Computational Studies of Electron Affinities, Acidities, and Bond\nDissociation Energies of Boron-Containing Species:  The\nCH₃(CH₂)<i>_n</i>_-1BH₂, CH₂F(CH₂)<i>_n</i>_-1BH₂, and CH₃(CH₂)<i>_n</i>_-1BHF Series

Abstract

Computational studies of three series of boron-containing compounds, CH₃(CH₂)<i>_n</i>_-1BH₂, CH₂F(CH₂)<i>_n</i>_-1BH₂,\nand CH₃(CH₂)<i>_n</i>_-1BHF, have been carried out to probe the structural characteristics of the parent compounds\nand the radicals and anions derived by loss of their terminal C−H. The energies of these species yield\nelectron affinities, acidities, and bond dissociation energies that are explained in terms of the structural\ncharacteristics, which vary with the value of <i>n</i>. One-carbon species are considered in terms of\nhyperconjugative effects. The radical species with <i>n</i> > 1 reveal important structures with strong tendencies\nfor termini to interact. Carbon 2p radicals and empty 2p boron centers interact to give a cyclic radical\nspecies for BH₂CH₂CH₂ and BH₂CH₂CHF and bent radicals with short distances between the termini for\nlarger values of <i>n</i>. The bent and zigzag BH₂(CH₂)<i>_n</i>_-1CH₂ radicals for <i>n</i> ≥ 3 are quite close in energy. All\nanions with <i>n</i> ≥ 2 are cyclic and considerably lower in energy than zigzag species. Single-point energies\nof parent, radical, and anionic species allow the determination of electron affinities (EA), acidities (Δ<i>H</i>_acid),\nand bond dissociation energies (BDE). The EAs have a noticeable break between <i>n</i> = 3 and 4, a\nconsequence of the strain energy of the <i>n</i> = 2 and 3 anions. The Δ<i>H</i>_acid values exhibit trends also related\nto the strained nature of the <i>n</i> = 2 and 3 anions. The values of the terminal C−H BDEs are typical of\nC−H bonds with the exception of the CF-substituted series, where the somewhat smaller values indicate\nthe C−H bond-weakening effect of fluorine substitution.