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
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
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
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
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
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
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
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 magnetogyric ratio of the particular nucleus under consideration, an empirically determined … Read more
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 momentum that determine its properties. All nuclei have a spin quantum … Read more