How to Find Proton NMR (Hydrogen-1 NMR or 1H NMR)
Equivalency and Peak Assignment
*NMR is important in determining molecular structure (Carbon skeleton)
KEY TERMS
● NMR Spectrum: Plot of photon quantity (y-axis) vs photon energy (x-axis)
● NMR signal: Peaks in the NMR spectrum plot
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Proton: Particle or atom bearing a positive (+) charge, H is referred to as a proton
Equivalent protons: Protons with magnetic environment that are identical in EVERY
way, can’t be distinguished from each other based on relative position
Nonequivalent protons: Protons with magnetic environments that are not identical in
one or more ways, can be distinguished based on relative position
○ It is easier to find ONE difference (nonequivalent) than to test ALL similarities (equivalent)
C has 3 H attachments
C has 2 H attachments
C has 1 H attachment
*Recognizing these methane groups will be one of the keys in finding equivalency
Determining Equivalency
Equivalent protons will be considered as groups or sets. Different groups are nonequivalent to
each other. Equivalent groups will be adjacent to the same things and will be indistinguishable
from their counterparts
*Maximum number of signals = number of protons (hydrogens)
(1) Similar atoms/molecules attached to the same atom will always be equivalent
Keep an eye out for methyl (CH3) and methylene (CH2) groups. All protons in those groups will
be equivalent.
DO NOT BE CONFUSED INTO THINKING IN A 2-D FASHION WHEN FINDING
EQUIVALENCY!
Methyl group in 3-D is tetrahedral and if:
One signal = All 3 Hydrogens are equivalent
Two signals = 2 of 3 Hydrogens are equivalent
Three signals = All 3 Hydrogens are nonequivalent
H’s (baby blue balls) all equidistant from C (black plastic
piece) and Cl (green plastic piece)
If the carbon was spun along the axis of rotation connected
to chlorine the hydrogens would all appear “unchanged”
● That is, if you’re rotating along a single bond in an
acyclic molecule (often shows equivalency)
We can now extrapolate this concept to more complicated
structures such as:
Notice how the 3 purple methyl groups are labeled as equivalent, implying that all the H’s contained in
those 3 methyl groups are equivalent. All 3 methyls have a common carbon, making all those hydrogens
equidistant to that carbon, therefore equivalent
Having a difficult time imagining the molecule? Here it is in 3-D!
(2) Use lines of symmetry
Another useful method for finding equivalency is to use lines of symmetry in addition to
concepts from rule (1). This is especially useful when finding equivalence in cyclic structures.
*Make note of the benzene ring
*PI BONDS ARE
MISLEADING IN CYCLIC
STRUCTURES SINCE PI BONDS MOVE AROUND AS PARTIAL PI BONDS IN RESONANCE
Lines of symmetry across a ring such as benzene would be more accurately portrayed as:
Putting rules 1 and 2 together:
(3) Consider the surroundings
Always be conscious of what the protons are adjacent to as differing adjacent groups means
inequality.
The number of signals in the NMR spectrum can be determined by finding the number of
nonequivalent proton groups in a molecule.
1 group = 1 signal
Assigning Groups and Signals to Peaks
Considering the previous examples:
When it comes to using proton NMR, the number of peaks can be determined. However, there
are limitations:
1) proton NMR can’t pinpoint the position of the signals on the x-axis
2) proton NMR can’t give the relative intensity of signals
3) proton NMR can’t split signals
Works Cited
All information and related pictures taken from lecture slides provided in the chem 14C lecture
supplement CD by Dr. Steven Hardinger.