Homolytic bond energy values

The crosses refer to anions where the donor atom is from the third, fourth, or fifth row. These data points fall regularly below the line I > Br Cl- SeH- > SH- AsH2- > PH2- and GeH3- > SiH3-. These nucleophiles are all weaker bases than their ease of oxidation would suggest. This behavior is easily traced to low values of the homolytic bond energy of HX. 

For simple aliphatic amines, like the methylamines, there is a linear inverse correlation between proton affinities and vertical IPs ". A low IP value should therefore indicate high proton affinity and vice versa. The proton affinity PA(B) of a molecule B is related to the homolytic bond energy D(B+—H) in the conjugate acid, as indicated by equation 8. If the homolytic bond dissociation energy is assumed to be constant for a particular functional group, e.g. N—H, the proton affinity wiU exhibit a linear correlation with the quantity IP(H) — IP(B), and such correlations have been reported for the proton affinities and the nitrogen lone-pair ionizations . ... 

Bordwell et al., 1988, 1989) and Amett (Amett et al., 1990a,b, 1992 Venimadhavan et al., 1992) have employed thermodynamic cycles consisting of heterolysis of a molecule and redox processes of the resulting ions to evaluate homolytic dissociation energies of C—H, C—C, C—N, C—O and C—S bonds in solution. In a similar way, knowledge of the A//het(R-R ) values allows determination of the heat of homolysis of carbon-carbon bonds [A/fhomo(R"R )] using (27). The results are summarized in Table 4. 

Knowledge of homolytic bond dissociation energies (BDEs) is critically important for understanding radical chemistry. The bond energies of organic compounds have been reviewed extensively, but we will use recommended R-H BDE values for organic compounds given in a recent excellent... 

The pKi, values of a series of para- and meffl-substituted benzaldoximes and phenyl methyl ketoximes, ArCR=NOH (R=H, Me), have been measured in DMSO. The aldoximes exhibit pK. = 20.05 + 3.21ap. The homolytic bond dissociation energy of the O-H bond has been estimated as 88.3 (aldoximes) and 89.2kcal mol" (ketoximes) by relating the pK to the oxidation potential of the conjugate base (i.e. ox for ArCR=NO- ArCR=NO ). 

Several directly measured values of AH° for homolytic dissociation of a metal-metal-bonded carbonyl in solution have been obtained (9). This was for the complexes [(n3-C3H5)Fe(CO)2 )2 where L = CO or a number of different P-donor ligands. The low value AH = 56.5 kJ mol-1 when L = CO was not unexpected for such a sterically crowded molecule. The P-donor substituents increased the steric crowding and displaced the equilibria in favor of the monomers but the effect seemed to be controlled more by AS° than AH°. In general metal-metal bond energies, however they may have been estimated, are too large to allow for direct measurement of equilibrium constants in solution in this way. 

Carbon-centered radicals play an important role in organic synthesis, biological chemistry, and polymer chemistry. The radical chemistry observed in these areas can, to a good part, be rationalized by the thermodynamic stability of the open shell species involved. Challenges associated with the experimental determination of homolytic bond dissociation energies (BDEs) have lead to the widespread use of theoretically calculated values. These can be presented either directly as the enthalpy for the C-H bond dissociation reaction described in Equation 5.1, the gas-phase thermodynamic values at the standard state of 298.15K and 1 bar pressure being the most commonly reported values. 

In QSAR 1.113, 62% of the variance is accounted for by and 28% is explained by log P. It appears that free-radical-mediated toxicity is responsible for the growth-inhibitory effects of the phenols. Homolytic bond dissociation energies related to the homolytic cleavage of the OH bond in the following reac-tion (X—C6H4OH + CeHsO. X—CgH O. + CeHgOH) have been used in lieu of values. The net result is similar, as seen in QSAR 1.114(242). 

Since the dissociation energy of X 2 ArF (reaction 6) represents a homolytic bond cleavage with the transition of a closed-shell species into two open-shell fragments, the Z) value... 

In a landmark paper, Breslow and coworkers described the determination of pA), values of weak hydrocarbon acids by use of thermochemical cycles involving electrochemical reduction data for triarylmethyl, cycloheptatrienyl, and triphenyl- and trialkylcyclopropenyl cations and radicals [9aj. Later, they derived pATa data from standard oxidation potentials and bond-dissociation energies [9b, c]. The methodology was further developed by Nicholas and Arnold [10a] for the determination of cation radical acidities, and later modified and extensively used by Bordwell and coworkers [10b, c] so that homolytic bond-dissociation energies and cation radical... 

The electrical conductivity a of liquid sulfur increases with temperature except near the viscosity maximum of ca. 170 °C where a minimum of the conductivity is observed. Above 200 °C the plot of log a vs 1/T was found by several authors to be linear but the slopes of these linear relationships as well as the absolute conductivities vary considerably [118-122]. On the assumption that the conductivity at these temperatures is intrinsic, values of about 1.6 eV were derived for the activation energy at high temperatures (up to 900 °C) [121, 122], an energy which is much higher than the activation energy for the formation of free spins by homolytic bond dissociation (see above). 

H-shift (Reaction (3.15)) was considered feasible based on (i) the analogy to the well-known, solvent-assisted 1,2-H-shift within alkoxy radicals [19] and (ii) the exo-thermicity based on the homolytic bond dissociation energies (BDEs) of the N-H (406 kj mol ) and the C-H bond (363 kj mol ) (representative values for the Gly anion [47]). However, both pulse radiolysis and y-radiolysis experiments concluded that the 1,2-H-shift in aminyl and amidyl radicals derived from amino acids and peptides must be rather slow (kis 1.2 x 10 s ) [37, 40]. 

A simple consideration of typical values for N-N bond energies shows that they are in the range 1.5 to 3.0 eV (taken from hydrazine derivatives) [184], Therefore, both the 248-nm (5 eV) and the 308-nm (4 eV) excimer lasers are capable of breaking this bond directly. For aryl dialkyl triazene compounds, which exhibit a similar structural unit, a radical pathway of decomposition was reported [119, 140, 153]. The first step in this pathway is the homolytic bond cleavage between the N-N bond, creating a labile diazo... 

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