Total dissociation energy

This computed quantity is also sometimes synonymously referred to as the total dissociation energy IDg). 

Ab initio energies are total dissociation energies (dissociation to electrons and atomic nuclei) and AMI and PM3 energies are standard heats of formation. Is one of these kinds of energy more useful Why or why not ... 

The summation of the energies for the four discrete steps, which gives the total dissociation energies, was found to be a more realistic measure of the activity of the metal in methane dissociation. The calculations of these sums carried out by Au et revealed that the total dissociation of methane on Rh is thermodynamically more favoured on Rh than on other transition metals, and the values varied in the order Rh < Ru < Ir < Os Pt < Pd. This trend is similar to that obtained by Schmidt et in the partial oxidation of CH4... 

In the first order approximation (assuming the additivity of interaction components) the total dissociation energy of tiie cluster XYn can be expressed in terms of the interaction energy... 

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. 

Even within a particular approximation, total energy values relative to the method s zero of energy are often very inaccurate. It is quite common to find that this inaccuracy is almost always the result of systematic error. As such, the most accurate values are often relative energies obtained by subtracting total energies from separate calculations. This is why the difference in energy between conformers and bond dissociation energies can be computed extremely accurately. 

Physical Properties. Properties of some alkyl peroxyesters are Hsted in Table 13 and the properties of some alkyl areneperoxysulfonates are given in Table 14. Mass spectra (226), total energies, and dipole moments (227) oxygen—oxygen bond-dissociation energies (44,228) and boiling points, melting points, densities, and refractive indexes (44,168,213) have been reported for a variety of tert-huty peroxycarboxylates. 

The top table gives the predicted total energies for each molecule, and the bottom table lists the computed dissociation energies and AH. All three model chemistries do pretty well on all three phases of the process, with G2 and CBS-Q generally modeling it very accurately. The CBS-Q values are the most accurate, and they take only about half as long to compute as G2 theory. CBS-4 performs well for O2 and for the overall AH at substantially less cost about one sixth the cost of G2 and one third the cost of CBS-Q. ... 

The factor Dg can either be determined from the dissociation energy and the ground state vibration energy or from thermodynamic data. The heat of formation of H atoms from H2 molecules can be found in the literature, but some care should be exercised in considering the total energy content of H atoms and H2 molecules under standard conditions. 

Extraction of the speed distribution is achieved in an analogous manner by integrating over all angles for each speed. The speed distributions can be further transformed, using the law of conservation of momentum, into total translational energy distributions for the O3 — O2(X3S ) + 0(3Pj) dissociation. 

Fig. 13. The total translational energy distributions for the dissociation of 03 to 0(3Pj) + 02(X%-) at 226, 230, 233, 234, 240 and 266nm. The vibrational levels of the 02(X3S ) fragment are indicated by the combs. The dotted curves represent the uncertainty in the signal intensity arising from counting statistics.

Substituting R0 back into Equation (3.9) gives the total energy, U = -121/50 = 33 eV., compared with the experimental value of 32 eV., and the dissociation energy is D = 5.7 eV., compared with the experimental 4.7 eV. 

Next, we discuss the J = 0 calculations of bound and pseudobound vibrational states reported elsewhere [12] for Li3 in its first-excited electronic doublet state. A total of 1944 (1675), 1787 (1732), and 2349 (2387) vibrational states of A, Ai, and E symmetries have been computed without (with) consideration of the GP effect up to the Li2(63 X)u) +Li dissociation threshold of 0.0422 eV. Figure 9 shows the energy levels that have been calculated without consideration of the GP effect up to the dissociation threshold of the lower surface, 1.0560eV, in a total of 41, 16, and 51 levels of A], A2, and E symmetries. Note that they are genuine bound states. On the other hand, the cone states above the dissociation energy of the lower surface are embedded in a continuum, and hence appear as resonances in scattering experiments or long-lived complexes in unimolecular decay experiments. They are therefore pseudobound states or resonance states if the full two-state nonadiabatic problem is considered. The lowest levels of A, A2, and E symmetries lie at —1.4282,... 

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