Infrared Spectra of Protonated Pyrene and Its Neutral Counterpart in Solid <italic>para</italic>-Hydrogen

Protonated polycyclic aromatic hydrocarbons (HPAHs) have been reported to have infrared (IR) bands at wavenumbers near those of unidentified infrared (UIR) emission bands from interstellar objects. We produced 1-C16H11 + and 1-C16H11 upon electron bombardment during matrix deposition of p-H2 containing pyrene (C16H10) in a small proportion. Intensities of absorption features of 1-C16H11 + decreased after the matrix was maintained in darkness or irradiated with light at 365 nm, whereas those of 1-C16H11 increased. The observed line wavenumbers and relative intensities of 1-C16H11 + and 1-C16H11 agree satisfactorily with the scaled vibrational wavenumbers and IR intensities predicted with the B3PW91/6-311++G(2d,2p) method. Our method, being relatively clean with negligible fragmentation, is applicable to larger HPAH; it has the advantages of producing excellent IR spectra covering a broad spectral range with narrow lines and accurate intensities, so that structural identification among various isomers is feasible. SECTION: Spectroscopy, Photochemistry, and Excited States T infrared emission spectra of galactic and extragalactic objects show features at 3.3, 6.2, 7.7, 8.6, and 11.2 μm, the so-called unidentified interstellar infrared (UIR) bands. These features are generally attributed to polycyclic aromatic hydrocarbons (PAHs) and their derivatives, even though no definitive PAH responsible for the UIR bands has been positively identified. Because proton sources are abundant in space, protonated PAHs, designated as HPAHs, are expected to be likewise abundant. Furthermore, because HPAHs are stable closed-shell molecules, they become effective candidates for carriers of the UIR bands from the perspective of interstellar chemistry. Laboratory experiments have demonstrated that HPAHs are formed readily through protonation of PAH or attachment of a H atom to PAH cations. Comparisons of the astronomical spectra with those of various HPAHs obtained from experiments or theory support the hypothesis that HPAHs are possible carriers of the UIR and the diffuse interstellar bands (DIBs). Although UIR bands from various objects are similar, detailed investigations have revealed significant variations in peak position and relative intensities of these bands from one source to another. Hence, the characterization of the infrared (IR) spectrum of individual HPAHs is important in understanding these variations. Recording IR spectra of HPAHs in laboratories is challenging, largely because of the difficulties in generating HPAHs in sufficient quantity for spectral interrogation. Two major spectral methods are employed to yield IR spectra of HPAH. One employs IR multiphoton dissociation (IRMPD) of HPAH on monitoring the loss of either the H atom or H2 molecule with an ion cyclotron resonance mass spectrometer. Another method measures the single-photon IR photodissociation (IRPD) action spectra of cold HPAH tagged with a weakly bound ligand, such as Ar, by monitoring the loss of the ligand. Because of the small perturbation by Ar, the action spectrum of the Ar-tagged HPAH is similar to the IR spectrum of the bare cation. Although the Ar tagging IRPD method provides much improved spectra over those from IRMPD, a critical limitation of the Ar tagging method is its difficulty in tagging large protonated PAHs because of their large internal energy. So far, the largest protonated species detected with this method is protonated naphthalene. According to an astrochemical model, PAH molecules containing 20−80 carbon atoms are photochemically more stable in interstellar clouds. Hence, a new method for investigating the IR spectra of the large protonated PAHs with high spectral resolution is essential. Pyrene (C16H10), the smallest peri-condensed PAH, comprises four benzenoid rings. Five distinct sites of pyrene are possible for protonation or hydrogenation, as shown in Received: May 2, 2013 Accepted: May 28, 2013 Published: May 28, 2013 Letter