Crystallographic Information Framework

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Core dictionary (coreCIF) version 2.4.5




   _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 parenthesis.
     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 carbon is present or
     not. 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 alphabetical 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.


Example 1 - based on data set TOZ of Willis, Beckwith & Tozer [Acta Cryst. (1991), C47, 2276-2277].
    _chemical_formula_moiety            'C18 H25 N O3'
    _chemical_formula_sum               'C18 H25 N O3'
    _chemical_formula_weight            303.40

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

Type: null

Category: category_overview