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 cation leads primarily to sequential loss of one to five hydrogens. Subsequent photolysis of the odd mass dehydrogenated species induces further fragmentation to lower mass products. In the present paper, results from density functional calculations are used to explain the experimental findings. These results show that dehydrogenation is predicted to occur first from the sp3 carbon on the five-membered ring and then from only one of the six-membered rings. The predicted infrared spectrum of this C13H5+ (m/z 161) species is shown to match well with a matrix isolation spectrum of a photolyzed fluorene sample. The conclusion is drawn that the C13H5+ (m/z 161) ion retains its fluorene-like framework and does not isomerize upon dehydrogenation. Photolysis of this C13H5+ (m/z 161) ion does appear to lead to isomerization. Plausible photodecomposition pathways leading ...