CHAPTER 10 Using Nuclear Magnetic Resonance Spectroscopy to Deduce Structure
10-1 Physical and Chemical Tests
Some common physical tests are melting point and boiling point. Chemical tests such as elemental analysis can help define the structure of an unknown compound. However, there are literally millions and millons of organic compounds and different structures can have the same physical and chemical properties.
10-2 Defining Spectroscopy
Spectroscopy involves the analysis of the absorption of radiation. Absorption of infrared energy results in stretching and bending of bonds. Visible and ultraviolet light are absorbed and result in the excitation of electrons from lower to higher lying orbitals. In the presence of a magnetic field, certain nuclei absorb electromagnetic radiation resulting in a re-alignment with the applied magnetic field.
10-3 Proton Nuclear Magnetic Resonance
A proton spectrum provides information as to:
- electron density (further left is more electron defficient)
- number of hydrogens (from area of peaks)
- number of neighboring hydrogens (from peak multiplicity)
10-4 Using MNR Spectra to Analyze Molecular Structure: The Proton Chemical Shift
The energy required to reorient a hydrogen nucleus in a magnetic field depends on the electron density surrounding the hydrogen.
10-5 Tests for Chemical Equivalence
Chemically equivalent hydrogens will appear in an NMR spectrum at the same place. However, chemically different hydrogens may or may not appear as distinct signals.
The intensities of signals for hydrogen atoms in an NMR spectrum are directly proportional to the number of hydrogens contribution to each signal.
10-7 Spin-Spin Splitting: The Effect of Nonequivalent Neighboring Hydrogens
A hydrogen atom with a neighboring hydrogen is exposed to two magnetic fields as the neighbor either adds to or substracts from the applied magnetic field. This results in two unique signals. Two neighbors created three unique magnetic fields, and three neighbors result in four unique fields. This multiplicity is very useful in deducing the structure of a compound from its NMR spectrum.
10-8 Spin-Spin Splitting: Some Complications
Non-first order effects can complicate the analysis of NMR spectra.
10-9 Carbon-13 Nuclear Magnetic Resonance
Carbon-13 represents only about 1% of the carbon in a sample, so instrumentation must be more sensitive than used for proton spectroscopy. On the other hand, the probability of carbon atoms coupling with each other is small as it is unlikely that a structure will have two carbon-13 atoms next to each other.
The range covered by C13 is much wider than is that for protons and, as a result, very rarely do unique carbon atoms overlap. Thus, C13 spectra can be used to determine the number of unique carbon atoms.
C13 also couples to protons and from this, it can be determined the number of hydrogen atoms attached to each carbon.