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 1 H frequency) 11.74 Tesla (500 MHz 1 H frequency) NMR is insensitive because it is low energy: 1 H versus 13 C NMR response
Chemistry 2600 Lecture Notes Chapter 15 Nuclear Magnetic Resonance Spectroscopy Page 2 of 23 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 fieldindependent units and are then called the chemical shift ( FrequencyA FrequencyRef ) inhz δ A = = ( Chemical shift) in ppm Spectrometer Frequency in MHz ( ) The standard reference compound for 1 H and 13 C 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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