CARBON-13 NMR SPECTROSCOPY• Analyse carbon-13 NMR spectra to make

predictions about the different types of carbon atoms present.

• Predict the chemical shifts of carbons in a given molecule.

DEFINITIONS

Chemical shift

Retention time

Rf value

Nucleon

Adsorption

Phase

CARBON-13 NMR SPECTROSCOPY

What is the relative atomic mass of carbon? 12.011

How much of naturally occurring carbon is carbon -13? 1.1%

Why might carbon-13 be used for NMR spectroscopy? It has an odd number of nucleons, resulting in a

residual magnetic spin

Carbon – 13 NMR spectroscopy allows the identification of carbon atoms in an organic molecule

TYPICAL CARBON – 13 SHIFTS

Chemical shifts indicate the chemical environment of the carbon atoms present

More electronegative atoms or groups cause a significant chemical shift

The range of chemical shifts values is between 0 – 230ppm so carbon atoms will typically have a separate signal (unless they are in an identical environment)

On your data sheet!

INTERPRETING CARBON – 13 NMR SPECTRA

From a carbon-13 spectrum, the most important information is: The number of different carbon environments

(from the number of peaks) The types of carbon environment (from the

chemical shift)

NB the size of the peak tells us nothing about the number of carbon atoms responsible!

WORKED EXAMPLE

Carbon-13 NMR spectra of propan-1-ol and propan-2-ol

1. How many carbon environments for each compound?

Propan-1-ol = 3 peaks: 3 C atoms in 3 environments

Propan-2-ol = 2 peaks: 3 C atoms in 2 environments

WORKED EXAMPLE

1. What are the environments of each carbon atom? (Use the chemical shift values)

Propan-1-ol 1 peak at δ = 64ppm for C-O 1 peak at δ = 15 ppm for C-C 1 peak at δ = 27 ppm for C-C Propan-2-ol 1 peak at δ = 64 ppm for C-O 1 peak at δ = 28 ppm for C-C

QUESTIONS

Complete questions 1 and 2 on page 87 Complete question 3 on page 105