NMR Spectroscopy Theory: Introduction

An important tool of the chemist for the determination of molecular structure is Nuclear Magnetic Resonance Spectroscopy, or NMR. NMR spectra are acquired on a special instrument called an NMR Spectrometer. The instrument and concepts are related to those used for Magnetic Resonance Imaging or MRI in medical diagnostics. The technique is challenging to learn but wonderfully powerful for the determination of organic and biochemical structure.

Some nuclei exist in discrete nuclear spin states when the nuclei reside in an external magnetic field. Nuclear magnetic resonance spectroscopy observes transitions between these spin states. Important examples are the nucleus of hydrogen (the proton) and the nucleus of the 13C isotope of carbon. Nuclei that do not exist in spin states are the 12C isotope of carbon (the major isotope), and the 16O isotope of oxygen (again the major isotope). Consequently, 12C and 16O nuclei are transparent to NMR spectroscopy. The abundance of the 13C isotope of carbon in nature is only 1.1% relative to 98.9% 12C isotope. In spite of this low abundance, 13C nuclear spin transitions can be readily observed.

Other nuclei commonly present in organic molecules such as chlorine, bromine, iodine, and nitrogen nuclei have nuclear spin but are transparent for reasons beyond the scope of this presentation. Important exceptions are fluorine and phosphorous for which the major isotopes 19F and 31P have nuclear spin and spin transitions are readily observed.

For organic structure determination, the two most important types of NMR spectra are the proton and carbon spectra. They give information about the number of hydrogens and carbons in a molecule and how those hydrogens and carbons are connected together as well as information about functional groups. NMR complements IR for determination of structures of organic compounds. Remember IR spectra help identify functional groups in a molecular structure.

Below is a photo of the Varian 300 mHz NMR instrument at CU Boulder. The large round cylinder on the left is the liquid helium-cooled magnet. The sample is placed in a small glass tube and dropped into this magnet. The computer screen can be seen just to the right of and partially behind the magnet; the box to the far right houses the source of the radiofrequency radiation.

Next section: Spin states, magnetic fields, and electromagnetic radiation

Copyright information: Original content © University of Colorado, Boulder, Chemistry and Biochemistry Department, 2011. The information on these pages is available for academic use without restriction.