Infrared spectra of protonated polycyclic aromatic hydrocarbon molecules: Azulene

Abstract

The infrared ͑IR͒ spectrum of protonated azulene ͑AzuH + , C 10 H 9 + ͒ has been measured in the fingerprint range ͑600-1800 cm −1 ͒ by means of IR multiple photon dissociation ͑IRMPD͒ spectroscopy in a Fourier transform ion cyclotron resonance mass spectrometer equipped with an electrospray ionization source using a free electron laser. The potential energy surface of AzuH + has been characterized at the B3LYP/ 6-311G‫ءء‬ level in order to determine the global and local minima and the corresponding transition states for interconversion. The energies of the local and global minima, the dissociation energies for the lowest-energy fragmentation pathways, and the proton affinity have been evaluated at the CBS-QB3 level. Comparison with calculated linear IR absorption spectra supports the assignment of the IRMPD spectrum to C4-protonated AzuH + , the most stable of the six distinguishable C-protonated AzuH + isomers. Comparison between Azu and C4-AzuH + reveals the effects of protonation on the geometry, vibrational properties, and the charge distribution of these fundamental aromatic molecules. Calculations at the MP2 level indicate that this technique is not suitable to predict reliable IR spectra for this type of carbocations even for relatively large basis sets. The IRMPD spectrum of protonated azulene is compared to that of isomeric protonated naphthalene and to an astronomical spectrum of the unidentified IR emission bands.

Infrared spectroscopy of ionized corannulene in the gas phase

“…Density functional theory (DFT) methods have been extensively applied to planar PAH systems 8 and they have proven to yield calculated harmonic IR frequencies which are in good agreement with experimental IR vibrational spectra, both for neutral and radical cation forms 9,10 as well as for positively charged PAH derivatives. 11,12 These methods have also been applied to PAHs containing a five-membered ring, 13 but the performance of these methods for nonplanar corannulene in its cationic state has not been verified against experimental data.…”

Laboratory Spectroscopy of Protonated PAH Molecules Relevant For Interstellar Chemistry

“…Subsequent IRMPD spectra reported for isolated C 6 H + 7 in the fingerprint range (Fig. 1) using CLIO and monitoring H 2 loss (Jones et al 2003;Dopfer et al 2005) and higher-resolution IRPD spectra of Ar-tagged C 6 H + 7 ions (Douberly et al 2008) (Dopfer 2005;Lorenz et al 2007;Dopfer 2008;Knorke et al 2009;Zhao et al 2009); the spectra of benzene H + and naphthalene H + were recorded using CLIO, whereas all other spectra were obtained using FELIX; bands marked by asterisks are largely suppressed due to reduced laser power. Right: IR spectra of coronene, coronene + and coronene H + compared to an UIR spectrum (Knorke et al 2009).…”

“…Significantly, apart from benzene H + , the IRMPD spectra in Figure 1 correspond to the first spectroscopic data of these H + PAH in the gas phase, providing very valuable information about the geometric and electronic structures as well as the chemical reactivity and stability of these fundamental ions. For an illustration of the information content of these spectra, the reader is referred to a recent analysis of the azuleneH + spectrum using quantum chemical calculations (Zhao et al 2009). A similar analysis was performed for all H + PAH spectra in Figure 1 (Zhao 2010) and will be published elsewhere.…”

“…While this contribution focuses on the general appearance of the IR spectra and the comparsion with an UIR spectrum representative of a highlyionized region of the interstellar medium (Tielens 2008; Fig. 1), a full account of the potential energy surfaces with structural and vibrational analyses is given elsewhere (Zhao et al 2009;Zhao 2010). Significantly, apart from benzene H + , the IRMPD spectra in Figure 1 correspond to the first spectroscopic data of these H + PAH in the gas phase, providing very valuable information about the geometric and electronic structures as well as the chemical reactivity and stability of these fundamental ions.…”

“…The second spectroscopic strategy couples high-intensity free electron laser (FEL) sources (FELIX and CLIO), operating in the complementary 50-2500 cm −1 range, with ion cyclotron resonance (ICR) mass spectrometry to drive IRMPD of isolated, strongly-bound H + PAH by monitoring either H-atom or H 2 loss (Jones et al 2003;Dopfer et al 2005;Lorenz et al 2007;Zhao et al 2009;Knorke et al 2009). In general, the spectral resolution observed in the IRMPD spectra (circa 30 cm −1 ) is lower than in IRPD spectra due to the finite bandwidth of the FEL, the broader rotational contour (as the ions probed in the ICR cell are at room temperature), spectral congestion due to overlapping transitions, and spectral broadening arising from the multiple-photon character of the IRMPD process.…”