Bridge-Bond Dissociation Energy

The energy of dissociation of diborane into two BH3 molecules is a matter of great interest. Nevertheless, all attempts to measure this quantity directly from a study of the supposed equilibrium 

however, a direct study of the equilibrium is not feasible, Z)(H3B BH3) must be estimated by less direct means. Several such methods are now available, but they have not led to concordant results. Although limits had already been proposed—namely, 15 (22) and 50 kcal rnole (52)—the first serious attempt to fix D more precisely stands to the credit of McCoy and 

Furthermore, the only published estimate of the difference, which rests on assumed values for the entropies of the methyldiboreuies amd rather imprecise equilibrium data, is 3.5 kcal rnole in favor of diborane. The previously cited value for D(Me2BH -HBMe2) thus corresponds to approximately 22 kcal mole for D(H3B BH3). Admittedly the precision must remain low until the spread in the dissociation energies of the methyldi-boranes is more accurately known, but the approach is fundamentally sound, in that it does not rest on assumed mecheuiisms but depends on strictly reversible equilibria that involve the stable molecule BMe3 in place of the highly reactive, nonobservable species 

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With respect to the parent ethylenebromonium ion and expressed as heterolytic bond dissociation energies D (R +—Br"). Data from Staley et al. ( 1977). Calculations (Galland et al., 1990) indicate that this ion is not bridged but open. 

M-H bond dissociation energies, 1, 289 for water-solubilization, 1, 827 Dichloro silafluorene, synthesis, 3, 436 Dichloro silaindene, synthesis, 3, 436 Dichlorostibines, synthesis, 3, 916 Dicobaloximes, bridged, 7, 39... 

Bridged species such as 76 are well documented in rhodium porphyrin chemistry.240-241 An acetylene bonded to one metal-centered radical is presumed to be trapped by addition of a second metal-centered radical. Lower bond dissociation energies of cobalt relative to rhodium would disfavor species such as 76 and facilitate the reaction with metal—hydride intermediates to form a trans product. 

Key Words Lewis acid adducts, Radical oxidations, Epoxidation, Hydrogen peroxide, Bond dissociation energy, Catalyst durability, Methyltrioxorhenium, Cross-bridged cyclam, Mn(IV), Late transition metal. Propylene oxide. Titanium silicalite (TS-1) catalyst, Ethylanthrahydroquinone/H2 process, Polyoxometallates, Mn(IV) catalyst. Hydrogen abstraction. Rebound mechanism, Isotopic label, t-BuOOH, Peroxide adduct. 2008 Elsevier B.v. 

In this way a hydrated H3O ion is formed with an exceptionally strong hydrogen bond (dissociation energy about 100 kJ mol ). A similar mechanism is valid in transport of OH ions, which also occurs along the hydrogen bridges ... 

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