Homolytic bond dissociation energies calculating

Local HSAB principle can also be used to calculate the relative homolytic bond dissociation energies (BDE). For the homolytic dissociation of para-substituted phenols ... 

The semiempirical AMI MO method has been used to calculate heats of formation of a series of m- and p-substituted benzene and toluene derivatives ArY and ArCHaY, and their phenyl or benzyl cations, anions, and radicals heterolytic and homolytic bond dissociation energies (BDEs) and electron transfer energies for the ions have also been calculated and the relationship A//het = A//et-I-AWhomo has been confirmed (it being noted that A//homo is insensitive to ring substituents). The linear relationship found between and the appropriate HOMO or LUMO... 

It is clear that proper description of the energetics of homolytic bond dissociation requires models that account for electron correlation. Are correlated models also needed for accurate descriptions of relative homolytic bond dissociation energies where the relevant reactions are expressed as isodesmic processes A single example suggests that they may not be. Table 6-15 compares calculated and measured CH bond dissociation energies in hydrocarbons, R-H, relative to the CH bond energy in methane as a standard ... 

Overall, the performance of Hartree-Fock models is very poor. In most cases, activation energies are overestimated by large amounts. This is not surprising in view of previous comparisons involving homolytic bond dissociation energies (see Table 6-2), which were too small. In terms of mean absolute deviation from the standard (MP2/6-311+G ) calculations, STO-3G yields the poorest results and 3-2IG the best results. 6-3IG and 6-311+G models provide nearly identical activation energies (just as they did for transition-... 

Solve the HMO equations for the orbitals and orbital energies of the C—C and C— bonds assume that h(O) = h(C) — hco, hcc = hco, and Sco = 0. Sketch the results in the form of an interaction diagram. Which bond is stronger Calculate the homolytic bond dissociation energies in units of hcc - What is the net charge on O, assuming that it arises solely from the polarization of the bond ... 

The homolytic bond dissociation energies (BDEs) of phenohc O—H bonds has been the subject of a computational study focusing on substituent effects by ab initio and density functional theory (DFT) methods.6 Consistent overestimation of the BDEs by MP2 and MP4 calculations was associated with spin contamination in the reference UHF wave functions, whilst the DFT calculations (particularly the B3LYP/6-31G level of theory) were relatively unaffected. Ab initio calculations of the photosensitized C—C BDEs of /f-phenethyl ethers has revealed a significant configurational... 

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. 

For the first two peptides, CysS radicals abstract hydrogen atoms from the or-carbon of glycine with 7 = (1.0 to 1.1) x 10 s , while the reverse reaction proceeds with = (8.0 to 8.9) x 10 s . For the latter peptide, CysS radicals abstract hydrogen atoms from the ce-carbon of alanine with = (0.9 to 1.0) x 10 s while the reverse reaction proceeds with k -j = 1.0 x 10 s" The order of reactivity, Gly > Ala, is in accordance with previous studies on intermolecular reactions of thiyl radicals with these amino acids. The fact that < k y suggests that some secondary structure prevents the adoption of extended conformations for which calculations of homolytic bond dissociation energies would have predicted k j > k y. 

Density functional theory has been used to calculate the P-E bond energies and orbital populations of trimethylphosphine chalcogenides and related compounds. The results indicate that Me3P=CH2 has a 7i-bond order of 0.5. The equilibrium acidities in DMSO solution and gas-phase homolytic bond dissociation energies of tributylphosphonium ylide precursors have also been determined. ... 

Homolytic bond dissociation energies (BDEs) for the detachment of a hydrogen atom (radical), are shown in Table 18. This type of comparison is expected to be less accurate than PAs, because different numbers of pairs of electrons are involved on the two sides of equation 266. For example, the gas-phase BDE of H20 is calculated from Table 17 at 114 kcal mol"1, where the experimental value, corrected for zero-point vibrational energy which was not taken into account in the theoretical calculation, is 126 kcal mol"15. Thus, as expected, the calculation underestimates BDEs because of the relatively poorer description of the B—H system relative to B- + H A larger basis set and higher level of theory are required to achieve better accuracy for BDEs68. Nonetheless, we can examine the BDE values as a function of substituent X, and compare sulphur-bonded to oxygen-bonded H atom dissociation. 

Calculated ionization energies, electron affinities, proton affinities, homolytic bond dissociation energies and other thermochemical quantities are tabulated and discussed. Agreement with experiment, where available, is very reasonable. 

Which approach is correct Usually our measure of the correctness of any calculation is how accurately it reproduces a known physical property. In the case of H2, a relevant property is the homolytic bond dissociation energy. The simple VB calculation described here gives a value of 3.14 eV (72.4 kcal/mol) for H2 dissociation. The simple MO calculation gives a value of 2.70 eV (62.3 kcal/mol). The experimental value is 4.75 eV (109.5 kcal/mol). Obviously, neither calculation is correct unless one takes order of magnitude agreement as satisfactory in that case, both calculations are correct. 

The homolytic bond dissociation energies of a variety of covalent bonds have been determined experimentally or calculated from related data. Some of these DH° values are listed in Table 10.1. 

A How to Use Homolytic Bond Dissociation Energies to Calculate Heats of Reaction... 

The S-S bond dissociation energies of H2S2, H2S3 and H2S4 have been studied by Steudel et al. at the GCSD(T)//6-311 G(2df,p) level [42]. The calculated enthalpies AH° for the dissociation at the central bonds at 298 K are 247, 201 and 159 kJ mol respectively. The lower stability of the tri- and tetrasulfanes towards homolytic S-S cleavage is attributed to the stability of the generated HSS radical which is stabilized by the formation of a three-electron n bond. 

Energies for a selection of homolytic bond dissociation reactions of two-heavy-atom hydrides are provided in Table 6-2. These have been drawn from a larger collection found in Appendix A6 (Tables A6-1 to A6-8). A summary of mean absolute deviations from G3 calculations (based on the full collection) is provided in Table 6-3. 

Table 6-3 Mean Absolute Deviations from G3 Calculations in Energies of Homolytic Bond Dissociation Reactions... 

This inability of Hartree-Fock calculations to model correctly homolytic bond dissociation is commonly illustrated by curves of the change in energy as a bond is stretched, e.g. Fig. 5.19. The phenomenon is discussed in detail in numerous expositions of electron correlation [82]. Suffice it to say here that representing the wavefunction as one determinant (or a few), as is done in Hartree-Fock theory, does not permit correct homolytic dissociation to two radicals because while the reactant (e.g. H2) is a closed-shell species that can (usually) be represented well by one determinant made up of paired electrons in the occupied MOs, the products are two radicals, each with an unpaired electron. Ways of obtaining satisfactory energies,... 

Bond dissociation energies, described in Chapter 5, measure the energy required to homolytically break a bond. They are not the same as dissociation enthalpies, which measure the ability of a compound to dissociate heterolytically. Bond dissociation energies can be used to calculate dissociation enthalpies in the gas phase if other quantities are also known. 

The RSE is calculated here as the difference between the homolytic C-C bond dissociation energy in ethane (5) and a symmetric hydrocarbon 6 resulting from dimerization of the substituted radical 2. By definition the C-C bonds cleaved in this process are unpolarized and, baring some strongly repulsive steric effects in symmetric dimer 6, the complications in the interpretation of substituent effects are thus avoided. Since two substituted radicals are formed in the process, the reaction enthalpy for the process shown in Equation 5.5 contains the substituent effect on radical stability twice. The actual RSE value is therefore only half of the reaction enthalpy for reaction 5.5 as expressed in Equation 5.6. 

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