NMR Good Notes

Chemistry 2600 Lecture Notes Chapter 15 Nuclear Magnetic Resonance Spectroscopy Page 1 of 23

Structure Determination in Organic Chemistry: NMR Spectroscopy

• Three main techniques are used to determine the structures of unknown organic molecules.

• These methods are completely general, and can be used for molecular compounds of all the elements

• These methods are used in conjunction with the chemist’s knowledge of chemical formula, chemical structure and composition of functional groups

Basic principles of Nuclear Magnetic Resonance

The Experiment • Some nuclei have nuclear magnetic moments; just as importantly, some do not

• These moments may be dipoles or quadrupoles

• Some commonly observed dipolar NMR nuclei in two classes: (A) Nuclei that are the major isotopes of the element in the natural form (B) Nuclei that are only minor constituents of the natural form of the element

• Ignoring the theory of the NMR response entirely, we observe that nuclear dipoles are differentiated in the presence of a strong magnetic field.

• Field strengths: 5.87 Tesla (250 MHz 1H frequency) 11.74 Tesla (500 MHz 1H frequency)

• NMR is insensitive because it is low energy: 1H versus 13C NMR response


The compound

• Nuclei in different locations in a molecule experience different local magnetic fields

• The unique resonance frequencies of nuclei in different types of nuclei are expressed in field-independent units and are then called the chemical shift

( )( )

( )RefAA

Frequency Frequency inHzChemical shift in ppm

Spectrometer Frequency in MHzδ

−= =

• The standard reference compound for 1H and 13C NMR is tetramethylsilane, TMS

• Rapidly moving nuclei experience an average local magnetic field

• Rotation about single bonds is normally sufficient to render attached nuclei equivalent

• Equivalent nuclei act as a single group rather than as individual nuclei: “intensity”

• Double and triple bonds and aromatic compounds render attached nuclei rigid

• Symmetry is used to identify “equivalent” and “inequivalent” nuclei in the NMR experiment

• Spin-spin splitting between inequivalent nuclei is always expressed in Hz and is field independent

The chemist • NMR is too complex to comprehend without advanced study

• Structure determination problems are solved by using pure logic

• Practice is the best teacher






















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