Bond dissociation enthalpy thermochemical cycle

Figure 5.1 Thermochemical cycle (f = 298.15 K), showing how solution and gas-phase bond dissociation enthalpies are related. AS0 VH° are standard enthalpies of solvation.

Figure 5.3 Thermochemical cycle, illustrating how the enthalpy of reaction 5.10 can be given as a bond dissociation enthalpy balance.

Figure 5.5 Thermochemical cycles relating O-H bond enthalpy contributions ( s) with bond dissociation enthalpies (DH°) in phenol and ethanol. ER are reorganization energies (see text).

We are now left with the evaluation of E s (Cr—CO), the Cr-CO bond enthalpy contribution in Cr(CO)6. The third thermochemical cycle in figure 5.6 shows how this bond enthalpy contribution can be evaluated from the Cr-CO mean bond dissociation enthalpy (107.0 0.8 kJ mol-1 see section 5.2) and the reorganization energy ER(CO ). 

Finally, the R-H bond dissociation enthalpy in the gas phase can be obtained by using the general thermochemical cycle shown in figure 5.1 (R = A and H = B), which includes the solvation enthalpies of RH and R ... 

In the gas phase, homolytic bond dissociation enthalpies (D//) relate the thermochemical properties of molecules to those of radicals while ionization potentials (IP) and electron affinities (EA) tie the thermochemistry of neutral species to those of their corresponding ions. For example, Scheme 2.1 represents the relationships between RsSiH and its related radicals, ions, and radical ions. This representation does not define thermodynamic cycles (the H fragment is not explicitly considered) but it is rather a thermochemical mnemonic that affords a simple way of establishing the experimental data required to obtain a chosen thermochemical property. 

For any species containing -XH groups there is a relationship between the redox and acidity properties and the homolytic X-H bond dissociation enthalpy as illustrated by the thermochemical cycle in scheme 1. 

Thermodynamic cycles involving standard electrode potentials obtained by cyclic voltammetry have also been used to provide thermochemical information on organometallic compounds. This so-called electrochemical method leads to Gibbs energies of reaction in solution, from which bond dissociation enthalpies may be derived using a number of auxiliary data that are often estimated. For example, the derivation of a metal-hydrogen bond dissociation enthalpy in an L MH species requires (i) an estimate of the reduction potential of in the same solvent where the experiments were carried out (ii) an estimate of the solvation entropies of L MH, L M, and H and (iii) the knowledge of the pK of... 

To find the bond enthalpy term, start by writing an equation for the dissociation of gaseous Sip4 into gaseous atoms, and then set up an appropriate thermochemical cycle that incorporates Af ff°(SiF4,g). 

The methyl radical has a small electron affinity (1.8 0.7kcalmol and this has been combined with the bond dissociation energy (BDE) of methane and the ionization potential of H atoms to give the enthalpy for the deprotonation of methane using the thermochemical cycle (equation 24). For most alkyl radicals the electron affinities are... 

The bond dissociation energies D(F2N-F), D(FN-F), and D(N-F) have been obtained from appearance potentials (AP) of fragment ions in mass spectra combined with ionization potentials (IP) of the neutral precursors, thermochemical values for atomization enthalpies (AHgt), and standard enthalpies of formation (AH°) of NF3 and NFg [1 to 3]. They have also been obtained from thermochemical cycles alone [4,5]. D(F2N-F) was also directly determined from the equilibrium constant of the gas-phase reaction NF3 Np2+ F [6]. The values are compiled in Table 4. 

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