Chapter 13 - NMR basics

Chapter 13: Spectroscopy

Nuclear Magnetic Resonance (NMR) Spectroscopy

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:

effect of an applied magnetic field on nuclear spins