¹³C Nuclear Magnetic Resonance Spectroscopy Tutorial

Key Concepts

• Nuclear Magnetic Resonance Spectroscopy, or NMR Spectroscopy, can be used to identify any isotope, unless the isotope has both an even number of protons and an even number of neutrons.
• The nuclei of many elements, such as ¹³C, spin generating a magnetic field.
  When placed in a strong external field they align themselves either with or against the field.
  When the sample is irradiated with radio waves the energy absorbed corresponds to the difference between these two magnetic alignments.
• ¹³C NMR Spectroscopy (carbon nuclear magnetic resonance spectroscopy) is used to identify the structure of organic (carbon) compounds.

¹³C NMR spectra provide information about:

1. The Number of Signals: each chemically different carbon in a structure is also magnetically different.
   CH₃ groups are chemically different to CH₂ groups and to CH groups.
     eg, CH₃CH₂CH₂Cl contains 3 chemically different environments for the carbon atoms:
           CH₃
           CH₂ adjacent to CH₃
           CH₂ covalently bonded to Cl
2. The Position of the Signal with respect to an internal standard (chemical shift): tetramethylsilane, (CH₃)₄S or TMS, is often used as an internal standard since almost all ¹³C signals appear downfield from the TMS signal.
   On the δ scale, the TMS signal is 0 ppm
   Tables of chemical shifts are derived from measurements of a large number of samples and represent a "normal" range.
   ¹³C signals affected by highly electronegative elements are shifted further downfield.

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Example: Number of Signals

The number of signals in the ¹³C NMR spectrum tell us how many different chemically different groups (or environments) there are for the carbon atoms in the structure.

The number of signals does not tell us how many carbon atoms there are in total in the structure.

Compare the ¹³C NMR spectra of ethanol, CH₃CH₂OH, and ethoxyethane (diethyl ether), CH₃CH₂OCH₂CH₃, shown below:

ethanol CH3CH2OH	ethoxyethane (diethyl ether) CH3CH2OCH2CH3
2 signals = 2 different carbon atom environments	2 signals = 2 different carbon atom environments
C from CH3 gives one signal C from CH2 gives one signal	C from CH3 gives one signal (both CH3 groups chemically equivalent) C from CH2 gives one signal (both CH2 groups chemically equivalent)

Example: Chemical Shift

ethanol CH3CH2OH	ethoxyethane (diethyl ether) CH3CH2OCH2CH3
2 signals = 2 different carbon atom environments	2 signals = 2 different carbon atom environments
C from CH3 signal at 18 ppm C from CH2 signal 58 ppm	C from CH3 signal 14 ppm C from CH2 signal 66 ppm

Worked Example: ¹³C NMR Spectrum Example

What is the probable structure (structural formula) for this molecule?

1. Find the number of signals in the spectrum:
   3 signals means there are 3 different chemical environments for the carbon atoms in the structure.
   Since there are 4 carbon atoms in the molecular formula, 2 of the carbon atoms must be in the same chemical environment.
2. Since there are only 4 structural isomers for this alcohol, we can draw each structure and see whether it would produce 3 signals:

   1-butanol (butan-1-ol)	H | H | H | H | H- - C - C - C - C - OH | H | H | H | H	1 signal: terminal CH3 1 signal: CH2 inbetween CH3 and CH2 1 signal: CH2 inbetween CH2 and CH2OH 1 signal: CH2-OH 4 signals in total
   2-butanol (butan-2-ol)	H | H | H | H | H- - C - C - C - C - H | H | H | OH | H	1 signal: terminal CH3 attached to CH2 1 signal: CH2 inbetween CH3 and CHOH 1 signal: CHOH inbetween CH2 and CH3 1 signal: terminal CH3 attached to CHOH 4 signals in total
   2-methylpropan-2-ol (2-methyl-2-propanol)	CH3 | H3C - C - CH3 | OH	1 signal: 3 identical CH3 carbons 1 signal: C-OH bonded to the 3 identical CH3 groups 2 signals in total
   2-methylpropan-1-ol (2-methyl-1-propanol)	CH3 | H | H3C - C - C - OH | H | H	2 signals: CH3 bonded to CH 1 signal: CH bonded to 2 CH3 and CH2OH 1 signal: CH2-OH 3 signals in total

The only structural formula that fits the ¹³C NMR spectrum is that of 2-methylpropan-1-ol (or 2-methyl-1-propanol).