• Basic principles of NMR
• Chemical shift scale
• Shielding in H-NMR
• Table of H-NMR chemical shifts
• H-NMR spectra I
• Coupling in H-NMR
• H-NMR spectra II
• Interpreting H-NMR
• Interpreting C-NMR
• C-NMR spectra

In NMR, EM radiation is used to "flip" the alignment of nuclear spins from the low energy spin aligned state to the higher energy spin opposed state. The energy required for this transition depends on the strength of the applied magnetic field (see below) but it is small and corresponds to the radio frequency range of the EM spectrum.

As this diagram shows, the energy required for the spin-flip depends on the magnetic field strength at the nucleus. With no applied field, there is no energy difference between the spin states, but as the field increases so does the separation of energies of the spin states and therefore so does the frequency required to cause the spin-flip, referred to as resonance.

The basic arrangement of an NMR spectrometer is shown to the left.  The sample is positioned in the magnetic field and excited via pulsations in the radio frequency input circuit. The realigned magnetic fields induce a radio signal in the output circuit which is used to generate the output signal.  Fourier analysis of the complex output produces the actual spectrum.The pulse is repeated as many times as necessary to allow the signals to be identified from the background noise.

• An NMR spectrum is a plot of the radio frequency applied against absorption.
• A signal in the spectrum is referred to as a resonance.
• The frequency of a signal is known as its chemical shift.

It is often convienient to describe the relative positions of the resonances in an NMR spectrum.  For example, a peak at a chemical shift, d, of 10 ppm is said to be downfield or deshielded with respect to a peak at 5 ppm, or if you prefer, the peak at 5 ppm is upfield or shielded  with respect to the peak at 10 ppm.
 

 Typically for a field strength of 4.7T the resonance frequency of a proton will occur around 200MHz and for a carbon, around 50.4MHz.  The reference compound is the same for both, tetramethysilane (Si(CH₃)₄).

What would be the chemical shift of a peak that occurs 655.2 Hz downfield of TMS on a spectrum recorded using a 90 MHz spectrometer ? 

At what frequency would the chemical shift of chloroform (CHCl₃, d=7.28 ppm) occur relative to TMS on a spectrum recorded on a 300 MHz spectrometer ? 

A 1 GHz (1000 MHz) NMR spectrometer is being developed, at what frequency and chemical shift would chloroform occur ? 

Shielding in H-NMR

The magnetic field experienced by a proton is influenced by various structural factors.
Since the magnetic field strength dictates the energy separation of the spin states and hence the radio frequency of the resonance, the structural factors mean that different types of proton will occur at different chemical shifts. This is what makes NMR so useful for structure determination, otherwise all protons would have the same chemical shift.

The various factors include:

• inductive effects by electronegative groups
• magnetic anisotropy
• hydrogen bonding

The electrons around the proton create a magnetic field that opposes the applied field. Since this reduces the field experienced at the nucleus, the electrons are said to shield the proton. It can be useful to think of this in terms of vectors....

Since the field experienced by the proton defines the energy difference between the two spin states, the frequency and hence the chemical shift, d /ppm, will change depending on the electron density around the proton. Electronegative groups attached to the C-H system decrease the electron density around the protons, and there is less shielding (i.e. deshielding) so the chemical shift increases. This is reflected by the plot shown in the graph to the left which is based on the data shown below.

Magnetic Anisotropy

The word "anisotropic" means "non-uniform".  Magnetic anisotropy means that there is a "non-uniform magnetic field". Electrons in p systems (e.g. aromatics, alkenes, alkynes, carbonyls etc.) interact with the applied field which induces a magnetic field that causes the anisotropy.  As a result, the nearby protons will experience 3 fields: the applied field, the shielding field of the valence electrons and the field due to the p system. Depending on the position of the proton in this third field, it can be either shielded (smaller d) or deshielded (larger d), which implies that the energy required for, and the frequency of the absorption will change.

Hydrogen Bonding

Protons that are involved in hydrogen bonding (this usually means -OH or -NH) are typically observed over a large range of chemical shift values.  The more hydrogen bonding there is, the more the proton is deshielded and the higher its chemical shift will be. However, since the amount of hydrogen bonding is susceptible to factors such as solvation, acidity, concentration and temperature, it can often be difficult to predict.

HINT : It is often a good idea to leave assigning  -OH or -NH resonances until other assignments have been made.

Experimentally, -OH and -NH protons can be identified by carrying out a simple D₂O (deuterium oxide, also known as heavy water) exchange experiment.

• Run the regular H-NMR experiment
• Add a few drops of D₂O
• Re-run H-NMR experiment
• Compare the two spectra a look for peaks that have "disappeared"

Consider the alcohol case for example:   R-OH  +  D₂O   <=>   R-OD   +   HOD
During the hydrogen bonding, the alcohol and heavy water can "exchange" -H and -D each other, so the alcohol becomes R-OD.
Although D is NMR active, it's signals are of different energy and are not seen in the H-NMR, hence the peak due to the -OH disappears.  (Note that the HOD will appear...)
 

Suggest a reason why the acidic protons in a carboxylic acid appear so far downfield (about 12 ppm) ?  

H-NMR Chemical shifts

• The chemical shift is the position on the d scale (in ppm) where the peak occurs.
• Typical d /ppm values for protons in different chemical environments are shown in the figure below.
• There are two major factors that influence chemical shifts

(a) deshielding due to reduced electron density (due electronegative atoms) and
(b) anisotropy (due to magnetic fields generated by p bonds).

Note that the figure shows the typical chemical shifts for protons being influenced by a single group.
In cases where a proton is influenced by more than one group, the effects are essentially cumulative.

H-NMR Spectra I

The time has arrived to look at a few H-NMR spectra.....

Coupling in H-NMR

So far the H-NMR spectra that we have looked at have all had different types of protons that are seen as singlets in the spectra.  This is not the normal case.... spectra usually have peaks that appear as groups of peaks due to coupling with neighboring protons, for example, see the spectra of 1,1-dichloroethane shown below.

Before we look at the coupling, lets review the assignment of the peaks first:

• d = 5.9 ppm, integration = 1H deshielded : agrees with the -CHCl₂ unit
• d = 2.1 ppm, integration = 3H, agrees with -CH₃ unit.

 Coupling arises because the magnetic field of vicinal (adjacent) protons influences the field that the proton experiences.

To understand the implications of this we should first consider the effect the -CH group has on the adjacent -CH3.
	The methine -CH can adopt two alignments with respect to the applied field. As a result, the signal for the adjacent methyl -CH3 is split in two lines, of equal intensity, a doublet.
Now consider the effect of the -CH3 group has on the adjacent -CH .
	The methyl -CH3 protons give rise to 8 possible combinations with respect to the applied field. However, some combinations are equivalent and there are four magnetically different effects. As a result, the signal for the adjacent  methine -CH is split into four lines, of intensity ratio 1:3:3:1, a quartet.

• The proximity of  "n" equivalent H on neighbouring carbon atoms, causes the signals to be split into "n+1" lines.
• This is known as the multiplicity or splitting or coupling pattern of each signal.
• Equivalent protons (or those with the same chemical shift) do not couple to each other.
• If the neighbours are not all equivalent, more complex patterns arise.
• To a first approximation, protons on adjacent sp³ C tend to behave as if they are equivalent.

• d = 5.9 ppm, quartet, integration = 1H, deshielded

grees with the -CHCl₂ unit next to a -CH₃ unit (n = 3, so n + 1 = 4 lines)
• d = 2.1 ppm, doublet, integration = 3H

grees with -CH₃ unit, next to a -CH- (n = 1, so n + 1 = 2 lines)

The coupling constant, J (usually in frequency units, Hz) is a measure of the interaction between a pair of protons. In a vicinal system of the general type, Ha-C-C-Hbthen the coupling of Ha with Hb, Jab, MUST BE EQUAL to the coupling of Hb with Ha, Jba, therefore Jab = Jba.  The implications are that the spacing between the lines in the coupling patterns are the same as can be seen in the coupling patterns from the H-NMR spectra of 1,1-dichloroethane (see left).

Pascal's Triangle
 

The relative intensitites of the lines in a coupling pattern is given by a binomial expansion or more conviently by Pascal's triangle. To derive Pascal's triangle, start at the apex, and generate each lower row by creating each number by adding the two numbers above and to either side in the row above together.  The first six rows are shown to the right. So for H-NMR a proton with zero neighbours, n = 0,  appears as a single line, a proton with one neighbors, n =1 as two lines of equal intensity, a proton with two neighbours, n = 2, as three lines of intensities 1 : 2 : 1,   etc.

What would the multiplicity and the relative intensitites be for the secondary H in propane ? 

Summary

• Individual resonances are split due to coupling with "n" adjacent protons
• Number of lines in coupling pattern, L = n + 1

In reality, coupling patterns are often more complex than the simple n+1 rule since the neighbouring protons are often not equivlalent to each other (i.e. there are different types of neighbours) and therefore couple differently.  In these cases, the "n+1" rule has to be refined so that each type of neighbour causes n+1 lines.
For example for a proton with two types of neighbor, number of lines, L = (n₁ + 1)(n₂ + 1).
However, in many cases the lines overlap with each other and the result is further distortion from the "ideal" pattern.
Coupling patterns involving aromatic or alkene protons are often complex.

H-NMR Spectra II

Now for a few more spectra, this time where there are coupling patterns.....

Interpretting ¹H-NMR Spectra

Let's summarize what can be obtained from a ¹H NMR spectrum:
 

How many types of H ?	Indicated by how many groups of signals there are in the spectra
What types of H ?	Indicated by the chemical shift of each group
How many H of each type are there?	Indicated by the integration (relative area) of the signal for each group.
What is the connectivity ?	Look at the coupling patterns. This tells you what is next to each group

Chemical shift

• The chemical shift is the position on the d scale (in ppm) where the peak occurs.
• Typical d /ppm values for protons in different chemical environments are shown in the figure below.
• There are two major factors that influence chemical shifts (a) deshielding due to reduced electron density (due electronegative atoms) and (b) anisotropy (due to magnetic fields generated by p bonds).

Note that the figure shows the typical chemical shifts for protons being influenced by a single group. In cases where a proton is influenced by more than one group, the effects are essentially cumulative.

Integration

• The area of a peak is proportional to the number of H's that the peak represents
• The integral measures the area of the peak
• The integral gives the relative ratio of the number of H for each peak

• The proximity of other "n" H atoms on neighboring carbon atoms, causes the signals to be split into "n+1" lines.
• This is also known as the multiplicity or splitting of each signal.

An example of an H NMR is shown below.

Based on the outline given above the four sets of information we get are:

5 basic types of H present in the ratio of 5 : 2 : 2 : 2 : 3.
These are seen as a 5H "singlet" (ArH), two 2H triplets, a 2H quartet and a 3H triplet. Each triplet tells us that there are 2H in the adjacent position, and a quartet tells us that there are 3H adjacent.
(Think of it as the lines you see, L = n + 1, where n = number of equivalent adjacent H)
This tells us we that the peaks at 4.4 and 2.8 ppm must be connected as a CH₂CH₂ unit.
The peaks at 2.1 and 0.9 ppm as a CH₂CH₃ unit. Using the chemical shift charts, the H can be assigned to the peaks as below:

C-NMR Spectroscopy

It is useful to compare and contrast H-NMR and C-NMR as there are certain differences and similarities:

• ¹³C has only about 1.1% natural abundance (of carbon atoms)
• ¹²C does not exhibit NMR behavior
• ¹³C nucleus is about 400 times less sensitive than H nucleus to the NMR phenomena
• Due to low abundance, we do not usually see ¹³C-¹³C coupling
• Chemical shift range is normally 0 to 220 ppm
• Chemical shifts measured with respect to tetramethylsilane, (CH₃)₄Si (i.e. TMS)
• Similar factors affect the chemical shifts in ¹³C as seen for H NMR
• Long relaxation times (excited state to ground state) mean no integrations
• "Normal" ¹³C spectra are "broadband, proton decoupled" so the peaks show as single lines
• Number of peaks indicates the number of types of C

Here is the simple correlation table of ¹³C chemical shifts:

Interpretting C-NMR Spectra

The following information is to be gained from a typical  broadband decoupled ¹³C NMR spectrum:
 

How many types of C ?	Indicated by how many signals there are in the spectra
What types of C ?	Indicated by the chemical shift of each signal

Here are some examples of ¹³C-NMR spectra.