Solo modes are associated with long straight molecular edges. Duos and trios, on the other hand, correspond to corners.
Quartets are due to pendant rings attached to the structure.
The numbers in the molecular structures indicate the number of adjacent CH groups per aromatic ring It is now possible to quantify the relative amounts of the various types of CH groups on the periphery of the interstellar PAHs which dominate the emission in this wavelength range and derive the types and sizes of interstellar PAH structures implied. This is achieved by analysing the peak positions and integrated band strengths listed in Table 2 with the laboratory data summarised in Table 3 . In the following we assume that the 11.2 m emission band arises from the the solo vibrations, the 12.0 m band from duo modes, the 12.7 m from trios and the 13.5 m from quartets. From the intensities of the individual features listed in Table 2 for NGC 7027 we see that the observed solo to duo, solo to trio and solo to quartet intensity ratios are 23.8, 4.0, 14.7 respectively. We conclude that in NGC 7027 the ratio of the number of solo CH bonds to CH bonds in duo, trio and quartet groups are 3.8, 1.5 and 7.1.