Useful Concepts

Nuclear Magnetic Resonance (NMR) Spectroscopy

NMR occurs due to the absorbance of radio frequency radiation to cause the "flipping" of nuclear spins from low to high energy spin states.
While not all nuclei are NMR active (e.g. ¹²C and ¹⁶O are inactive), the most important nuclei for organic chemists are ¹H and ¹³C (both with nuclear spin = 1/2).
¹H (or proton) is the most common, and the one we will spend most time talking about.

H NMR:

There are 4 levels of information to be gained from proton NMR related to different aspects of the spectrum:

1. How many types of H ? ..........Look at how many basic groups of signals there are.
2. How many H of each type ? ....Look at the integration (relative area) of each group.
3. What is each type ? .................Look at the chemical shiftof each group and relate to tables of typical values.
4. What is the connectivity ? ........Look at the coupling patterns. This tells you what is next to each group

Integration = Measure of the area of the peaks which is proportional to the number of H that the peak represents.

Chemical shift = Position on the scale (in ppm) where the peak occurs. For ranges of typical values, see the figure below. There are two major factors that cause different chemical shifts (a) deshielding due to reduced electron density (due electronegative atoms) and (b) anisotropy (due to π bonds).

Coupling = Due to the proximity of "n" other equivalent H atoms, causes the signals to be split into (n+1) lines.
 

Deshielding: The electrons around the proton create a magnetic field that opposes the applied field. This reduces the field experienced at the nucleus and therefore decreases the freqency required for the absorption. Therefore the chemical shift (delta /ppm) will change depending on the electron density around the proton. Since electronegative groups decrease the electron density, there will be less shielding (ie. deshielding) and the chemical shift will increase.
 

Anisotropy: This just means "a non-uniform" magnetic field. Electrons in pi systems (e.g. benzene, alkenes, alkynes, carbonyls etc.) interact with the applied field and are caused to "circulate" within the pi system. This motion 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 pi system. Depending on the position of the proton in this third field, it can be either shielded or deshielded, which implies that the energy required for, and the frequency of the absorption will change.

Be aware that the exact substitution pattern around a particular H causes changes in the chemical shift and therefore ranges of values are given in the tables and the above figure. You will normally be given copies of the tables in exams, but you will save yourself lots of time, and avoid confusing yourself, if you know approximately where the various types of H occur.

¹³C NMR

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 behaviour
• 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 (note that X is any electronegative atom, for example O, N or Cl)

© Dr. Ian Hunt, Department of Chemistry