  In this work we report on high-resolution IR absorption studies that provide
a detailed view on how the peripheral structure of irregular polycyclic
aromatic hydrocarbons (PAHs) affects the shape and position of their 3 {\mu}m
absorption band. To this purpose we present mass-selected, high-resolution
absorption spectra of cold and isolated phenanthrene, pyrene, benz[a]antracene,
chrysene, triphenylene, and perylene molecules in the 2950-3150 cm-1 range. The
experimental spectra are compared with standard harmonic calculations, and
anharmonic calculations using a modified version of the SPECTRO program that
incorporates a Fermi resonance treatment utilizing intensity redistribution. We
show that the 3 {\mu}m region is dominated by the effects of anharmonicity,
resulting in many more bands than would have been expected in a purely harmonic
approximation. Importantly, we find that anharmonic spectra as calculated by
SPECTRO are in good agreement with the experimental spectra. Together with
previously reported high-resolution spectra of linear acenes, the present
spectra provide us with an extensive dataset of spectra of PAHs with a varying
number of aromatic rings, with geometries that range from open to
highly-condensed structures, and featuring CH groups in all possible edge
configurations. We discuss the astrophysical implications of the comparison of
these spectra on the interpretation of the appearance of the aromatic infrared
3 {\mu}m band, and on features such as the two-component emission character of
this band and the 3 {\mu}m emission plateau.
