Bond dissociation energy from thermochemical data

The indirect method was put forward by Stevenson in it the appearance potentials of the same ion produced from two different but related molecules are measured, and their difference is combined with thermochemical data to give the required dissociation energ) It is thus possible to calculate bond dissociation energies from appearance potentials and thermochemical data without a knowledge of ionization potentials. The essence of the method is to produce as the non-ionized partner in a dissociative ionization process of the type... 

RSE values can also be calculated from experimentally measured X - H bond dissociation energies or heats of formation (where available). In order to be directly comparable to the RSE values calculated at the ROMP2 or G3(MP2)-RAD level described above, this requires thermochemical data for the species in Eqs. 1-4 at 0 K. One straightforward approach is the back correction of experimentally measured heats of formation at 298.15 K to 0 K values using thermochemical corrections calculated using the rigid ro-tor/harmonic oscillator model in combination with scaled DFT or UMP2 frequencies [19,23]. 

In the case of hexafluoroacetone, the C-F bond dissociation energy must be so high that any photodecomposition of type C would not be anticipated. There are no thermochemical or electron impact data available from which it would be possible to assess the relative importance of the processes of types A and B and this is unfortunate for the chemical evidence is conflicting. 

For the simple radicals these data are known from studies of flame reactions. Furthermore the dissociation energies of chemical bonds have been determined by thermochemical measurements. Table 20.3 provides the full information on these bond dissociation energies. 

Thermodynamic data in the area of transition metal chemistry is available, but additional studies would be desirable. One of the early indications that C—Ti bonds are not notoriously weak was obtained from the heats of combustion of Cp2Ti(CH3)2 and Cp2Ti(C6H5)2 with subsequent estimation of the a-bond dissociation energies (250 kJ/mol-1 and 350 kJ/mol 1, respectively)49. From heats of alcoholysis of a number of titanium, zirconium and hafnium compounds, and heats of solution of the products as well as subsidiary data, Lappert estimated heats of formation (AHf°) and thermochemical mean bond energy terms (EM X) of metal—X bondsso> (Table 2). 

Table 1. Carbon-carbon bond dissociation energies BDEc-c) in neutral compounds and their radical cations [kjmol" ]. Data are gas-phase numbers from thermochemical cycle calculations by Dinnocenzo [22]...

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 G—N dissociation energies, computed from thermochemical data, show a considerable scattering, which is probably due to errors in A// - of the organic azides. D(H—N3) and D(G—N3) lie close to that of other single bonds between the same atoms but somewhat... 

This relationship may be used to determine an unknown bond dissociation energy Z)(R—X ), in another molecule RX, provided that the heats of formation AH (RX. g) and J f (X, g) are known. Thus, from D(Cli —IL), AH (ll,g) and AH) m, g) value for A(CH, g) can be obtained. Used in conjunction with / /(CHgCbg) and J Ty(Cl,g) this can give a value for X>(CH3—Cl). An experimental determination of this dissociation energy would serve as a cross-check on the reliability of the thermochemical data used. 

AHf" (X) AHf" (RX). Some authors list bond strengths for OK, but here the values for 298K are given because more thermodynamic data are available for this temperature. Bond strengths, or bond dissociation energies, are not equal to, and may differ considerable from, mean bond energies derived solely from thermochemical data on molecules and atoms. 

An early report on computational estimation of the thermodynamic properties of (difluoramino)alkanes, outside of the context of the American and Russian research programs, was by inel (Bogazi i University, Turkey), who estimated heats of formation, entropies, and bond dissociation energies of several simple difluoramines [141,142], derived from pubhshed experimental data on perfluorinated amines and thermochemical relationships. 

---