Electronic spectra of 1-methyl and 2-methyl phenanthrenes and their radical cations

Electronic spectra of phenanthrene (P), 1-methyl phenanthrene (1-MeP), 2-me thyl phenanthrene (2-MeP) and their monopositive ions are investigated expe rimentally as well as theoretically. The ions were produced by photo-oxidat ion of the hydrocarbons in boric acid matrix. The electronic absorption spe ctrum of 2-methyl phenanthrene cation (2-MeP+) is entirely new. For the int erpretation of the electronic spectra of neutral and ionized MePs, semi-emp irical AM1 (Austin Model 1) calculations are carried out for the first time . The bathochromic shifts in the spectral bands of the neutral and ionized MePs are attributed to 'conjugative' effect. The present experiments reveal that the 448 nm band of 1-methyl phenanthrene cation (1-MeP+) and the 486 nm band of 2-MeP+ show close matching with the respective 450 nm and 488 nm 'diffuse interstellar bands'. This suggests the possibility of the existen ce of such ionic species in the interstellar matter. (C) 2000 Elsevier Scie nce B.V. All rights reserved.