Theoretical study of the photodissociation and hydrogenation of the fluorene cation

Previous Fourier transform ion cyclotron resonance mass spectrometry (FTICR /MS) experiments have shown that UV/visible photolysis of the fluorene cati on leads primarily to sequential loss of one to five hydrogens. Subsequent photolysis of the odd mass dehydrogenated species induces further fragmenta tion to lower mass products. In the present paper, results from density fun ctional calculations are used to explain the experimental findings. These r esults show that dehydrogenation is predicted to occur first from the sp(3) carbon on the five-membered ring and then from only one of the six-membere d rings. The predicted infrared spectrum of this C13H5+ (m/z 161) species i s shown to match well with a matrix isolation spectrum of a photolyzed fluo rene sample. The conclusion is drawn that the C13H5+ (m/z 161) ion retains its fluorene-like framework and does not isomerize upon dehydrogenation. Ph otolysis of this C13H5+ (m/z 161) ion does appear to lead to isomerization. Plausible photodecomposition pathways leading from this (and other) specie s to the observed low-mass products are shown to be possible only if it is assumed that the fluorene framework opens to a monocyclic ring. Unusual geo metries, such as a "tadpole" shape (three-membered ring attached to a linea r carbon chain) for the C5H3+ species, a three-membered ring fused to a six -membered ring for the C7H5+ product and monocyclic rings for the all-carbo n C-9(+) and C-11(+) product ions are computed to be the most stable for th ese observed products.