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Isotopically Modified Compounds

R-8.3.2 Selectively labelled compounds

An isotopically labelled compound is designated as selectively labelled when a mixture of isotopically substituted compounds is formally added to the analogous isotopically unmodified compound in such a way that the position(s) but not necessarily the number of each labelling nuclide is defined. A selectively labelled compound may be considered as a mixture of specifically labelled compounds.

A selectively labelled compound may be (a) multiply labelled when in the unmodified compound there is more than one atom of the same element at the position where the isotopic modification occurs, for example, H in CH4; or there are several atoms of the same element at different positions where the isotopic modifications occurs, for example, C in C4H8O; or (b) mixed labelled when there is more than one labelling nuclide in the compound, for example, C and O in .

Note: When there is only one atom of an element that can be modified in a compound, only specific labelling can result.

R-8.3.2.1 A selectively labelled compound cannot be described by a unique structural formula; therefore it is represented by inserting the nuclide symbols preceded by any necessary locant(s) (letters and/or numbers) but without multiplying subscripts, enclosed in square brackets directly before the usual formula or, if necessary, before parts of the formula that have an independent numbering. Identical locants are not repeated. When different nuclides are present, the nuclide symbols are written in alphabetical order according to their symbols, or when the atomic symbols are identical, in order of increasing mass number.

Examples to R-8.3.2.1

Mixture of isotopically
substituted compound		 when
added to		 Isotopically
unmodified
compound		 gives
rise to		 Selectively
labelled
compound


or any two or more of the above		 CH4
Note: The method of writing formulae as given by the above rule may also be of use if a compound is represented by its molecular formula rather than its structural formula, e.g., .

R-8.3.2.2 The name of a selectively labelled compound is formed in the same way as the name of a specifically labelled compound, except that the multiplying subscript following the atomic symbols are generally omitted. Identical locants corresponding to the same element are not repeated. The name of a selectively labelled compound differs from the name of the corresponding isotopically substituted compound in the use of square brackets surrounding the nuclide descriptor rather than parentheses and in the omission of repeated identical locants and multiplying subscripts.

Examples to R-8.3.2.2

Mixture of isotopically
substituted compounds		 when added to is named

CH4


In a selectively labelled compound formally arising from mixing several known isotopically substituted compounds with the analogous isotopically unmodified compound, the number or the possible number of labelling nuclide(s) for each position may be indicated by subscripts to the atomic symbol(s). Two or more subscripts referring to the same nuclide symbol are separated by a semicolon. For a multiply labelled or mixing labelled compound, the subscripts are written successively in the same order as the various isotopically substituted compounds are considered. The subscript zero is used to indicate that one of the isotopically substituted compounds is not modified at the indicated position.

Examples to R-8.3.2.2

Mixture of isotopically
substituted compounds		 when
added to		 Isotopically
unmodified
compound		 gives
rise to		 Selectively
labelled
compound


Next:
R-8.3.3 Nonselectively labelled compounds
R-8.3.4 Isotopically deficient compounds
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This HTML reproduction is as close as possible to the published version [see IUPAC, Commission on Nomenclature of Organic Chemistry. A Guide to IUPAC Nomenclature of Organic Compounds (Recommendations 1993), 1993, Blackwell Scientific publications, Copyright 1993 IUPAC]. If you need to cite these rules please quote this reference as their source.

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