A study of the gas phase ion chemistry of organosilicon compounds in a flowing afterglow

David Jeffrey Hajdasz, Purdue University

Abstract

This thesis examines the reactivity, thermochemistry and structure of gas phase organosilicon cations and anions. The work encompasses three areas of organosilicon gas phase ion chemistry: hypervalent silylhydrides, beta silicon carbenium ions and electrophilic substitution reactions of silicenium ions. The hydride affinities of five organosilanes, as well as that of silane itself, were measured to be on the order of 20 kcal/mole. We have found that the hydride affinities of the silicon containing compounds by can be roughly ordered by the number of alkyl groups on the silicon atom, i.e., increasing the number of alkyl groups decreases the hydride affinity. Furthermore, at some point during their lifetimes, these ions posses ligands that are chemically and structurally equivalent as was demonstrated by the statistical yields for hydride and deuteride transfer to CO$\sb2$ from the deuterium labeled silylhydride ions. The beta silicon carebenium ions formed in this work have been determined to posses either an open beta silicon or bridged structure. Furthermore, the proton affinities of vinyl- and allyltrimethylsilane were determined to be 199 and 210 kcal/mole respectively. From the proton affinity measurements and other experiments we have determined the heats of formation of protonated vinyl- and allyltrimethylsilane to be 140 and 125 kcal/mole respectively. We have also studied several electrophilic substitution reactions of organosilanes. Here, we used a variety of techniques, such as ligand switching, energy resolved mass spectrometry and collision induced dissociation, to probe the ionic structures and reaction pathways. In addition, we have investigated the mechanism for symmetric electrophilic substitution reactions involving labeled (CH$\sb3$)$\sb3$Si$\sp+$ ions and several organosilanes. The symmetric electrophilic substitution reactions were not observed to occur in the flow tube. However, each of the stabilized adducts that were formed undergo CID to produce equal amounts of the labeled and unlabeled (CH$\sb3$)$\sb3$Si$\sp+$ ions. Thus, the two (CH$\sb3)\sb3$SI$\sp+$ groups most likely become equivalent in the adducts as a result of collisional activation.

Degree

Ph.D.

Advisors

Squires, Purdue University.

Subject Area

Analytical chemistry

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