Analysing Structure of Organic Compounds Using Carbon-13 NMR Spectroscopy

This is part of the HSC Chemistry course under the topic Analysis of Organic Substances. 

HSC Chemistry Syllabus

Investigate the processes used to analyse the structure of simple organic compounds addressed in the course, including but not limited to:

• proton and carbon-13 NMR

• mass spectrometry

• infrared spectroscopy (ACSCH130)

How to Use Carbon-13 NMR to Analyse Structure of Organic Compounds.

This video explains how carbon-13 NMR spectroscopy can be used to analyse organic molecules. It explores what information a carbon-13 NMR spectrum provides about the structure of organic molecules.

What is Carbon-13 NMR?

• Carbon-13 NMR is an nmr method which analyses the carbon-13 nuclei instead of the far more abundant carbon-12 nuclei because the latter does not possess a net magnetic spin and thus, cannot be analysed using NMR.

• Carbon-13 nuclei in different chemical environments require different radiofrequency EMR to transition to a higher energy state. This frequency is compared to the frequency absorbed by a reference molecule (e.g. TMS), and used to calculate the chemical shift according to the formula:

 $$\delta = \frac{V_{sample} - V_{ref}}{V_{ref}}$$

 

• The number of carbon-13 nuclei in different chemical environments are represented by the total number of signals in the NMR spectrum.

 

Reference chemical – tetramethysilane (TMS) is added to a test sample Four carbon atoms, but only one chemical environment → one signal C–Si bonds are stable and chemically unreactive →  no unwanted reaction in a sample mixture C–Si chemical environment produces a low signal → easy to distinguish from almost all organic molecules.

 

	C1 and C2 are bound to different atoms and thus, are in different chemical environments. C1 has a greater chemical shift due to its direct attachment to an electronegative oxygen atom (C–O).
	C1 and C2 are bound to different atoms and thus, are in different chemical environments. C1 has a very high chemical shift value because it is greatly de-shielded by the adjacent oxygen atom.
	Four carbon nuclei in distinct chemical environments. C2 has the highest chemical shift due to it being a C=O ester carbon (160-185 ppm) C3 has the second highest chemical shift due to it being a C–O ester carbon (50-90 ppm)
	More than one carbon atom in a molecule can share the same chemical environment. The first and last carbon atom in pentane share a chemical environment as they are bonded to the same atoms and have the same relative position in the molecule (terminal carbon). The second and fourth carbon atom share a chemical environment as they are bonded to the same atoms and have the same relative positing in the molecule (2nd from terminal carbon).
	In contrast to pentane, the isomer methyl butane shows four signals in its carbon-13 NMR spectrum as there are four carbon chemical environments. The first and the methyl carbon atom share a chemical environment as they are bonded to the same atoms and have the same relative position in the molecule (terminal carbon). 13C NMR is useful in distinguishing isomers like pentane and methylbutane.