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.
Biochemistry 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. ==Cumomer==