Carbenium ion stabilities in the gas phase and solution. An ion cyclotron resonance study of bromide transfer reactions involving alkali ions, alkyl carbenium ions, acyl cations, and cyclic halonium ions

Abstract

Ion cyclotron resonance techniques are used to measure relative heterolytic bond dissociation energies D(R/sup +/ - Br/sup -/) in the gas phase for a series of alkali cations, alkyl carbenium ions, acyl cations, and cyclic halonium ions. D(R/sup +/ - Br/sup -/) for adamantyl cation is found to be less than for tert-butyl cation. A proton affinity for norbornene of D(B - H/sup +/) = 198.8 +- 2 kcal/mol is determined from which D(R/sup +/ - Br/sup -/) = 146.8 +- 2.3 kcal/mol is calculated for norbornyl cation, 16 kcal/mol less than for cyclopentyl cation. In several cases additional thermochemical data useful in assessing absolute D(R/sup +/ - Br/sup -/) heterolytic bond dissociation energies are provided by photoionization mass spectrometry. Relative enthalpies of solvation for carbenium ions are estimated via appropriate thermochemical cycles by combining the gas phase data with heats of ionization in a HSO/sub 3/F/SbF/sub 3/ medium. 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 appreciablemore » effect in attenuating the observed range.« less