One starts with the IUPAC "preferred name" (sometimes referred to as the "trivial name") used for certain of the simple PAHs, e.g., anthracene, phenanthrene, pyrene, and fluoranthene. More complicated structures are then built up by the addition of, for example, benzo, dibenzo, or naphtho groups to the skeleton of the simple PAH.
The sequence of steps to follow when drawing a structure of a fused PAH ring system is the following:
f. Define the molecular structure for which a preferred name exists in the IUPAC list (1979).
2. Place the maximum number of rings in a horizontal row.
3. Place the greatest number of remaining rings above and to the right of a vertical axis drawn through the center of the molecule (i.e., in the upper right-hand quadrant). Structure I shown earlier for benzo[a]pyrene illustrates these points.
4. After properly orientating it, number the carbon atoms in the molecule in a clockwise direction starting with the carbon atom not common to another ring (i.e., not involved with fusion of two rings) that is in the most counter-clockwise position of the uppermost ring that is farthest to the right (see structures for PAHs I-V and Table 10.1). Carbon atoms common to two or more rings are not numbered.
5. Letter the faces of the ring in alphabetical order.
6. Starting with "a," letter the faces of the rings in alphabetical order beginning with that side between carbon atoms 1 and 2. Continue clockwise around the molecule, noting, however, that ring faces common to two rings are not lettered.
See fluoranthene, V, and benzo[&]fluoranthene, XXVI.
Since addition of another ring is defined by the bond(s) involved in the fusion, in this case the k face of fluoranthene, the name becomes benzo[&]fluo-ranthene.
The numbering in the final molecule can be different from the one used in the original compound, since the addition of more rings may change its orientation in the frame of reference. For example, consider anthracene (VIII), 9,10-dimethylanthracene (IX), and 7,12-dimethylbenz[a]anthracene (X):
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