Index

Macromolecular dictionary (mmCIF) version 2.0.09

Category CHEMICAL_FORMULA

Name:
'chemical_formula'

Description:

   Data items in the CHEMICAL_FORMULA category would not, in
   general, be used in a macromolecular CIF. See instead the
   ENTITY data items.

   Data items in the CHEMICAL_FORMULA category 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, _chemical_formula.structural and
   _chemical_formula.sum. For the data item
   _chemical_formula.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 _chemical_formula.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 should be:  C, then H, then the other
      elements 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:

Example 2 - based on data set TOZ of Willis, Beckwith & Tozer [(1991). Acta Cryst. C47, 2276-2277].
 
    _chemical_formula.entry_id          'TOZ'
    _chemical_formula.moiety            'C18 H25 N O3'
    _chemical_formula.sum               'C18 H25 N O3'
    _chemical_formula.weight            303.40



Category groups:
    inclusive_group
    chemical_group
Category key:
    _chemical_formula.entry_id

Mandatory category: no