Dissociation energies, estimation

The new dinitrogen complex [Ni(CO)3N2] can be generated in a pressure cell by UV photolysis of tetracarbonylnickel in liquid krypton, doped with N2 at 114K. The decomposition of this complex was followed over the temperature range 122-127 K and a value of the Ni—N2 bond dissociation energy estimated at lOkcal moN1.2474... 

Compounds containing Mo—Mo, W—W, or Related Bonds.— Some tentative spectral evidence has been obtained for the formation of Mo2 dimers when isolated molybdenum atoms are allowed to diffuse through an Ar matrix.273 The kinetics of the formation and dissociation of W2 and WRe adatom species have been studied by field ion microscopy and their dissociation energies estimated as 30 5 and 14 + 2 kJ mol-1, respectively, at ca. 400K.395... 

The M H dissociation energies see Bond Dissociation Energy) estimated from ion beam experiments are quite... 

Table 5.2. Selected M-L bond dissociation energy estimates (gas-phase values unless Indicated otherwise, D = BDE in kcal/mol).

Mn) is much less than D(CHjC(=0)-Mn) although the analogous D(R-H) and D(R-CH3) are approximately equal. Bond dissociation energies estimated for R=CH3 and CF3 for the cation show that reaction (48) is somewhat easier than (47) ... 

For many purposes, for example the estimation of approximate heats of formation (p. 63), it is sufficient to have an average value. This average of the bond dissociation energies is called the average thermochemical bond energy or (more commonly) simply the bond energy. ... 

Emphasis was put on providing a sound physicochemical basis for the modeling of the effects determining a reaction mechanism. Thus, methods were developed for the estimation of pXj-vahies, bond dissociation energies, heats of formation, frontier molecular orbital energies and coefficients, and stcric hindrance. 

A more useful quantity for comparison with experiment is the heat of formation, which is defined as the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states. The heat of formation can thus be calculated by subtracting the heats of atomisation of the elements and the atomic ionisation energies from the total energy. Unfortunately, ab initio calculations that do not include electron correlation (which we will discuss in Chapter 3) provide uniformly poor estimates of heats of formation w ith errors in bond dissociation energies of 25-40 kcal/mol, even at the Hartree-Fock limit for diatomic molecules. 

Somewhat surprisingly perhaps, it has been found that [l.l.l]propellane is considerably less reactive than [2.2.1]propellane. Use the theoretically calculated enthalpy data below to estimate the bond dissociation energy of the central bond in each of the three propellanes shown. How might this explain the relative reactivity of the [1-1.1]- and [2.2. Ijpropellanes ... 

From this value and known C—H bond dissociation energies, pK values can be calculated. Early application of these methods gave estimates of the p/Ts of toluene and propene of about 45 and 48, respectively. Methane was estimated to have a pAT in the range of 52-62. Electrochemical measurements in DMF have given the results shown in Table 7.3. These measurements put the pK of methane at about 48, with benzylic and allylic stabilization leading to values of 39 and 38 for toluene and propene, respectively. The electrochemical values overlap with the pATdmso scale for compounds such as diphenyl-methane and triphenylmethane. 

Bond dissociation energies such as those in Table 12.6 are also useful for estimation of the energy balance in individual steps in a free-radical reaction sequence. This is an... 

It is estimated that each year approximately 150 million tonnes of nitrogen are fixed biologically compared to 120 million tonnes fixed industrially by the Haber process (p. 421). In both cases N2 is converted to NH3, requiring the rupture of the N=N triple bond which has the highest dissociation energy (945.41 kJmol )... 

Where no data exist, one wishes to be able to estimate thermochemical quantities. A simple and convenient method to do that is through the use of the method of group additivity developed by Benson and coworkers15,21 22. The earlier group values are revised here, and new group values calculated to allow extension of the method to sulfites and sulfates. In addition, a method based on the constancy of S—O bond dissociation energies is applied. 

However, the fact that the derived S—O bond dissociation energy in sulfuric acid is identical to that found in the acid derivatives, strongly supports the estimated enthalpy of formation for gas-phase sulfurous acid given by Benson18. 

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