown below:
In NMR, EM radiation is used to "flip" the
alignment of nuclear spins from the low
energy spin aligned state to the higher
energy spin opposed state. The energy
required for this transition depends on the
strength of the applied magnetic field (see
below) but in is small and corresponds to
the radio frequency range of the EM
spectrum.
As this diagram shows, the energy required for the spin-flip
depends on the magnetic field strength at the nucleus. With
no applied field, there is no energy difference between the
spin states, but as the field increases so does the separation
of energies of the spin states and therefore so does the
frequency required to cause the spin-flip, referred to as
resonance.
The basic
arrangement of an
NMR spectrometer
is shown to the left.
The sample is
positioned in the
magnetic field and
excited via
pulsations in the
radio frequency
input circuit. The
realigned magnetic
fields induce a radio
signal in the output
circuit which is used
to generate the
output signal.
Fourier analysis of
the complex output
produces the actual
spectrum.The pulse
is repeated as many
times as necessary
to allow the signals
to be identified from
the background
noise.
Chapter 13: Spectroscopy
Chemical Shift
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An NMR spectrum is a plot of the radio frequency applied against absorption.
A signal in the spectrum is referred to as a resonance.
The frequency of a signal is known as its chemical shift.
The chemical shift in absolute terms is defined by the frequency of the resonance
expressed with reference to a standard compound which is defined to be at 0 ppm. The
scale is made more manageable by expressing it in parts per million (ppm) and is
indepedent of the spectrometer frequency.
It is often convienient to describe the relative positions of the resonances in an NMR
spectrum. For example, a peak at a chemical shift, , of 10 ppm is said to be downfield
or deshielded with respect to a peak at 5 ppm, or if you prefer, the peak at 5 ppm is
upfield or shielded with respect to the peak at 10 ppm.
Typically for a field strength of 4.7T the resonance frequency of a proton will occur
around 200MHz and for a carbon, around 50.4MHz. The reference compound is the
same for both, tetramethysilane (Si(CH3)4 often just refered to as TMS).
What would be the chemical shift of a peak that occurs 655.2 Hz downfield of TMS on a
spectrum recorded using a 90 MHz spectrometer ?
At what frequency would the chemical shift of chloroform (CHCl3, =7.28 ppm) occur
relative to TMS on a spectrum recorded on a 300 MHz spectrometer ?
A 1 GHz (1000 MHz) NMR spectrometer is being developed, at what frequency and
chemical shift would chloroform occur ?
A Few Basic Principles of NMR
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NMR relies on a phenomenon of physics: Many common atoms found in matter,
such as the hydrogen atom, will resonate, or absorb energy at a specific radio
wave frequency (RF), when placed in a strong magnetic field.
Resonance happens because the nuclei of these atoms have weak magnetic
properties, referred to as “spin”. In a strong magnetic field, the spins of these
nuclei tend to align in two or more possible energy states, with or against the
direction of the magnetic field.
The frequency of radio waves required to produce “resonance” depends upon the
way the atoms are connected together and their relative positions in space.
The chemical shift phenomenon: Each atom in a molecule resonates at a slightly
different frequency based on the other elements around it. This gives scientists an
idea of how the atoms are joined together in the molecule, besides how many
different atoms there are in the molecule.
Stronger magnetic fields lead to richer, more detailed information content and
increased sensitivity, allowing the detection and characterization of smaller
amounts of material.