Isotopomers

Overview

Isotopomers (isotopic isomers) are isomers having the same number of each isotopic atom but differing in their positions. For example, CH₃CHDCH₃ and CH₃CH₂CH₂D are a pair of constitutional isotopomers. An isotopomer should not be confused with an isotopologue.

¹³C NMR

The peaks seen in a ¹³C NMR spectra are a result of different Isotopomers. For example consider ethanol (CH₃CH₂OH). Ethanol will have 2 simple isotopomers one in which the ¹³C atom is on the methyl group i.e. ¹³CH₃CH₂OH and one isotopomer in which the ¹³C atom is on the methylene i.e. CH₃¹³CH₂OH.

This gives rise to 2 peaks in the ¹³C NMR spectrum.

Note the ¹²C atoms do not show up in the ¹³C NMR.

Also given that ¹³C is only present in 1% abundance, there is a very small chance of also seeing the isotopologue in the ¹³C NMR, wherein both carbon atoms are ¹³C atoms (i.e. ¹³CH₃¹³CH₂OH). These atoms would couple, giving satellites off the main peaks in a normal ¹³C-NMR spectrum and forming the basis for INADEQUATE (¹³C–¹³C correlation) experiments.

Uses

Isotopmers have been used to determine reaction pathways e.g. A m-Benzyne to o-Benzyne Conversion Through a 1,2-Shift of a Phenyl Group. Blake, M. E.; Bartlett, K. L.; Jones, M. Jr. J. Am. Chem. Soc. 2003, 125, 6485

See also

• Mass (mass spectrometry)