Structural formulae enclosing marks

IR-4.2.3 Structural formulae and the use of enclosing marks in formulae IR-4.2.4 Formulae of (formal) addition compounds IR-4.2.5 Solid state structural information IR-4.3 Indication of ionic charge IR-4.4 Sequence of citation of symbols in formulae IR-4.4.1 Introduction IR-4.4.2 Ordering principles IR-4.4.2.1 Electronegativity IR-4.4.2.2 Alphanumerical order IR-4.4.3 Formulae for specific classes of compounds IR-4.4.3.1 Binary species... 

As soon as the compound has even a slightly more complex structure, it becomes necessary to use enclosing marks in line formulae to separate subgroups of atoms. Different enclosing marks must be used for repeating units and sidechains in order to avoid ambiguity. 

The basic rules for applying enclosing marks in structural formulae are as follows ... 

The chemical structure of the CRU is enclosed in parentheses or brackets. While dashes representing chemical bonds may be omitted within the formula unless necessary for clarity, at the ends of the CRU, dashes must be shown. They are drawn across the enclosing marks. 

Blocks should be arranged in sequence as they are to appear in the text, with their location clearly indicated. Structures that are mentioned in the text should be numbered in sequence using arable numbers, marked for bold-face type, enclosed in parentheses, and centered beneath the formulae. In the text, structures should be referred to by these numbers. Formulae that are not mentioned in the text need not be numbered. 

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