  We propose to evaluate the contribution of polycyclic aromatic hydrocarbon
molecules that carry side groups to the mid-infrared emission spectra. Within
this framework, the IR absorption spectra of 2- and 9-vinylanthracene were
measured in Ar matrices at 12 K and in CsI and polyethylene pellets at room
temperature. The laboratory spectra were analyzed with the support of
simulations based on the density functional theory. For each PAH molecule,
eight IR spectra were computed by combining the B3LYP functional with as many
different basis sets, namely 4-31G, 4-31G(d), 6-31G, 6-311G, 6-31G(d),
6-31G(d,p), 6-31+G(d,p), and 6-31++G(d,p). The comparison of the theoretical
spectra with the laboratory data allowed us to determine the most suitable
combinations for modeling the IR spectra of neutral PAH molecules that carry a
vinyl side group. It was concluded from the examples of 2- and 9-VA that the
optimum basis set is 6-31G unless a steric interaction has to be taken into
account, in which case the optimum basis set is 6-31G(d). Thus, in the presence
of such an interaction, the use of d-type polarization functions is
recommended. We discuss the possibility for neutral vinyl-substituted PAHs to
contribute to the mid-infrared emission spectra and find that their specific
features do not match with the mid-infrared aromatic emission bands.
