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

  • 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 13C 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

  • 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

  • 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.