Studies of acid-base ion-molecule chemistry using ion cyclotron resonance techniques and photoelectron spectroscopy

Citation

Staley, Ralph Horton (1976) Studies of acid-base ion-molecule chemistry using ion cyclotron resonance techniques and photoelectron spectroscopy. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/32NW-ER15. https://resolver.caltech.edu/CaltechTHESIS:11052009-095120323

Abstract

Thermochemical properties related to molecular basicity are examined for series of substituted amines and nitriles using ion cyclotron resonance techniques and photoelectron spectroscopy. Proton affinities PA(B), defined for a base B as the heterolytic bond dissociation energy for removing a proton from the conjugate acid BH^+, and adiabatic ionization potentials, IP(B), are determined. In previously examined series of bases linear relationships have been established between proton affinities and adiabatic first ionization potentials which correspond to removal of an electron from a lone pair localized at the site of protonation. Nitrile proton affinities are found to be linearly related not to the first CN π ionization potentials but rather to the adiabatic N lone pair σ ionization potentials at higher energy. This relationship provides a useful chemical means for the assignment of bands in photoelectron spectra; specific examples are considered and assigned. Homolytic bond dissociation energies D(B^+-H) are obtained using the relation PA(B)-D(B^+-H) = IP(H)-IP(B). Systematic effects of substitution on PA(B), D(B^+-H), and IP(B) are identified and discussed in terms of the intrinsic factors affecting molecular basicity. Ion cyclotron resonance techniques are also used to measure relative heterolytic bond dissociation energies D(R^+-Br^-) in the gas phase for a series of alkali cations, alkyl carbenium ions, acylcations, and cyclic halonium ions. D(R^+-Br^-) for adamantyl cation is found to be less than for tert-butyl cation. A proton affinity for norbornene of D(B-H^+) = 198.2 ± 2 kcal/mol is determined from which D(R^+-Br^-) = 146.8 ± 2.3 kcal/mol is calculated for norbornyl cation, 14 kcal/mol less than for cyclo-pentyl cation. Relative enthalpies of solvation are estimated via appropriate thermochemical cycles by combining the gas phase data with heats of ionization in fluorosulfuric acid. The results show that solvation enthalpies are related to ion size with smaller ions being better solvated. Relative stabilities of cyclic bromonium ions are the same in the gas phase and solution. Stability increases with increasing ring size and in the three-membered rings with methyl substitution. However, solvent has an appreciable effect in attenuating the observed range.

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