Accurate Borane Sequential Bond Dissociation Energies by High-Level ab Initio Computational Methods

Abstract

Ab initio molecular orbital calculations at the G-2 and CBS-4 compound levels of theory were used to determine the sequential homolytic bond dissociation energies (BDE's) for a series of B−H, B−C, and B−F bonds in a variety of cyclic and acyclic boranes. The calculated average BDE's agreed very well with the limited experimental data available. However, the first sequential BDE's, which are the most relevant for understanding borane reactivity, were substantially higher than the average BDE's. In general, first BDE's were found to be larger for B−C and B−H bonds in organoboranes than for C−C and C−H bonds in hydrocarbons, even though average B−H and B−C BDE's are lower than average C−H and C−C BDE's. In all the boron substitution patterns examined, B−H and B−C bonds were found to be of almost identical strength, while B−F bonds were found to be much stronger. Moreover, the strengths of B−H and B−C bonds were found to be essentially independent of the electronegativity, π-donating ability, and conjugative ability of the other substituents on boron. Thus, for instance, a phenyl group was found not to stabilize the odd electron of borane radicals and hence not to lead to reduced B−H or B−C bond strengths. However, B−H bonds of four-coordinate boron were slightly weaker than those of three-coordinate boron.

Reaction of Benzene and Boron Atom: Mechanism of Formation of Benzoborirene and Hydrogen Atom

“…59 In agreement with these authors, we find that 10 prefers a planar structure ( 2 A′) with a bent geometry at boron, ∠(C-B-H) ) 128.2°. Compared to the reactants, 10 is very low in energy (-61.2 kcal mol -1 ), making it the most stable C 6 H 6 B species considered in this study.…”

The structure and energetics of triplet [B, C, F, H2]

“…More computational work has been carried out on some of the boron-containing fragments relevant to the [B, C, F, H 2 ] species. Much of the recent work has focused on the thermochemical properties of these fragment species, in particular their atomization energies, enthalpies of formation, bond dissociation enthalpies BDHs, and excitation energies [8][9][10][11][12][13][14][15][16]. Most recently, Grant and Dixon [9] have reported these thermochemical data for H (3Àn) BX n compounds for which X is F, Cl, Br, I, NH 2 , OH and SH (where 1 n 3), using the composite ab initio molecular orbital theory approach Feller et al are developing, which allows them to calculate thermochemical In the current paper, we discuss our high level quantum chemical results for the structure and energetics of triplet (and hence open-shell) isomers corresponding to the stoichiometry of one boron, carbon, and fluorine apiece, and two hydrogens.…”

“…[10] and references cited therein.) Earlier calculations on these and related cyclic and acyclic borane molecules and radicals were performed by Raabe et al [11], Poon and Mayer [12], and Rablen and Hartwig [13]. Barreto et al.…”

“…Most of the above articles include a discussion of trends in sequential bond dissociation enthalpies and/or trends in bond dissociation enthalpies along the periodic table. Comparison of B-X bond dissociation enthalpies, X = H, C, F, Cl, Br, I, in analogous compounds has shown that (1) BDHs decrease down the group from F to I [9,11], (2) B-H and B-C bond strengths are similar in magnitude but much smaller than B-F bond strengths [9,[12][13][14][15], (3) the strengths of B-H and B-C bonds tend to be less dependent on the other boron substituents than do boron-halogen bonds [9,13,14], and (4) replacing a halogen substituent with hydrogen generally increases the BDHs of boron-halogen bonds [9,11,14]. However, because sequential adiabatic BDHs often have large fluctuations resulting from reorganization energy in the product fragments, the authors of some of these articles suggest that intrinsic bond strengths should be compared by evaluating diabatic BDHs [9,12,17] or the electron density at the bond critical point [15].…”

A computational study on the capability of borane‐cyclic boryl anion adducts to act as hydrogen atom donors

“…Unfortunately, the homolytic bond dissociation energy (BDE) of the BÀ ÀH bond is too high to release a hydrogen atom in a radical reaction. 1 Via a computational approach, Rablen computed that a Lewis base can aid in the dissociation of the BÀ ÀH bond by complexation with BH 3 .…”