I'd like to use Hill notation (i.e. CH, then alphabetical), but I am not sure how to include isotopologues (i.e. carbon-13) in this notation.

For example, what would be the preferred form of ethylamine (with 13C)?

  1. C13CH7N
  2. CH713CN
  3. 12C13CH7N
  4. 12CH713CN
  5. Other

I'm leaning toward using #2 but was trying to find the std practice, but I didn't see anything about isotopologues. Thanks.


I can't point to a definitive authority, but in most of the papers I read, #5 other is used most often. Specifically for ethylamine:


I've always assumed the reason for this choice is to accentuate the presence of the isotopically unusual atoms. I suppose a good test for my theory would be formulas with multiple, isotopically distinct atoms of an element such as iron, for which the most abundant isotope is not the lightest.

Hopefully other answers will be along shortly to point to something more authoritative, such as a IUPAC report, guideline, or document.

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  • $\begingroup$ I've been using that formula notation, for the reason you say. To accentuate the presence of 13C. If nobody has a more authoritative source, I'll use that notation, calling it a modified Hill system if need be, with C, H, then alphabetical by element, with isotopologues listed in reverse by natural abundance (i.e. 14C13C12C). Thanks. $\endgroup$ – ngreen08 Sep 27 '15 at 13:43
  • $\begingroup$ Sorry, just, what does this formula exactly mean? I'm not really experienced in working with isotopologues. Especially, what does the superscript before an atomic symbol mean? $\endgroup$ – Eenoku Jun 30 '19 at 23:31

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