Laboratory IR spectroscopy of protonated hexa-peri-hexabenzocoronene and dicoronylene

Abstract The mid-infrared emission spectra of a large variety of astronomical objects are dominated by the aromatic infrared bands (AIBs), which are now widely accepted to originate from gaseous polycyclic aromatic hydrocarbons (PAHs). It is believed that the astrophysically most relevant molecules are at least 40–50 carbon atoms in size. Still, the large majority of laboratory experiments have been performed on smaller PAHs, mainly for reasons of experimental limitations and availability. Here, we show that combination of atmospheric pressure chemical ionization (APCI) with a direct insertion probe (DIP) inlet gives efficient access to larger, ionic PAHs for action spectroscopy studies. We present the gaseous IR spectra of two astrophysically relevant large PAHs, hexa-peri-hexabenzocoronene ( C 42 H 19 + ) and dicoronylene ( C 48 H 21 + ) in their protonated form. Compared to their radical cation analogs, the protonated species have a lower dissociation threshold as they can expel a neutral hydrogen radical leaving behind the resonance-stabilized radical cation; provided that the mass spectrometer can resolve precursor and product ions at one amu difference, this generates good quality spectra under multiple-photon dissociation conditions. Quantum-chemical computations at the density functional level are used to support experiments. Despite the apparent similarity of different protonation isomers, their IR spectra are predicted to be remarkably distinct. This facilitates a straightforward identification of the isomers formed experimentally. For both species studied, protonation occurs on the peripheral CH moiety in the ’bay region’ of the molecules. We compare the spectra of the protonated species with those of their radical cation analogs reported previously and briefly discuss the astrophysical relevance.