Solution phase bond dissociation enthalpy

According to the definition of the A-B bond dissociation enthalpy, reactants and products in reaction 5.1 must be in the gas phase under standard conditions. That is to say that those species are in the ideal gas phase, implying that inter-molecular interactions do not exist. DH (A-B) refers, therefore, to the isolated molecule AB, and it does not contain any contribution from intermolecular forces. Though this is obviously the correct way of defining the energetics of any bond, there are many literature examples where bond dissociation enthalpies have been reported in solution. 

In order to distinguish solution from gas-phase bond dissociation enthalpies, we shall use the subscript sin. Thus, DHjsln( A-B) represents the standard enthalpy of the reaction in solution, where the only event is the cleavage of the A B bond, at a given temperature ... 

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 5.2 Experimental data for the PhO-H bond dissociation enthalpy, in solution (only photoacoustic calorimetry values) and in the gas phase. A recommended gas-phase value is indicated by the solid line and its error limit by dashed lines. Adapted from [75],...

Note that the values in table 5.2 are not absolute solution phase bond dissociation enthalpies. Because the purpose of the exercise is to probe the substituent... 

There is an additional advantage in using relative solution phase bond dissociation enthalpies. In most cases, solution phase bond dissociation enthalpies do not refer to standard states (see section 2.3), and the required corrections are hard to predict. When we consider relative bond dissociation enthalpies in a series of similar molecules in solution it is likely that the unknown corrections to standard states are nearly constant. 

Though some more traditional thermodynamicists will be dismayed by the concept of solution phase bond dissociation enthalpy, the fact is that the database involving these quantities is growing fast. When used judiciously, they may provide important chemical insights—as is indeed the case for the stability of the O-H bond in phenolic compounds. Although solution phase bond dissociation enthalpies are not true bond dissociation enthalpies, because they include some contribution from intermolecular forces, a series of solution values like those in table 5.2 may be (and often is) taken as a good approximation of the trend in the gas-phase. 

As the enthalpy of reaction 13.34 can be given as a solution phase bond dissociation enthalpy difference,... 

In this chapter, we have collected and discussed the available data in both gas and liquid phases related to Scheme 2.1. Emphasis will be given to homolytic bond dissociation enthalpies of silanes. Generally, Df/values are extrapolated from the gas phase to solution without concerning solvent effects (particularly in the... 

In organometallic reactions it is often possible to make measurements or reasonable estimates of vaporization and solution enthalpies, in order to derive the enthalpy of reaction in the gas phase. Yet, the situation where only one bond dissociation enthalpy is unknown is rather uncommon. Despite these shortcomings, classical reaction-solution calorimetry, being a well tested, inexpensive, reliable technique, has provided an outstanding contribution to our present knowledge in the area of transition metal organometallic thermochemistry. 

Fig. 6 and 7 display typical results for the effect of solvent on the excitation spectra, fluorescence and lifetimes. We used these observations to measure bond dissociation energy of the different species by varying the excess vibrational energy of the parent jet-cooled molecule type modes. The threshold effect found for the dissociation of the 1 1 IQ (methanol) is evident and occurs at ca. 3 kcal mol , in agreement with solution-phase enthalpies. (The effect of higher degree of solvation on the dynamics will be discussed elsewhere.)... 

The crystal structure of acetic acid shows that the molecules pair up into dimers connected by hydrogen bonds. The dimers can also be detected in the vapour at 120 °C. They also occur in the liquid phase in dilute solutions in non-hydrogen-bonding solvents, and a certain extent in pure acetic acid, but are disrupted by hydrogen-bonding solvents. The dissociation enthalpy of the dimer is estimated at 65.0-66.0 kJ/mol, and the dissociation entropy at 154-157 J mol K This dimerization behaviour is shared by other lower carboxylic acids. 

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