Glossary – B

Band Gap: This is the difference in energy between bands formed from the overlap of orbitals of different symmetry, though it is typically taken to be the energy difference between the highest fully occupied and the lowest fully unoccupied bands. Band: A band is formed when there is extended overlap of molecular orbitals in a … Read more

Glossary – A

Absorption spectroscopy: Generally, spectroscopy that relies upon measuring the radiation absorbed by a sample as it is excited to higher energy levels. Absorption spectroscopy: Generally, spectroscopy that relies upon measuring the radiation absorbed by a sample as it is excited to higher energy levels. Achiral: Lacking chirality. i.e. having no stereocentre. Acidic Proton: A hydrogen … Read more

Electron Spin Resonance (ESR) Spectroscopy

This is a spectroscopic technique related to NMR that makes use of the fact that electrons also have an intrinsic spin angular momentum. For electrons, the spin quantum number, S, is equal to ½. This number specifies the magnitude of the total spin angular momentum for an electron to have the numerical value / 2 … Read more

Practical aspects of NMR

High resolution NMR spectra are routinely obtained today, but there are various requirements which must be fulfilled to produce high quality spectra from which a great deal of useful data can be obtained. Today, most NMR is of the pulse variety, in which all the nuclei in the sample are placed in a static magnetic … Read more

Relaxation times and Linewidths in NMR Spectra

We have already established that a bulk sample containing numerous spin ½ nuclei in a static magnetic field possesses a net magnetisation in the z direction (as defined by the direction of the magnetic field). Application of a brief electromagnetic pulse in the xy plane produces a magnetisation component in the xy plane which decays … Read more

Pulse Techniques in NMR

In the early days of NMR spectroscopy, it was common to hold the magnetic field at a constant strength and then irradiate the sample with highly monochromatic electromagnetic radiation (i.e. electromagnetic radiation spanning only a small frequency range). This radiation could then be swept through different frequencies, and the frequencies at which nuclei in the … Read more

Patterns of Splitting in an NMR Spectrum

As we have already stated, the NMR spectrum of an AX system (two spin-½ nuclei, A and X, spin coupled together) consist of two lines, each split into a pair of component lines that are known as a doublet. If we consider the resonance of the A nucleus, then it is relatively simple to see … Read more

Fine Structure in an NMR Spectrum

As mentioned previously, it is most common for the lines in an NMR spectrum to be split into several components. This is referred to as the fine structure of the spectrum, and it leads to the NMR spectrum of ethanol (C2H5OH) having the following appearance: The appearance of such a spectrum makes it hard to … Read more

Contributions to the Chemical Shift

Broadly speaking, we have established that protons in inequivalent positions in a molecule will have different resonance frequencies. This explains the appearance of the NMR spectrum of ethanol: (Note that this representation would be a very low resolution spectrum – normally the three lines shown above would be split into different components from which further … Read more

Chemical Shift

Nuclear magnetic moments interact with the local magnetic field, which will not necessarily be identical to the applied magnetic field. The applied field can induce motion of electrons in orbitals, which gives rise to an additional magnetic field at the nucleus. This extra field δB is proportional to the applied field, so it is common … Read more