Flowing afterglow and computational studies of the thermochemistry and reactivity of gas -phase Lewis acid -base complexes

Jianhua Ren, Purdue University

Abstract

The thermochemistry, reactivity and reaction mechanisms of several classes of Lewis acid-base complexes were studied by using the Flowing afterglow-triple quadrupole technique. Detailed structural information, electronic properties and energetic quantities were obtained by molecular orbital calculations. Experimental approaches based on thermochemical cycles were developed for the determination of the gas-phase acidities and hydride ion affinities of Lewis acid-base complexes that are not sufficiently stable for equilibrium methods. CID threshold energy measurements are essential for these approaches. An absolute acidity scale of Lewis acid-base complexes was established for the BH3 and/or BF3 complexed alkyl sulfides, amines, phosphines, carbonyl compounds, epoxides and carboxylic acids. The results demonstrate that coordination with BF3 has essentially transformed simple carbonyl compounds into superacids with gas-phase acidities comparable or greater than that of the hydrogen atom. Quantitative models were developed for Lewis acid activation with respect to acidities and hydride ion affinities of Lewis acid-base complexes. The acidity of the [special characters omitted]-CH bonds in alkyl sulfides, amines and phosphines can be increased by 18 to 20 kcal/mol by borane coordination. Complexation of BF3 (or BH3) to the oxygen atom of carbonyl compounds or epoxides can enhance the [special characters omitted]-CH acidity by up to 50 kcal/mol and the hydride ion affinity by up to 67 kcal/mol. Coordination of BH3 to the [special characters omitted]-heteroatoms in carboxylic acids can increase the OH acidity by 12 to 15 kcal/mol. The acidity enhancements caused by Lewis acid complexation are directly related to the differences in the Lewis acid-binding energies of the neutral molecules and the corresponding anions. Deprotonation of BH3 and/or BF3 coordinated dimethylsulfide, trimethylamine and trimethylphosphine produces stable [special characters omitted]-carbanions that do not rearrange to the corresponding borate isomers. Complexation of a Lewis acid to the heteroatoms of alkyl sulfides, amines and ethers can induce substitution and elimination reactions that will not occur without the complexation.

Degree

Ph.D.

Advisors

Squires, Purdue University.

Subject Area

Organic chemistry|Chemistry|Analytical chemistry

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