Bonding interactions through hydrogen

Daniel J Goebbert, Purdue University

Abstract

Hydride reductions are some of the most important reactions in synthetic organic chemistry. Typically, a hydride transfer reagent, such as LiAlH 4 or NaBH4, is dissolved in solution and undergoes reactions with carbon-heteroatom unsaturated bonds. The classic example of such reactivity is the synthesis of alcohols from ketones. We have found, using experimental methods, that the hydride affinity of AIH3 is similar to that of BH3. Studies of AIH4− and BH 4− show selective reactivity for both ions, and gas-phase reactions with ketones fail to show formation of the hydride transfer product. In order to explain the differences in reactivity of LiAlH4 compared to NaBH4 we used theoretical studies to show that NaBH4 is a more selective reducing agent because Na+ is a weaker Lewis acid than Li+. Ion-molecule reactions at high pressures result in the formation of a large number of cluster ions. Our instrument is well suited for studies of proton bound clusters, and we adopted a standard technique for measuring proton affinities of molecules, the Cooks kinetic method, and applied it to larger clusters. Using this method we have been able to measure the proton affinity of water dimer and trimer, and acetic acid dimer. Unfortunately attempts to measure proton affinities of methanol-water or methanol dimer were unsuccessful. We were able to use cluster proton affinities to calculate neutral cluster binding energies, in this case, hydrogen bond energies. Therefore, we have demonstrated a new application for kinetic method experiments, determining cluster binding energies of neutral systems. Ion molecule reactions, not only lead to cluster formation, but in many instances, chemical reactions are observed. In our study of diacetylene radical cation reactions, we used a variety of methods to interrogate the products formed from the reaction with ethylene. Based on reactivity and energetic methods we determined the primary reaction product, the adduct ion, was benzene radical cation. Similarly we used these methods to show that ionized 1,5-Hexadyine rearranges to the benzene cation structure.

Degree

Ph.D.

Advisors

Wenthold, Purdue University.

Subject Area

Analytical chemistry

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