<b>An Electron Impact Investigation of Some Organoboron Difluorides</b>

Steele, W. C.; Nichols, L. D.; Stone, F. Gordon A.

doi:10.1021/ja00866a019

Cited by 21 publications

(7 citation statements)

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“…In the pure silane pyrolysis, since reactions (2) and (9) can be eliminated on energetic grounds,? it seems reasonable that silylene may react via the reaction sequence (lo), (lo'), (10") 9 AH: = 49.6 kcal SiH2 + M + S i + H2 + M Eg = 59.3 kcal Recent calculations [ll] suggest that singlet H3SiSiH is the lower-energy form The reaction (10) sequence can explain several low temperature pyrolysis observations of P & W [4]. Thus in the very early stages of reaction, no appreciable total pressure changes were observed.…”

Section: (7)mentioning

confidence: 99%

“…Thus, with regard to reaction (2), it appears that an upper limit for the bond dissociation energy of SiH has been fairly reliably set at DH$(Si-H) = 70.6 kcal/mole [8]. This value, coupled with the first and second bond dissociation energies in silane of 94 kcal/mole [9] and (60.1-E3) kcal/mole,* requires that DHt(SiH-H) = EZ L 83.5 -E3 kcal/mole. Since any subsequent reaction to reaction (1) is limited to an activation energy of less than about 56 kcal/mole [ 11, reaction (2) cannot occur under our pyrolysis conditions.…”

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confidence: 99%

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Kinetics and mechanism of the silane decomposition

Newman

O’Neal

Ring

et al. 1979

Int J of Chemical Kinetics

184

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The homogeneous gas-phase decomposition kinetics of silane has been investigated using the single-pulse shock tube comparative rate technique (T = 1035-1184°K, P~d = 4000 Torr).The initial reaction of the decomposition SiH4 ! + SiHz + H2 is a unimolecular process in its pressure fall-off regime with experimental Arrhenius parameters of logkl (sec-I) = 13.33 f 0.28-52,700 f 1400/2.303RT. The decomposition has also been studied at lower temperatures by conventional methods. The results confirm the total pressure effect, indicate a small but not negligible extent of induced reaction, and show that the decomposition is first order in silane at constant total pressures. RRKM-pressure fall-off calculations for four different transition-state models are reported, and good agreement with all the data is obtained with a model whose high-pressure parameters are logAl (sec-') = 15.5, El(,) = 56.9 kcal, and AE;;, = 55.9 kcal. The mechanism of the decomposition is discussed, and it is concluded that hydrogen atoms are not involved. It is further suggested that silylene in the pure silane pyrolysis ultimately reacts with itself to give hydrogen: 2SiH2 -(SiZHd)* -(SiH3SiH)* -SizHz + Hz. The mechanism of H -D exchange absorbed in the pyrolysis of SiD4-hydrocarbon systems is also discussed.In a prior paper [l], we reported preliminary single-pulse shock tube kinetic results on the silane decomposition. We showed that the initial reaction of the decomposition is molecular Hz elimination [reaction (l)]+ (1)Production of hydrogen atoms in the overall reaction was suggested by large yields of HD found in the pyrolysis of SiD4 in the presence of excess toluene (see Table I). Silylene decomposition [reaction (2)] was postulated to be the source of the D atoms [l]. However, we now believe that this conclusion n + The simple bond rupture process, SiH4 -SiH3 + H, was eliminated as a possible initiation reaction because its high activation energy (93 kcal) would require a chain process with chain lengths in excess of lo6 in order to match observed reaction rates. Such long chains under shock conditions are clearly impossible. Thus one calculates that on average fewer than 50 collisions between silane (0.01%) and product molecules occur in a typical 200 Fsec shock period.

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Section: (7)mentioning

confidence: 99%

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confidence: 99%

Kinetics and mechanism of the silane decomposition

Newman

O’Neal

Ring

et al. 1979

Int J of Chemical Kinetics

184

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“…However, the available experimental data are limited, and are usually average bond energies derived from heats of formation . Even here, bomb calorimetry is plagued by the formation of multiple products, − and determinations derived from the heats of hydroboration reactions must correct for products with differing regiochemistry. , The values that are most relevant to reaction chemistry are individual, sequential bond energies. These values correspond to the average bond energies derived from heats of formation only if each, sequential bond energy is similar.…”

Section: Introductionmentioning

confidence: 99%

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

1996

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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.

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“…However, experimental bond dissociation energies (BDE's) are often difficult to obtain . For instance, the presence of multiple isomers and uncertainty regarding the true coordination numbers of borane species contribute to ambiguity in the interpretation of experimental results . Furthermore, bomb calorimetry, which has been used to derive BDE's, yields information only about average bond energies…”

Section: Introductionmentioning

confidence: 99%

Large Effect on Borane Bond Dissociation Energies Resulting from Coordination by Lewis Bases

Rablen

1997

J. Am. Chem. Soc.

100

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Ab initio molecular orbital calculations at the G-2 and CBS-4 levels of theory were used to determine homolytic bond dissociation energies (BDE's) for the B−H bonds in a series of donor−acceptor complexes of borane. The B−H bonds of four-coordinate boron were found to be weaker than those of three-coordinate boron. The effect of complexation on BDE varied from very small, in the cases of NH3 and H2O, to as much as 30−50 kcal/mol, in the cases of formaldehyde, CO, and HCN. The BDE's were not found to correlate with the strength of coordination. However, they were closely correlated with the degree to which spin density in the radical was delocalized away from boron and onto the associated Lewis base. The presence of either a π system or, to some extent, a second-row element such as phosphorus or sulfur promoted such delocalization. Delocalization of spin density onto carbon appeared to be particularly favorable, and to correlate with particularly low B−H BDE's. The BDE's in 4-coordinate complexes of borane with various larger ligands, including typical ethers and amines, followed patterns almost identical with those in the smaller species, and could be understood according to the same principles.

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An Electron Impact Investigation of Some Organoboron Difluorides

Cited by 21 publications

References 0 publications

Kinetics and mechanism of the silane decomposition

Kinetics and mechanism of the silane decomposition

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

Large Effect on Borane Bond Dissociation Energies Resulting from Coordination by Lewis Bases

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