Section 9.5.3. Content and arrangement of the table

For an element pair X—Y, the primary ordering is alphabetic by element symbols according to the rows of Fig. 9.5.3.1(a); i.e. X changes slowest, Y fastest. The complete sequence runs from As—As to Te—Te with bonds involving carbon in the natural position: As—CC—CC—Te. Within a given X—Y pair, a secondary ordering is based on the coordination numbers (j) of X and Y, and on the nature of the bond between them. The bond definition is of the form X(j)—Y(j), with j decreasing fastest for Y, slowest for X, and with all single bonds preceding any multiple bonds. For carbon, the formal hybridization state replaces (but is equivalent to) the coordination number and it is for this element that the ordering rules are most clearly required. The ordering of the most populous C—C, C—N, C—O sections is illustrated in Fig. 9.5.3.1(b). The 13 possible C—C combinations follow the sequence Csp³—Csp³, Csp³—Csp², Csp³—C_ar, Csp³—Csp¹, Csp²—Csp², Csp²—C_ar, Csp²—Csp¹, C_ar—C_ar, C_ar—Csp¹, Csp¹—Csp¹, Csp²=Csp², C_arC_ar, Csp¹≡Csp¹. The symbol C_ar represents aryl carbon in six-membered rings, which is treated separately from Csp² throughout the table. The symbol is used to indicate a delocalized double or aromatic bond according to context.

The chemical environment of each bond is normally defined by a linear formulation of the substructure. The target bond is set in bold type, e.g. C_ar—C≡N (aryl cyanides); C—CH₂—O—C_ar (primary alkyl aryl ethers); (C—O)₂ —P(O)₂ (phosphate diesters). Occasionally, the chemical name of a functional group or ring system is used to define bond environment, e.g. in naphthalene, C2—C3; in imidazole, N1—C2. To avoid any possible ambiguity in these cases, we include numbered chemical diagrams in Fig. 9.5.3.2. A combination of chemical name and linear formulation is often employed to increase the precision of the definition, e.g. NH₂—C=O in acyclic amides; C=C—C(=O)—C=O in benzoquinone. Finally, for very simple ions, the accepted conventional representation is deemed to be sufficient, e.g. in , , etc.

Figure 9.5.3.2| top | pdf | Alphabetized index of ring systems referred to in the table; the numbering scheme used in assembling the bond-length data is given where necessary.

The chemical definition of substructure may be followed by brief qualifying information, concerning substitution, conformational restrictions, etc. For example: Csp³—Csp³: in cyclobutane (any substituent); X—C—F₃ (X = C, H, N, O); C_ar—NH—Csp³ (Nsp³: pyramidal). Where the generic symbol X is unqualified, it denotes any element type, including hydrogen. If the qualifying information is too extensive, then it will be given as a table footnote (see below).

The `Substructure' column is designed to convey as much unambiguous information as possible within a small space. For Csp³, we have employed the short forms C* and C^#. C* indicates Csp³ whose bonds, additional to those specified in the linear formulation, are to C or H atoms only. C*—OH would then represent the group of alcohols CH₃—OH, —C—CH₂—OH, —C₂—CH—OH and —C₃—C—OH. C* is frequently used to restrict the secondary environment of a given bond to avoid the perturbing influence of, e.g., electronegative substituents. The symbol C^# is merely a space-saving device to indicate any Csp³ atom and includes C* as a subset.

The `Note' column refers to the footnotes collected in Appendix 9.5.1. These record additional information as follows: (a) additional details concerning the chemical definition of substructures, e.g. the omission of three- and four-membered rings; (b) statements of geometrical constraints used in obtaining the cited average, e.g. definition of planarity or pyramidality at N, torsional constraints in conjugated systems; (c) any peculiarities of a particular bond-length distribution, e.g. sample dominated by C* = methyl; (d) references to previously published surveys of crystallographic results relevant to the substructure in question. We do not claim that these references are in any way comprehensive and we would be grateful to authors for notification (to FHA) of any omissions. This will serve to improve the content of any future version of the table.