  We investigate the presence of silicon atoms adsorbed on the surface of
interstellar polycyclic aromatic hydrocarbons (PAHs) to form SiPAH
pi-complexes. We use quantum chemistry calculations to obtain structural,
thermodynamic and mid-IR properties of neutral and cationic SiPAH complexes.
The binding energy was found to be at least 1.5 eV for [SiPAH]+ complexes
whereas it is roughly 0.5 eV for their neutral counterparts. From the spectral
analysis of the calculated IR spectra, we found that the coordination of
silicon to PAH+ does not strongly affect the intensities of the PAH+ spectra,
but systematically introduces blueshifts of the C-C in-plane and the C-H
out-of-plane bands. The thermodynamic data calculated for [SiPAH]+ complexes
show that these species are stable and can be easily formed by radiative
association of Si+ and PAH species that are known to be abundant in
photodissociation regions. Their mid-IR fingerprints show features induced by
the coordination of silicon that could account for (i) the blueshifted position
of the 6.2 micron AIB and (ii) the presence of satellite bands observed on the
blue side of the 6.2 and 11.2 micron AIBs. From such an assignment, we can
deduce that typically 1% of the cosmic silicon appears to be attached to PAHs.
