Tyler P. Troy, Scott H. Kable, Timothy W. Schmidt Department of Chemistry, University of Sydney Scott A. Reid Department of Chemistry, Marquette University 67 th ISMS, Columbus, OH June 21, 2012 On the Electronic Spectroscopy of Closed Shell Aromatic Cations: Insights from Theory and Franck-Condon Analysis

Why closed shell cations? Closed shell cations derived from resonance stabilized radicals (RSRs) have attractive properties as potential carriers of the Diffuse Interstellar Bands  Lowest electronic transitions are in the visible to NIR region  Stronger electronic transitions than corresponding RSR  Low reactivity (resonance stabilization also important)  High Ionization Potentials (IP’s) Recently, spectra of several CSCs have been reported by Maier and co-workers using mass-selected matrix isolation. Bieske and co- workers are also studying these systems using Ar tagging approach. In order to model existing spectra and predict spectra for new species, we conducted a theoretical analysis and Franck-Condon simulation of their electronic spectra.

Maier’s Benzylium Spectrum

Methodology Ground state geometry optimization and frequency calculation performed at M06/aug-cc-pVDZ level Excited state optimization and frequency calculation performed at TDM06/aug- cc-pVDZ level. Calculations of vertical transition energies were also performed with complete active-space self-consistent field (CASSCF) methods combined with a second-order perturbation theory treatment (MRPT2). Optimized stuctures, harmonic frequencies, and l-matrices were used as input to calculate the Franck-Condon factors including Dushinsky mixing (PGOPHER program) parent RSR Cation –H  –e –

Electronic spectra of RSR’s and their CSC’s

Benzylium calculations S 0  S 1 : 411 nm, ƒ = 0.026

Simulation vs. (Maier’s) Experiment

Relation to DIB problem DIBs are largely uncorrelated along different lines of sight, suggesting the presence of multiple carriers ( Hobbs, L. M., York, D. G., Thorburn, J. A., et al. 2009, Astrophys. J., 705, 32 ) This precludes, of course, a spectrum with pronounced Franck- Condon envelope (e.g., benzylium) – the spectrum should be origin dominated Questions:  What about larger polyaromatic closed shell cations?  Is activity in the ring distortion mode quenched?

What about polyaromatic CSC’s? S 0  S 1 : 488 nm ƒ = 0.043

What about polyaromatic CSC’s? S 0  S 1 : 475 nm ƒ = Experimental Spectrum Of Maier and co-workers J. Am. Chem. Soc. 133, (2011)

Three ring CSC’s S 0  S 1 : 883 nm ƒ = S 0  S 2 : 439 nm ƒ = HOMO-1

Calculated IPs of CSC’s Experimental Data from Rabrenov ́ıc, M., Proctor, C. J., Ast, T., et al. 1983, J. Phys. Chem., 87, 3305 Jenniskens et al. showed from the strength of the λ6284 DIB at low reddening the carrier is resilient to photons of energy < 13.6 eV. (Astron. Astrophys. 281, 517 (1994)) In a later study, Sonnentrucker et al. conclude that λ5780, 5797 and 6613 DIBs possess ionization energies in the range 10.0 to 13.5 eV. (Astron. Astrophys. 327, 1215 (1997)) B3LYP/ G(d,p)

Conclusions Closed shell cations are attractive candidates for the DIB carriers The Franck-condon activity in the electronic spectra of two model CSC’s, benzylium and 1-napthylmethylium, has been modeled and the simulated spectra are in good agreement with recent experiments by Maier and co-workers The pronounced Franck-Condon activity in the ring distortion mode seen for benzylium is largely quenched in polyaromatic systems The calculated Ionization Potentials for a range of CSC’s fall in the eV range

Acknowledgments Funding: ARC Discovery funding scheme (TWS and SHK, Project Numbers DP and DP ) Way-Klingler Sabbatical Fellowship (MU, to SAR)