Thermochemical determinations and gas-phase transacetalization reactions studied by tandem mass spectrometry
Abstract
By using the kinetic method, the proton affinity, gas-phase basicity and gas-phase acidity of urea are determined to be 873.5 ± 5.0, 841.6 ± 5.0, and 1513.8 ± 12.0 kJ/mol, respectively. Both the gas-phase basicity and acidity of urea are predominantly controlled by resonance stabilization effects in the corresponding protonated and deprotonated amide moieties. The kinetic method is also used to investigate stereoelectronic effects on the unimolecular dissociation of SiFn+ (n = 1, 3)/pyridine clusters with a pentaquadrupole mass spectrometer. The SiFn+ (n = 1, 3)/pyridine clusters fragment to yield two monomers, and the logarithm of their relative abundance correlates linearly with the proton affinity of the unhindered pyridine ligand. Deviations from the linear correlation are observed for dimers composed of ortho-substituted pyridines and are attributed to a combination of auxiliary bonding and steric hindrance. Ab initio calculations confirm that agostic bonding enhances the central cation bonding to the ortho-substituted pyridine ligand in SiF+ adducts, while the steric effect weakens the central Si+-N bond in SiF3+ adducts The unique MS3 capability of the pentaquadrupole mass spectrometer also facilitates the exploration of ambiphilicity of the borinium and phosphonium ions by their reactions with cyclic acetals and ketals. Tricoordinated cyclic boron cations and 1,3,2-diheterophospholanium ions are synthesized by a highly exothermic reaction channel, which proceeds via initial cationic binding to a heteroatom followed by a consecutive ring opening and ring reclosing process. The cyclic structure of these B-O and P-O analogs of 1,3-dioxolanylium cations is confirmed by collision-induced dissociation and by comparison with authentic compounds. Generally, these product ions dissociate to reform the reactant ions in high yield, which forecasts the acidic hydrolysis of cyclic borates and phosphorus-containing esters in condensed phase.
Degree
Ph.D.
Advisors
Cooks, Purdue University.
Subject Area
Analytical chemistry
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