Article pubs.acs.org/JPCA
Electronic Spectra of Protonated Fluoranthene in a Neon Matrix and Gas Phase at 10 K A. Chakraborty, C. A. Rice, F.-X. Hardy, J. Fulara, and J. P. Maier* Department of Chemistry, University of Basel, Klingelbergstr. 80, CH-4056 Basel, Switzerland ABSTRACT: Four electronic systems with origin bands at 759.5, 559.3, 476.3, and 385.5 nm are detected in a 6 K neon matrix following deposition of mass-selected protonated fluoranthene C16H11+ produced from a reaction of neutral vapor and ethanol in a hot-cathode ion source. Two cationic isomers are identified as the carriers of these band systems. The 559.3, 476.3, and 385.5 nm absorptions are assigned to 4,3,2 1A′ ← X 1A′ transitions of isomer E+ (γ−) and the 2 1A′ ← X 1A′ system at 759.5 nm is of isomer C+ (α−) of protonated fluoranthene on the basis of theoretical predictions. The electronic spectrum of E+ was also recorded in the gas phase using a resonant 1 + 1 two-photon excitation−dissociation technique in an ion trap at vibrational and rotational temperatures of 10 K. The 3,2 1A′ ← X 1A′ transitions have origin band maxima at 558.28 ± 0.01 and 474.92 ± 0.01 nm. Both the 2 1A′ and 3 1A′ excited states have a distinct vibrational pattern with lifetimes on the order of 1 ps.
FT,17 only IR absorptions of FT+ in the gas phase and in a matrix are reported.18,19 Herein, the electronic absorption spectra of protonated fluoranthene (H+-FT) and its neutral radical counterpart in 6 K neon matrices are presented. The spectrum of H+-FT was also recorded in the gas phase at 10 K using a cryogenic 22-pole rf ion trap. These complementary techniques, as well as quantumchemical predictions, are used to infer the isomeric structures.
1. INTRODUCTION Polycyclic aromatic hydrocarbons (PAHs) and their derivatives have been proposed as the carriers of the unidentified infrared emission bands (UIRs)1 and possibly the diffuse interstellar absorptions (DIBs)2 because of their vibrational features and optical transitions. These bands are observed in galactic and extragalactic objects, indicating that the carriers are universal in the interstellar medium (ISM).3,4 To test this hypothesis, IR and electronic spectra of several PAH derivatives have been measured in the laboratory to probe for their existence in the ISM;5 however, this has been limited by the lack of spectroscopic data on PAH ions under conditions relevant to outer space, that is, gaseous.6,7 In this concern, techniques such as matrix-isolation and ion confinement in radio frequency (rf) traps have proven to be suited to study both positive and negative ions.8−10 Investigations of molecular spectroscopic properties of neon/argon matrix-isolated PAHs and their ions produced by in situ photoionization have been carried out.11 Mass-selected deposition of charged species in 6 K solid-neon has also been very successful.12 Such data are of general interest as ionized PAHs are considered to be reaction intermediates in combustion13 and a primary cause for the production of carcinogenic agents.14 Spectroscopic studies have been focused on one class of PAHs bearing six-membered rings and having absorption in the IR and visible. The recent identification of C60+15 in the ISM suggests the existence of polycyclic species containing both fiveand six-membered rings. Hence, electronic and IR spectra are required for the latter sort of molecules. A gas-phase electronic spectrum of protonated fluorene, the smallest polycyclic aromatic system containing a five-membered ring, has already been measured.16 Fluoranthene (FT) is the next PAH of this class, where naphthalene and benzene units are conjugated by a five-membered ring. Besides the photoelectron spectrum of © XXXX American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Ion Production. H+-FT was produced by chemical ionization (CI). FT vapor was injected into an ion source after resistively heating the solid precursor to 390 K. Protonated ethanol (EtOH2+) was used to react with FT vapor in the source. The proton affinities of EtOH and FT molecules are 776.4 and 828.6 kJ mol−1, respectively, indicating that protonation of FT from EtOH2+ is exothermic.20 H+-FT can also be generated by using protonated toluene in an exothermic reaction. 2.2. Matrix Isolation. Ions formed inside the source were extracted, transported to a electrostatic quadrupole bender (QB), and deflected 90°, separating neutrals from cations. The cation beam was guided to a quadrupole mass-filter, where H+FT (m/z = 203) was mass-selected (±1 u) and then codeposited with excess of neon atoms on a rhodium-coated sapphire plate held at 6 K. A mixture of neon with CH3Cl in a ratio 30 000:1 was used for efficient trapping of cations. Free Special Issue: Piergiorgio Casavecchia and Antonio Lagana Festschrift Received: December 14, 2015 Revised: February 1, 2016
A
DOI: 10.1021/acs.jpca.5b12232 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A electrons are produced by ions impinging on metal surfaces and scavenged by CH3Cl. Dissociative electron capture yields Cl−, balancing the positive charge in the matrix. Absorptions were recorded by a waveguide technique from 280 to 1100 nm with halogen and high-pressure xenon sources. Light transversed through 20 mm of the matrix was collected by optical fibers, wavelength dispersed by a spectrograph, and recorded by a CCD camera. Fluorescence was recorded after exciting the trapped species. A tunable laser with a bandwidth of ∼3 cm−1 and 2−30 mJ per pulse was directed 45° to the substrate surface. Any resulting fluorescence was monitored perpendicular to the matrix, focused into optical fibers, and detected in a similar fashion as in absorption. 2.3. Gas-Phase: Ion Trap. The experimental setup has been described.21 H+-FT was created in a CI source and injected into a 440 mm long hexapole divided in two. Positive species were confined within the second 6-pole and cooled for 50 ms, narrowing the kinetic energy distribution. After 50 ms of storage ions were released into a quadrupole mass selector (QMS), selecting m/z = 203 with a resolution of ±0.5 u. The ion beam was turned 90° by a QB, injected into an rf-only octupole ion guide, and transported to an rf ion trap. The (36 × 10) mm2 22-pole rf ion trap22 was filled with ∼105 ions per cycle. The sequence was 40 ms for filling, then 50 ms confinement, followed by laser irradiation and detection of photodissociation products. The ion trap is mounted on a closed-cycle cryostat reaching 3.2 K. Helium buffer gas was leaked into the trap via a piezo-valve,23 and a density of ∼4 × 1015 cm−3 was achieved after a few milliseconds, corresponding to about one collision per microsecond. After exiting the trap, ions were collimated by electrodes and deflected 90° by a QB into a second QMS. The latter selects C16H10+ fragments produced from laser irradiation. The ion beam was then focused by an einzel lens to a Daly detector. The experiment was synchronized with a dye laser (0.07 cm−1 bandwidth) at a repetition rate of 10 Hz. 2.4. Computational. There are five nonequivalent protonation sites in FT (Chart 1). Ground-state geometry optimization of these five isomers A+-E+ of H+-FT was carried out with B3LYP/cc-pVTZ theory implemented in Gaussian09. Four other isomers, except D+, lie energetically within 22 kJ mol−1 of each other, and E+ is the global minimum on the potential energy surface of H+-FT. Vertical excitation energies and oscillator strengths of H+-FT isomers were calculated at the equilibrium coordinates obtained from B3LYP/cc-pVTZ computations. Symmetry-Adapted Cluster/ Configuration Interaction (SAC−CI)24,25 method was employed with the Gaussian09 software26 using the cc-pVTZ basis set with 350 orbitals at an energy
