|Ph.D Student||Tenetov Elena|
|Subject||Gas Phase Reactions of pi-Conjugated Systems|
|Department||Department of Chemistry||Supervisor||DR. Chagit Denekamp|
This research deals with conjugation phenomena and charge delocalization in isolated charged species, comprising organic p-systems that were studied in the gas phase with the aid of a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer with an Electrospray Ionization Source (ESI FTICR MS).
A series of p-conjugated cationic compounds containing a well-defined charged site (a tertiary alkylammonium group) and a cycloheptatrienyl moiety tethered by both aromatic and non-aromatic linkers of different length and geometry has been synthesized. Fragmentation patterns of these cations have been studied by tandem mass spectrometry (MSMS) and Collision Induced Dissociation (CID) experiments.
The conjugated cationic models under discussion fragment in the gas phase with the loss of corresponding neutral alkylamines. The resulting cations ([M-Alk2NH]+) undergo either a heterolytic or homolytic cleavage of a C-C bond adjacent to a cycloheptatrienyl moiety, hence forming a tropylium cation of m/z 91 or eliminating a radical species of the same mass. This suggests that charge (or spin) migrates through the conjugated chain. In all cases cleavage of a C-C bond remote to the initially charged position is observed, while the heterolytic path is clearly favored. Although a tropylium cation is formed from a variety of precursor ions, the fragmentation efficiency is not the same for cationic compounds bearing conjugated linkers of different geometry and constitution. Structural effects on the fragmentation of different conjugated models were evaluated using breakdown curves.
In conclusion, the use of mass spectrometry for the evaluation of electronic properties of conjugated compounds is demonstrated. It is shown that conjugation and related properties can be studied in the gas phase, in order to deduce numerical parameters for charge delocalization ability.
Specifically we found the following: 1) aromatic systems are less efficient than the corresponding aliphatic polyenes for charge delocalization, but are far more stable. 2) Extension of the p-system enhances charge delocalization despite the possible conformational barriers for delocalization. 3) The best basic unit for delocalization is a 9,10-dihydrophenanthrenylene moiety, in which the dihedral angle between two phenyl rings is small (17 o).
In contrast to models with alkylammonium charged site pyridinium based models undergo a pronounced homolytic C-C cleavage. Theoretical calculations and substituent effect suggest that a triplet excited state is involved in this homolytic scission process.