Physics:Isotopomers

Short description: Molecules with same isotopes in different positions

Isotopomers or isotopic isomers are isomers with isotopic atoms, having the same number of each isotope of each element but differing in their positions. The result is that the molecules are either constitutional isomers or stereoisomers solely based on isotopic location. For example:

CH₃CHDCH₃ and CH₃CH₂CH₂D are a pair of constitutional isotopomers of propane.

(R)- and (S)-CH₃CHDOH are isotopic stereoisomers of ethanol.

(Z)- and (E)-CH₃CH=CHD are examples of isotopic stereoisomers of propene.

The term isotopomer was first proposed by Seeman and Paine in 1992 to distinguish isotopic isomers from isotopologues (isotopic homologues).

¹³C-NMR

In nuclear magnetic resonance spectroscopy, the highly abundant ¹²C isotope does not produce any signal whereas the comparably rare ¹³C isotope is easily detected. As a result, carbon isotopomers of a compound can be studied by carbon-13 NMR to learn about the different carbon atoms in the structure. Each individual structure that contains a single ¹³C isotope provides data about the structure in its immediate vicinity. A large sample of a chemical contains a mixture of all such isotopomers, so a single spectrum of the sample contains data about all carbons in it. Nearly all of the carbon in normal samples of carbon-based chemicals is ¹²C, with only about 1% abundance of ¹³C, so there is only about a 1% abundance of the total of the singly-substituted isotopologues, and exponentially smaller amounts of structures having two or more ¹³C in them. The rare case where two adjacent carbon atoms in a single structure are both ¹³C causes a detectable coupling effect between them as well as signals for each one itself. The INADEQUATE correlation experiment uses this effect to provide evidence for which carbon atoms in a structure are attached to each other, which can be useful for determining the actual structure of an unknown chemical.

Reaction kinetics

In reaction kinetics, a rate effect is sometimes observed between different isotopomers of the same chemical. This kinetic isotope effect can be used to study reaction mechanisms by analyzing how the differently massed atom is involved in the process.

In biochemistry differences between the isotopomers of biochemicals such as starches is of practical importance in archaeology. They offer clues to the diet of prehistoric humans that lived as long ago as paleolithic times. This is because naturally occurring carbon dioxide contains both ¹²C and ¹³C. Monocots, such as rice and oats, differ from dicots, such as potatoes and tree fruits, in the relative amounts of ¹²CO₂ and ¹³CO₂ that they incorporate into their tissues as products of photosynthesis. When tissues of such subjects are recovered, usually tooth or bone, the relative isotopic content can give useful indications of the main source of the staple foods of the subjects of the investigations.

See also

• Mass (mass spectrometry)