Nuclear Magnetic Resonance Spectroscopy
Practically all organic compounds contain H atoms. Therefore, proton NMR studies are of special value in determining the structures of organic compounds.
NMR in brief:
Recognise the different hydrogen environments in molecules and the number of hydrogen atoms in each.
NMR gives information about (1) the kinds of hydrogen environments (from the absorption peaks or signals) and (2) the number of H atoms in a hydrogen environment (integrated trace).
Essentially, electromagnetic radiation in the radio-wave region is absorbed. These (energy) absorption peaks in the NMR spectrum are represented along a horizontal chemical shift scale (rather than, for example, wavelength or frequency). The origin (zero) of this scale is set by measuring the sharp absorption of tetramethylsilane (TMS), (CH3)4Si, which is well away from most of those of interest to organic chemists. The extent to which an absorption (signal) differs from that of TMS is called its chemical shift.
Use 'chemical shift' tables to identify H environments. The relative area under each signal informs about the number of protons producing the signal. This is represented as an integrated trace that increases in steps in ratio to the number of H atoms in each environment (or the actual number of H atoms is stated).
The low-resolution NMR spectrum of ethanal is given as an example:
 | R-CHO | 10.0
|
| RCOCH3 | 2.2
|
How the NMR Spectrometer works in brief:
Again consider ethanal, CH3CHO, as an example. There are two different hydrogen environments in this molecule and therefore its NMR spectrum will have two absorption peaks.
Its 1H nuclei (protons) are spinning making them behave like tiny magnets that are randomly arranged. They all possess the same quantised amount of energy.
In the NMR spectrometer, the sample is placed in a strong external magnetic field.
The spinning protons behave like tiny magnets and align themselves either with or against the magnetic field.
A little more than 50% align with the magnetic field and possess slightly lower energy than those aligned against it.
There is a definite energy difference, DE, between those 1H nuclei aligned with and those aligned against the external magnetic field.
But there are four 1H nuclei in the ethanal molecule, so why doesn't each one experience the same magnetic field and so acquire one of the two possible energy states? If this were so, the instrument would not be able to distinguish between the different hydrogen environments.
None of the 1H nuclei experience the actual external magnetic field strength. This is because the electrons of nearby atoms cause small magnetic fields that oppose the external magnetic field. The 1H nuclei 'feel' an effective magnetic field that is slightly reduced in strength. 1H nuclei in the same environment will experience the same effective magnetic field, but the different environments will experience different effective magnetic field strengths because the atoms neighbouring them are different.
A simplified arrangement in a nuclear magnetic resonance spectrometer is shown below:
An NMR spectrometer can work in one of two ways:
- The radio wave frequency is kept the same and the magnetic field strength is changed so that DE for each different hydrogen environment in turn becomes the same. This then corresponds to the energy of the photon absorbed to change the alignment of a 1H nucleus from 'with' to 'against' the magnetic field. This idea is used below.
- The external magnetic field strength is kept the same and the sample is subjected to radio wave radiation across the range of quantised frequencies.
Whether the instrument measures the absorbed frequencies or the different external magnetic field strengths, this is expressed as a chemical shift value, based on the shift in absorption away from that of the reference compound, tetramethylsilane, (CH3)4Si. TMS is assigned the chemical shift value of zero.
An integrated trace represents the area under each absorption peak, informing about the number of 1H nuclei in each environment.
A table of chemical shift values can be used to identify the various hydrogen environments.
Learn more about NMR (if you like):
The energy levels of protons and neutrons in atomic nuclei are quantised like the energy levels of electrons in atoms.
As with electrons, protons and neutrons have two spin orientations, corresponding to quantum numbers of +½ (Ý) and -½ (ß). Like an unpaired electron, an unpaired proton or an unpaired neutron has a spin and behaves like a tiny magnet.
The proton spins and creates a tiny magnetic field.
Nuclei of atoms having an odd number of protons or neutrons or both have a net spin and therefore have a magnetic moment. Some important examples are 1H, 2D, 31P, 14N and 19F. However, spin pairing (Ý ß) occurs, so that nuclei with an even number of protons and an even number of neutrons do not have a net nuclear spin. For example, the nuclei of 12C, 16O, and 32S have no magnetic moment.
The nucleus of 1H is of particular importance. 1H has two spin states, +½ and -½. In the absence of a magnetic field, these spin states have the same energy. The magnetic fields of spinning protons are normally randomly arranged.
So, for equivalent H atoms in a molecule, when a strong magnetic field is applied, slightly more than 50% become aligned with the magnetic field and the others against it. Those aligned with the magnetic field have slightly lower energy than those aligned against it. The energy difference is DE.
For a given hydrogen environment, when radiation of a frequency corresponding to DE is applied, some of the nuclei in the lower energy level will move up to the higher energy level, absorbing the applied radiation.
Look at the low-resolution spectrum of Ethanal above...
Ethanal has two hydrogen environments (two kinds of protons) and so will show two absorptions. The relative areas under the peaks are 1:3. Clearly, the two kinds of protons do not experience exactly the same magnetic field. They 'feel' a different magnetic field. If this were not so then NMR spectroscopy would have very little use. In fact, neither kind of proton experiences the actual external magnetic field strength applied. This is because the electrons of nearby atoms cause a small magnetic field that opposes the applied field. The effective magnetic field experienced by a proton is therefore reduced. Because a proton does not feel the full applied field, it is said to be shielded.
Electronegative atoms have an effect...
Where a hydrogen atom is bonded to, or close to, an electronegative atom, the electronegative atom withdraws electron density from the H atom. The H atom is then less shielded and so experiences a greater applied field than would otherwise be so. This is happening to both hydrogen environments in the ethanal molecule, but the proton of the aldehyde group (-CHO) is least shielded and therefore feels the greater magnetic field. For this proton, in a given effective magnetic field, the difference in energy (DE) of the 'aligned with' and the 'aligned against' spin states is greater. This situation is illustrated below for the Ethanal molecule.
But here is an important difference..
Unlike an ultraviolet and infrared spectrometer, in an NMR spectrometer the frequency of the radio waves is kept constant. It is the magnetic field strength (H0) that is continuously changed. The NMR spectrum plots absorption against magnetic field strength at a fixed radio-wave frequency. The applied magnetic field strength needed to bring about the alignment 'with' and 'against' the magnetic field of the equivalent hydrogen nuclei of tetramethylsilane (TMS) is used to set the zero reference point of the horizontal magnetic field strength scale. For a particular hydrogen environment, the shift away from this point of the magnetic field strength required to bring about the absorption of radio waves is plotted. For this reason the scale is named chemical shift.
