orthonym.rules.isotope_letters

Note

Internal API. Names and behaviour may change between releases.

When an isotopic descriptor may cite a bare element letter.

An italic letter locant (‘O’, ‘N’) names a heteroatom that integer locants do not number: the hydroxyl oxygen of a carboxylic acid, the nitrogen of an amine or an amide. The Blue Book prints it bare where the parent holds one such atom, ‘(N-2H1)acetamide (PIN)’ NAMES,, the Blue Book) and ‘(O-2H, 18O)acetic acid (PIN)’,:44294, “the ‘18O’ is an isotopic descriptor and the ‘O’ is a locant”,:44296); its comparative table (:44506) prints a bare letter beside numerals, ‘(O,2,2,2-2H4)ethan-1-ol’, for a parent with one oxygen.

LOCANTS, Omission of locants, (:44180): “In preferred IUPAC names, locants

are omitted if no locants are necessary in unmodified names. However, if isotopic modification requires a locant to specify its position, then all locants must be specified and none are omitted.”

(:44202): “Locants are not omitted when there is a possibility of isomers”. Letter and/or numeral locants (:44208): “When locants are needed for defining the structure of

the parent structure or of a unit of structure as defined by its appropriate enclosing marks, then all locants must be cited for the parent structure or that structural unit.”

So a bare letter is cited only where it leaves no possibility of isomers: every atom of that element the letter could denote is the same position, one symmetry class of the structure. The class is judged on the structure without the labels the letter itself denotes (the isotopes of those atoms and of their hydrogens) and with every other label kept: the two hydroxyl oxygens of butanedioic acid are one class (‘(O-2H)butanedioic acid’), and so are they when only they carry labels; a 13C in the chain tells them apart, and then the letter is cited with the number of the atom it is borne by, ‘(2-13C,O1-18O) pentanedioic acid’ (‘O5’ is the other isomer). OPSIN 2.9.0 reads a bare letter as the first such atom, so the read-back cannot decide this; the rule does. One predicate for the decorator (rules.isotopes) and the scope composer (rules.isotope_scopes).

orthonym.rules.isotope_letters.letter_ranks(original, element)

Symmetry class of every atom of original (index of original -> class) in the structure without hydrogen atoms, without the isotopes of the atoms element’s letter can denote and of their hydrogens, and with every other isotope kept: a labelled hydroxyl oxygen, whose hydrogen is an explicit atom of original, ranks as the unlabelled hydroxyl oxygen it is, and a 13C elsewhere still tells the two ends of a diacid apart.

orthonym.rules.isotope_letters.denotable_atoms(original, element)

The atoms of original an italic element letter can denote: for ‘O’ the hydroxyl oxygens (single-bonded, one carbon neighbour; the carbonyl oxygen is the other oxygen of the acid), for ‘N’ the nitrogens that are not ring members, not aromatic and bear a hydrogen (an amine or an amide).

orthonym.rules.isotope_letters.distinct_classes(original, element, atoms=None)

The number of symmetry classes among atoms (default: every atom element’s letter can denote; letter_ranks), the quantity: more than one means citing the letter alone leaves a possibility of isomers. atoms index original.

orthonym.rules.isotope_letters.bare_letter_allowed(original, element, atoms=None)

Whether element’s italic letter may be cited without its number: the atoms it could denote (the default, the whole structure; atoms, those of one unit) are one symmetry class.