Ch.13
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Nuclear Magnetic Resonance spectroscopy
a form of spectroscopy that measures the absorption of radio-frequency by nuclei in a magnetic field. The energy absorbed causes nuclear spin transitions
Magnetic moment
the magnitude of a nuclear magnetic field
Chemical shift
the difference between the resonance frequency of the proton being observed and that of tetramethylsilane
_________ atoms deshield atoms and they move more downfield
electronegative
the farther the distance from the electronegative group, the ______ the effect
smaller
deshielded
when a nucleus is bonded to a group that withdraws part of the electron density from the bond, moving the absorption more downfield
What does the pi bonding in aromatic protons cause?
more deshielding and a farther downfield shift
what does pi bonding do in aldehyde protons?
deshield
what does the triple bond in acetylenic protons do?
shields the electrons
shielded
when a nucleus is surrounded by electrons whose induced magnetic field opposes the externally applied magnetic field
Hydrogen-bonded protons
hydrogen bonding will deshield the protons, lowering the field
carboxylic acid protons
the protons are deshielded by both the oxygen and the carbonyl group which will lower the field
chemically equivalent
atoms that can not be distinguished chemically
accidentally equivalent
atoms that are not equivalent by NMR yet absorbed at nearly the same chemical shift and are not resolved
Integration
the measurement of the area under a peak, proportional to the number of protons giving rise to that peak
spin-spin splitting
the interaction of the magnetic fields of two or more nuclei, usually through the bonds connecting them
What is the 3 bond rule?
protons within 3 bonds of each other will couple
N+1 Rule
if a signal is split by N equivalent protons, it is split into N+1 peaks
When will a proton split each other
• Nonequivalent • Bonded to the same carbon, or adjacent carbons
Coupling Constant (J)
the distance (Hz) between two adjacent peaks of a multiplet
If the J distance is the same, two molecules are _______
coupled
Complex Splitting
splitting by two or more different kinds of protons with different coupling constants