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H Bond Dissociation Energies (h + bond_dissociation_energy)
Selected Abstracts, -Substituent Effects on Si,H, P,H and S,H Bond Dissociation EnergiesCHINESE JOURNAL OF CHEMISTRY, Issue 3 2006Yao Fu Abstract CBS-Q and G3 methods were used to generate a large number of reliable Si,H, P,H and S,H bond dissociation energies (BDEs) for the first time. It was found that the Si,H BDE displayed dramatically different substituent effects compared with the C,H BDE. On the other hand, the P,H and S,H BDE exhibited patterns of substituent effects similar to those of the N,H and O,H BDE. Further analysis indicated that increasing the positive charge on Si of XSiH3 would strengthen the Si,H bond whereas increasing the positive charge on P and S of XPH2 and XSH would weaken the P,H and S,H bonds. Meanwhile, increasing the positive charge on Si of XSiH2· stabilized the silyl radical whereas increasing the positive charge on P and S in XPH· and XS· destabilized P- and S-centered radicals. These behaviors could be reasonalized by the fact that Si is less electronegative than H while P and S are not. Finally, it was demonstrated that the spin-delocalization effect was valid for the Si-, P- and S-centered radicals. [source] Determination of the electron affinities of ,- and ,-naphthyl radicals using the kinetic method with full entropy analysis.JOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 6 2001H bond dissociation energies of naphthalene, The C Abstract The C , H bond dissociation energies for naphthalene were determined using a negative ion thermochemical cycle involving the gas-phase acidity (,Hacid) and electron affinity (EA) for both the ,- and ,-positions. The gas-phase acidity of the naphthalene ,- and ,-positions and the EAs of the ,- and ,-naphthyl radicals were measured in the gas phase in a flowing afterglow,triple quadrupole apparatus. A variation of the Cooks kinetic method was used to measure the EAs of the naphthyl radicals by collision-induced dissociation of the corresponding ,- and ,-naphthylsulfinate adducts formed by reactions in the flow tube portion of the instrument. Calibration references included both , and , radicals, and full entropy analysis was performed over a series of calibration curves measured at collision energies ranging from 3.5 to 8 eV (center-of-mass). The measured EAs are 33.0 ± 1.4 and 31.4 ± 1.0 kcal mol,1 (1 kcal = 4.184 kJ) for the ,- and ,-naphthyl radicals, respectively. The gas-phase acidities for naphthalene were measured by the DePuy silane cleavage method, which utilizes the relative abundances of aryldimethylsiloxides and trimethylsiloxide that result from competitive cleavages from a proposed pentacoordinate hydroxysiliconate intermediate. The measured acidities are 394.0 ± 5.0 and 397.6 ± 4.8 kcal mol,1 for the ,- and ,- positions, respectively. The C , H bond dissociation energies calculated from the thermochemical cycle are 113.4 ± 5.2 and 115.4 ± 4.9 kcal mol,1 for the ,- and ,-positions, respectively. These energies are, to within experimental error, indistinguishable and are approximately the same as the first bond dissociation energy for benzene. Copyright © 2001 John Wiley & Sons, Ltd. [source] Solvation effects of H2O and DMSO on the O,H bond dissociation energies of substituted phenolsJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 4 2004Yao Fu Abstract Solvation effects on the O,H homolytic bond dissociation energies (BDEs) of substituted phenols were studied. It was demonstrated that the BDEs measured in solution in general do not equal the BDEs in the gas phase. Detailed theoretical analyses indicated that a long-range solvation effect (i.e. the interaction between the solvent and the overall dipole moment of the solute) and a short-range solvation effect (i.e. the hydrogen bonding between the solute and solvent) were both important for the O,H BDEs in water and in DMSO. Neither one of these two factors by itself could fully explain the experimentally observed solvation effect. However, a combination of these two factors, estimated through a semi-continuum solvation model, was shown to be reasonably successful in explaining the experimental results. Copyright © 2004 John Wiley & Sons, Ltd. [source] , -Substituent Effects on Si,H, P,H and S,H Bond Dissociation EnergiesCHINESE JOURNAL OF CHEMISTRY, Issue 3 2006Yao Fu Abstract CBS-Q and G3 methods were used to generate a large number of reliable Si,H, P,H and S,H bond dissociation energies (BDEs) for the first time. It was found that the Si,H BDE displayed dramatically different substituent effects compared with the C,H BDE. On the other hand, the P,H and S,H BDE exhibited patterns of substituent effects similar to those of the N,H and O,H BDE. Further analysis indicated that increasing the positive charge on Si of XSiH3 would strengthen the Si,H bond whereas increasing the positive charge on P and S of XPH2 and XSH would weaken the P,H and S,H bonds. Meanwhile, increasing the positive charge on Si of XSiH2· stabilized the silyl radical whereas increasing the positive charge on P and S in XPH· and XS· destabilized P- and S-centered radicals. These behaviors could be reasonalized by the fact that Si is less electronegative than H while P and S are not. Finally, it was demonstrated that the spin-delocalization effect was valid for the Si-, P- and S-centered radicals. [source] | ||||||||||