CHAPTER 15

Absorption of energy from different types of light on the electromagnetic spectrum
helps to determine the structure of organic molecule. Of the types of electromagnetic
radiation, nuclear magnetic spin transitions which occur at radio
frequencies is among the most sensitive and provides information about the way that
nuclei are bonded together. In order for a nuclei to be NMR active, it must have a odd atomic number or an odd mass number
Because of this, only certain nuclear spin states are allowed. Thus, not every nuclei is
NMR active and can be analyzed by this technique. In the presence of an applied magnetic field, the active nuclei will either align their
spins with or against the applied field. The interactions between nuclear spins and the
applied magnetic field are quantized which means that only certain orientations are
allowed. When a nucleus is aligned with the magnetic field it precesses at a characteristic
frequency (think like a spinning top!)

A H-1 NMR spectrum has chemical shift ranges from 0-12 ppm. The NMR active H
nuclei are affected by the electrons that surround them. These electrons create local
magnetic fields that can either align with or align against the nuclei from
the strength of the applied magnetic field. Deshielding: increases the nuclei’s exposure to an applied magnetic field. This
corresponds to higher chemical shift values
Shielding: decreases a nuclei’s exposure to the NMR magnet. Lower chemical shift values.


chemical shift is the numbered location in ppm of the type of H-1 nuclei. Chemical shift is independent of the strength of the NMR magnet and is defined by its
frequency shift from tetramethylsilane (TMS) a reference standard which has 0 ppm.

Equivalent Hydrogens:
In H-NMR, protons in the same sp3 chemical environment have the chemical
shift. Hydrogens (protons) are equivalent if:
They are bonded to the same hybridized carbon that had free bond rotation•
and are not next to a chiral center (more on that later). The lack of bond rotation in strained rings, alkenes, aromatic rings often causes•
protons attached to the same carbon to have different chemical environments

Integration Values and their Relationship to Equivalent Protons:
Integration values are exactly what you expect them to be from Calculus 2; the
calculated area under the curve of each peak in the NMR spectrum. But don’t worry,
we have computers that do this for us!!!
Can be represented as the s shaped integral curve (as seen below) where one can•
compare the height ratios between peaks (the mean way)
Can be represented as an integer above or below the peak

The integration value tells you the number of equivalent H’s making the peak

There are not many diagnostic regions for a variety of functional groups in H-1 NMR
since the chemical shift range is small.

Note: Chemical Shift values are approximations. The actual value for certain protons
can be a little more or less than the ranges given.

Chemical shift of protons and the amount the are shielded/deshielded depends on:
Electronegativity of nearby atoms- shift is downfield. This is a proximity effect. Protons next to an electronegative atom will become
deshielded and feel the strength of the applied magnetic field more leading to higher
chemical shift values. This is the opposite effect for protons attached to a carbon-carbon triple bond. These
protons require a lower frequency to make them resonate so they have smaller
chemical shift values and are found further upfield.

Magnetic induction with adjacent pi bonds in aromatic rings. When electrons in a conjugated pi system (like in aromatic rings) are
subjected to a magnetic field (like the NMR) they circulate and produce their
own magnetic field causing diamagnetic anisotropy
This creates different regular of different magnetic strengths and explains
why protons outside of the ring have higher chemical shifts. Simple Signal Splitting: the n+1 Rule and how it’s used to build structures:
“Neighboring” protons to a set of equivalent protons will split a peak into n+1 number
of lines where n is the number of neighboring protons.
In order for a set of protons to be considered “neighbors” they must be three bonds
away or less. Generally bond distances greater than four are not observed in standard H-1 NMR.

Common splitting patterns are: singlets(s), doublets(d), triplets(t), quartets(q)•
quintets, sextets, and septets.
The relative intensities or heights of each line is derived from Pascals Triangle.•
Each entry is the sum of the values before it•
Sometimes these don’t look “perfect” in a real•
spectrum but it will look close

Homotopic: these are protons that always have simple splitting patterns and follow the
n+1 rule. These protons also have identical chemical shift.
It is common to do a “deuterium replacement” test to see if protons are homotopic. If
switching one of the H’s to a D atom label, and no chiral center is produced, then the
protons are homotopic

Symmetry can also cause protons attached to different carbons to be equivalent.•
For example, two methyl groups can be equivalent.

Enantiotopic: these protons have simple splitting patterns that follow the n+1 rule
unless they are in a chiral environment. These types of protons do produce a chiral
center upon the deuterium replacement test. But, they have identical chemical shifts
and exhibit simple splitting unless they are in a chiral environment. Many -CH2 groups
are enantiotopic.

Diastereotopic: these protons do not exhibit simple splitting patterns and NEVER have
the same chemical shift. They are often found next to a chiral center
or attached to sp2 hybridized carbons such as alkene or benzene
Upon the deuterium replacement test, a chiral center is produced, and if it is already
next to a chiral center, then a diastereomer has been created thus those two protons
are in different chemical environments. Note: this test is only applicable to sp3

How to Solve an NMR Problem Given the Molecular Formula and Spectra:
Calculating the Index of Hydrogen Deficiency:1.
This calculation tells you how many double bonds/rings are present in a molecule•
and can help you narrow down potential functional groups before you look at the
NMR spectrum

fter calculating the IHD, look at the NMR spectrum and I make note of how many
peaks I have (non equivalent H’s) and where they are located. I usually start with the
one that is the furthest downfield because it is the most “interesting”
3. After identifying how many non equivalent H’s and if their chemical shifts indicate
functional groups present, next look at integration values and figure out if it’s a -CH,
-CH2, -CH3 etc.
4. Look at splitting patterns LAST. Since splitting is all about neighboring H’s, it makes
sense to examine at this last because you need to know what those H’s are neighbors
of. Use n-1 to determine the number of H neighbors.