Here is the basic principle of NMR spectroscopy:

1. we put the sample in a magnetic field
2. the nuclei distribute in two different energy levels – some oriented with the applied field (lower in energy – ɑ Spin state) and some are opposed to the magnetic field (higher energy state – β Spin state)
3. electromagnetic radiation matching this energy difference (radio frequency) is applied
4. each proton absorbs energy exactly matching the gap (resonance) between the ɑ and β states causing a spin-flip
5. protons relax back each releasing energy which is converted to the δ (ppm) value on the spectrum

 

 

What is crucial here is that the energy gap between the ɑ and β states is slightly different for each type of proton depending on their environment (neighboring atoms).

Without this energy difference, NMR would have been useless for structure determination because all the protons would resonate at the same frequency thus giving one signal.

So, let’s see how the proton environment affects the energy it absorbs or,

 

Why protons have different chemical shifts, why they give a signal at different ppm.

The Origin of Chemical Shift

Below are the main regions in the ¹H NMR spectrum that you need to know:

 

 

The energy axis is called a δ (delta) axis and the units are given in part per million (ppm). Most often the signal area for organic compounds ranges from 0-12 ppm.

The ppm value of a given proton depends on the energy difference of its ɑ and β states which also depends on how much energy it received from the magnetic field.

Now, the strength of the magnetic field that each proton “feels” and therefore jumps to a certain energy level in the β state depends on how exposed it is to the field. And the thing that protects the protons from being exposed to the field 100% is the electron cloud around them:

 

 

The higher the electron density, the less the given nucleus “feels” the magnetic field and, logically, the lower the electron density the more it is exposed to the magnetic field and more energy it absorbs.

 Now, what changes the electron density around a nucleus is the atom it is connected to. A neighboring electronegative atom pulls the electron density thus exposing the nucleus to a stronger magnetic field.

You can think about the analogy of being in the sun or a light; any object in the way of the light lowers its intensity, and depending on the positioning of an object, we experience the energy from the sun to a different extent.

Now, for the protons, these objects (the shield) are the electrons. The effect of electron-withdrawing groups on the chemical shift can be represented in the image below:

 

 

The higher the electron density, the better the shield and thus the more protected the nucleus. Now, the electron density depends on the neighboring groups: atoms, or groups containing electron-withdrawing groups lower density around adjacent nuclei and they experience a stronger magnetic field.

We will go over the absorption region for the protons in all the common functional groups but let’s address some important information and terminology before that.

 

Upfield and Downfield

The terms upfield and downfield refer to the low and high energy of the signals respectively. Yes, it sounds confusing since you’d expect the downfield to indicate a lower energy region and upfield as higher energy.

The wording has a historical origin. The first-generation NMR spectrometers used what is called “continuous wave field sweep” in which the frequency of rf radiation was kept constant and the magnetic field strength was slowly increased to detect which field strengths produce a signal. The more shielded protons required a stronger magnetic field to resonate and therefore the high energy was on the right side thus making in upfield and the terms upfield and downfield indicated high energy and low energy.

However, modern NMR instruments are designed to work by pulsed Fourier-transform NMR (FT-NMR) where the magnetic field is held constant and a short pulse covering the entire range of relevant radio frequencies is irradiated. So, the protons that are shielded appear to be at lower energy level and, therefore resonate at a lower frequency, and the ones deshielded experience a stronger magnetic field and resonate at higher energy which is opposite to what happened in older instruments. This doesn’t mean that the results were different, no they produce identical data, but the wording is mixed.

 

This was a historic throwback which you may not need to know, but you do want to remember that:

 

Downfield means higher energy – left side of the spectrum (higher ppm)

Upfield means lower energy – right side of the spectrum (lower ppm)

 

Chemical Shift Values

As already mentioned, the resonance frequency giving the signal in NMR, and indicating the types of protons, is shown on the x axis by δ (delta). The 0 ppm is a reference point where the protons of tetramethylsilane, (CH3)₄Si, also called TMS give signal.

There are a few reasons why TMS is used as a reference. First, it is a rear example of a carbon connected with a less electronegative element, silicon, which makes it shielded and therefore appear at lower ppm where other protons do not give signal and the peak at 0 ppm can be ignored when analyzing an NMR spectrum.

Second, it has a low boiling point which makes purifying the sample easier if needed.

The signals of all the other protons are reported in terms of how far (in Hertz) they are shifted from TMS signal and the chemical shift value (δ) is measured by the ratio of this shift in Hertz and the operational frequency of the spectrometer in MHz:

 

 

The operational frequency of most NMR instruments is in the MHz region and this is why the units are given in parts per million (ppm).

Now, you may ask, what is the purpose of dividing the shift by the frequency of instrument? Why not just report the shift in Hz?

 

Magnetic Strength and PPM Value

Couple of important reasons: The energy difference of the ɑ and β states for different protons is very little energy wise (for exampl, 0 – 1200 Hz). Again, this depends on the instrument, but it is still a low energy compared to the super powerful magnets that are used.  Nevertheless covering 0-1200 on a spectrum axis is not convenient when analyzing an NMR especially if the difference of two peaks is only a few Hz. However, this is rather a decorative obstacle.

The key reason here is to have a formula which allows a field-independent measure of the signals, meaning regardless what NMR is used, the chemical shift should always be the same for the given proton.

For example, ethanol, under certain conditions, gives two signals at 1.25 and 3.72 ppm with modern powerful NMR instrument and it did the same when the first instruments came about in mid last century. Having consistent results helps in many ways since research itself opens an infinite possibility of getting errors and surprises.

So, when switching to a more powerful NMR, let’s say from 300 MHz to 900 MHz, the shift of the signal from TMS changes but because we divide it by the operational frequency of the instrument, the ppm values stays the same:

 

 

Now, it’s a natural question to ask why would you use a more powerful NMR then if the results are the same? Especially when they are so expensive not only to purchase but also to maintain.

Well, first, the results are not the same, they are a lot better. We may not always need this improvement, but it is there; we don’t need a powerful binocular to watch a football game.

The only thing that stays identical is the ppm value for the given signal.

So, how are the results different?

The difference of a 300 MHz and 900 MHz NMR is the relationship of the ppm to Hz. For 300 MHz instrument, 1 ppm is equal to 300 Hz and for a 900 MHz instrument, it corresponds to 900 Hz.

Now, if two protons give almost identical signals with the 300 MHz instrument, let’s say the difference is only 15 Hz, it is very difficult to tell the signals apart since they are overlapping:

 

 

On the other hand, if we switch to the 900 MHz instrument, the difference of resonance frequencies of these protons triples as the magnetic field is tripled. So now, the difference of the signals is 45 Hz which is large enough to get them separated and yet keep the ppm values all identical:

 

 

In the next post, we will talk more specifically about the ppm values of the most common functional group.

 

Check Also

• NMR spectroscopy – An Easy Introduction
• NMR Chemical Shift
• NMR Chemical Shift Range and Value Table
• NMR Number of Signals and Equivalent Protons
• Homotopic Enantiotopic Diastereotopic and Heterotopic
• Homotopic Enantiotopic Diastereotopic Practice Problems
• Integration in NMR Spectroscopy
• Splitting and Multiplicity (N+1 rule) in NMR Spectroscopy
• NMR Signal Splitting N+1 Rule Multiplicity Practice Problems
• ¹³C NMR NMR
• DEPT NMR: Signals and Problem Solving
• NMR Spectroscopy-Carbon-Dept-IR Practice Problems