RESONANCE INTERACTIONS IN ACYCLIC SYSTEMS .2. Y-CONJUGATED ANIONS AND CATIONS

The first and second ionization energies of isobutene, acetone, acetic acid, and carbonic acid were calculated at the MP3/6-3I l++G**//6-31G* level giving values that agreed with the available experimental data. The first ionization energy decreased by 21 kcal/mol for each of the first two replacements of CH2 by O but decreased by only 10 kcal/mol for the third replacement. The second ionization energy was high and was the same for the first three compounds. It decreased for the third replacement of CH2 by O. It was concluded that the electronegativity of the terminal atoms determines the ionization energies, and that there was no evidence for Y aromaticity in the gas phase. In solution, the anions formed ion pairs or triplets with lithium, and this markedly reduced the energies of formation. The energies of reaction of isobutene with lithium hydride were calculated, and both the first and second reactions were found to be exothermic. The cations formed by the loss of one or two hydride ions from isobutene, and by the addition of a proton to guanidine, also were studied. The allyl cations had significant resonance stabilization, but no special stabilization was found for the dication. Guanidinium ion was found to have little if any resonance stabilization. The origin of the basicity of guanidine is discussed. © 1990, American Chemical Society. All rights reserved.