|Ph.D Student||Sandlers Yana|
|Subject||Study of the Reactive Cationic Intermediates|
in the Gas Phase
|Department||Department of Chemistry||Supervisor||DR. Chagit Denekamp|
|Full Thesis text|
The work is composed of two parts, both dealing with gas phase ion chemistry:
1) The purpose of this work was to study the relative reactivity and relative stability of electrophilic intermediates in a solvent free environment and compare these gas phase results to the corresponding rates in solution.
Ion-molecule reactions of diaryl carbenium ions with n-nucleophiles were carried out with the aid of an FT-ICR mass spectrometer. Rate constants were evaluated for each association process, then the constants were subjected to the correlation analysis.
It was found that the kinetics of electrophile-nucleophile gas phase reactions follow linear free energy relationship ln k=s (Egas + Ngas) where Egas is electrophilicity gas phase parameter, Ngas gas phase nucleophilicity parameter and s slope dependant parameter. Our results indicate that although the actual electrophilicity parameters are smaller in the gas phase they correlate well with values known from solution chemistry. DFT calculations show that the association between cations and neutral amine occurs without barrier however formation of stable ion-molecule complexes affects the kinetic behavior of the system and may explain the observed differences in cations reactivities.
2) Structural, protecting group and leaving group effects in the formation of oxocarbenium intermediates were studied in the gas phase. It is found that significant stabilization of oxocarbenium cations is achieved by protecting groups that interact with the cationic center via neighboring group participation despite the electron-withdrawing character of these moieties, while ethereal protecting groups do not facilitate the formation of oxocarbenium intermediates. Experimental findings are supported by DFT calculations that show the following order of stabilization by the group adjacent to the cationic center: RCO > SiR3 > R, where R is an alkyl group. This indicates that the SN1-like mechanism that is commonly proposed for this reaction is not always valid. Moderate leaving group effect is also detected in a series of thio-aryl glucopyranosides.
Furthermore, collision-induced dissociation of ammonium-cationized alpha and beta acetyl pyranosidic isomers were studied and stereochemical dependence of the reactivity towards elimination of acetic acid from the anomeric position was found. It is shown that isomers that contain trans diacetyloxy groups at positions 1 and 2 of the pyranoside are more reactive, allowing anomeric distinction according to the relative abundance of the oxocarbenium product ion of this reaction in the spectrum. The higher reactivity of trans isomers is rationalized by neighboring group assistance that is possible only in the trans configuration.