UC San Diego

Although my research has no common theme or defining area, a coherence connects the diverse topics insofar as one project leads logically to another. Thus, studies on mechanisms of hydrogen exchange in amides and amidines led to the influence of hydrogen bonding and to NMR methods for chemical kinetics, including 2D-EXSY spectroscopy. Another connection was the OH^--catalyzed NH exchange in amines that had supported the hypothesis of stereoelectronic control. We therefore analyzed that hypothesis critically, tested it, found counterexamples, and proposed an alternative hypothesis. We next addressed one-bond NMR coupling constants in ethers and the reverse anomeric effect. The latter studies required a highly accurate NMR titration method that we developed to measure the additional steric bulk resulting from protonation of a substituent. This method is also applicable to measuring secondary isotope effects on acidity, and we could demonstrate that they arise from n-σ* delocalization, not from an inductive effect. Other studies included kinetic isotope effects for both dissociation and H exchange of aqueous NH₄⁺, for C-N rotation in amides, and for a hydride transfer. The role of hydrogen bonding led us to the rotation of NH₄⁺ within its solvent cage and then to the symmetry of hydrogen bonds.