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 ● ● ● 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.