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Chemical Shift

Shielding

If you were to take just a bare proton by itself (no electrons around it) and placed it in a magnetic field (Bo), the proton would experience or "feel" the entire field strength Bo.  However, most nuclei (H's and C's) are embedded in orbitals with electrons around them.  Nuclei with more electrons around them feel less of the field Bo.  A simplified picture that is useful for remembering this effect is "shielding".  The electron density behaves like a shield, shielding the nuclei from the magnetic field.  More electron density means more shielded and less electron density means deshielded.  While this is a nice analogy that works well, the "real" physics is a bit more advanced.

Luckily for organic chemists different types of protons have different electron density, and we can tell them apart when we take an HNMR.

For example in chloroethane, there are 2 nonequivalent types of protons, colored blue and red.

Which protons would you think to have greater electron density around them, red or blue?  If you recall, chlorine withdraws electron density by induction and induction depends upon the distance.  The red protons feel the effect of the withdrawing group since they are closer and therefore have lower electron density.  We would say the red protons are deshielded.  The blue protons are further away and therefore have more electron density around them and are more shielded.

Chemical Shift is a measure of shielding!

Shielding is expressed in terms of a quantity called chemical shift (δ) and has units of parts per million (ppm) of the field strength.  Chemical shift value are relative to an internal standard which is normally tetramethylsilane (TMS).  The protons on TMS are more shielded than most other protons encountered in organic chemistry.  Silicone is less electronegative than carbon, and also has electrons in diffuse d orbitals.  TMS is given a chemical shift of 0 ppm and all other protons are relative to this.  A useful mnemonic is TMS stands for "The Most Shielded".  The most shielded end of the spectra is on the right where TMS is at 0 ppm, while the deshielded end is to the left on an NMR spectra.

tetramethylsilane (TMS)
0 ppm

 

Proton chemical shifts of protons can vary from about -1 (highly shielded) to 13 (highly deshielded).

The chemical shifts for many types of protons have been tabulated.  There are few very common proton chemical shifts you should commit to memory and they are listed below.

The following more inclusive table or a the table from you textbook book should also prove useful.

 

Factors Affecting Chemical Shift

There are a number of factors which influence chemical shift.

  • Inductive effects from adjacent EWG
  • Magnetic anisotropy in structures with π systems
  • Hydrogen bonding

Inductive effects from adjacent EWG.

The chemical shift of a proton is influenced by the electronegativity of the atoms attached to the carbons it is attached to.  The protons of fluoromethane have a chemical shift of 4.3 ppm, due to the decreased electron density around its protons from the highly electronegative fluorine. 

This effect is additive...the more electronegative groups, the more deshielding and the increase in chemical shift.

A similar trend is observed going across the periodic table. 

This effect depends on the distance from the EWG.  For example, the protons adjacent to the carbonyl group are deshielded more so than the protons two carbons from it.

Magnetic Anisotropy

Electrons in molecules with π systems (e.g. aromatics, alkenes, alkynes and aldehydes) can circulate creating a ring current(red lines below) when place in the applied magnet field Bo.  The ring current then induces or creates an induced magnetic field (blue lines).  As a result, the protons around the outside of the ring are deshielded since the field lines oppose the Bo field in that region.  Thus aromatic protons have a chemical shift of about 6.5-8.5 ppm.  A similar effect explains the highly deshielded protons observed in aldehydes (9-10 ppm), carboxylic acid (-OH 10-12ppm), and alkenes (5-6.5 ppm)

 

Hydrogen Bonding
Hydrogen bonding causes the deshielding of protons. Since hydrogen bonding effects are concentration and temperature-dependent you can usually recognize which protons are involved in hydrogen bonding by changing concentration or temperature.  With the exception of carboxylic acid OH groups, protons attached to oxygen and nitrogen generally resonate between 0.5 - 5 ppm.