Nuclear Magnetic Resonance Spectroscopy

Description

Nuclear magnetic resonance (NMR) is a property that some nuclei possess that is used to determine the structure of organic compounds. An NMR spectrometer uses a powerful magnet to align the nuclei and then measures the energy as the nuclei relax. A ¹H NMR spectrum shows how many protons (signals) there are in a similar chemical environment in the compound, how many neighboring protons (split) there are to those protons, and how close they are to an electronegative atom (shift).

A ¹³C NMR spectrum indicates how many different types of carbons are in a compound and the number of hydrogens on each carbon by the shift of the peaks and the type of peak in the distortionless enhancement of polarization transfer (DEPT) spectra. Two-dimensional NMR is used to plot one-dimensional NMR spectra against one another to determine even more information, such as which protons are coupled to each other and which protons are bonded to which carbons. Together, the information provided by NMR spectra acts as a confirmation tool if the structure of a compound is already known or it helps to identify an unknown compound.

At A Glance

• Nuclear magnetic resonance (NMR) is a property of some molecules that allows scientists to determine the structures of hydrocarbon compounds.
• An NMR spectrum contains four pieces of information that scientists use to determine structure: the number of signals, the chemical shift for each signal, integration, and splitting.
• To draw the spectra of a compound, first the number of unique signals is determined by examining the symmetry of the molecule and determining if there are any diastereotopic CH₂ in the molecule.
• The identity of a compound can be determined by examining each signal and determining the integration, the split, and the shift. Each signal is then stitched together with other signals until the compound is determined.
• Carbon nuclear magnetic resonance (NMR) shifts are larger and provide less information than proton NMR. Carbon NMR can distinguish four types of carbons based on their shifts: carbons connected to other carbons, carbons connected to heteroatoms, alkene and aromatic carbons, and carbonyl carbons.
• COSY, HETCOR, and 3-D NMR are types of multidimensional NMR techniques that can correlate to other types of NMR spectra to provide even more information about structures.