[IUCr Home Page] [CIF Home Page]

Core CIF Dictionary version 2.3




Category: category_overview

Data type: null


    _chemical_formula_moiety            'C18 H25 N O3'
    _chemical_formula_sum               'C18 H25 N O3'
    _chemical_formula_weight            303.40

Example 1 - based on data set TOZ of Willis, Beckwith & Tozer [(1991). Acta Cryst. C47, 2276-2277].

    _chemical_formula_iupac      '[Mo (C O)4 (C18 H33 P)2]'
    _chemical_formula_moiety     'C40 H66 Mo O4 P2'
    _chemical_formula_structural '((C O)4 (P (C6 H11)3)2)Mo'
    _chemical_formula_sum         'C40 H66 Mo O4 P2'
    _chemical_formula_weight      768.81

Example 2 - based on data set 9597gaus of Alyea, Ferguson & Kannan [(1996). Acta Cryst. C52, 765-767].


   _chemical_formula_ items specify the composition and chemical
  properties of the compound. The formula data items must agree
  with those that specify the density, unit-cell and Z values.

  The following rules apply to the construction of the data items
  _chemical_formula_analytical, *_structural and *_sum. For the
  data item *_moiety the formula construction is broken up into
  residues or moieties, i.e. groups of atoms that form a molecular
  unit or molecular ion. The  rules given below apply within each
  moiety but different requirements apply to the way that moieties
  are connected (see _chemical_formula_moiety).

  1. Only recognized element symbols may be used.

  2. Each element symbol is followed by a 'count' number. A count of
     '1' may be omitted.

  3. A space or parenthesis must separate each cluster of (element
     symbol + count).

  4. Where a group of elements is enclosed in parentheses, the
     multiplier for the group must follow the closing parentheses.
     That is, all element and group multipliers are assumed to be
     printed as subscripted numbers. [An exception to this rule
     exists for *_moiety formulae where pre- and post-multipliers
     are permitted for molecular units].

  5. Unless the elements are ordered in a manner that corresponds to
     their chemical structure, as in _chemical_formula_structural,
     the order of the elements within any group or moiety
     depends on whether or not carbon is present. If carbon is
     present, the order should be: C, then H, then the other
     elements in alphabetical order of their symbol. If carbon is
     not present, the elements are listed purely in alphabetic order
     of their symbol. This is the 'Hill' system used by Chemical
     Abstracts. This ordering is used in _chemical_formula_moiety
     and _chemical_formula_sum.

Copyright © 2003 International Union of Crystallography

IUCr Webmaster