General

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…

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…

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…

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…

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…

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…

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…

Energies of Nuclei in Magnetic Fields

The nuclear magnetic moment of a nucleus is denoted μ, and the component of this magnetic moment on the z axis, μz, is proportional to the component of the nuclear magnetic moment along this axis, mI. We thus write: where γ is called the…

Spin Angular Momentum

Spin angular momentum is a specific type of angular momentum possessed by some nuclei. As such, it obeys all the relations given for angular momentum under the quantum mechanics of rotation, here. i.e. there are two quantum numbers associated with the spin angular momentum…