Dissociation energy calculations

We will now look at how different types of wave functions behave when the O-H bond is stretched. The basis set used in all cases is the aug-cc-pVTZ, and the reference curve is taken as the [8, 8J-CASSCF result, which is slightly larger than a full-valence Cl. As mentioned in Section 4.6, this allows a correct dissociation, and since all the valence electrons are correlated, it will generate a curve close to the full Cl limit. The bond dissociation energy calculated at this level is 122.1 kcaPmol, which is comparable to the experimental value of 125.9 kcal/mol. 

Curves showing these two quantities as functions of rAB are given in Figure 42-2. It is seen that 1ps corresponds to attraction, with the formation of a stable molecule-ion, whereas pA corresponds to repulsion at all distances. There is rough agreement between observed properties of the hydrogen molecule-ion in its normal state and the values calculated in this simple way. The dissociation energy, calculated to be 1.77 v.e., is actually 2.78 v.e., and the equilibrium value of rAB, calculated as 1.32 A, is observed to be 1.06 A. 

Table. 2. Bond dissociation energies calculated for 2,2,5-trimethylheptane compared with corresponding experimental value (a) Ref. 32 (b) Ref. 33...

The (at least approximate) validity of Eqs. (6.12)-(6.14) is clearly demonstrated by the remarkable quality of intrinsic CN bond energies and of bond dissociation energies calculated by means of nitrogen net atomic charges deduced in this manner. 

Figure 21-11 Comparison of calculated exact" ab initio energies of H2 as a function of internuclear distance, r, with the energies calculated for simple MO and simple VB methods. The dissociation energy calculated by the ab initio procedure is in close agreement with the experimental value of 102 kcal. The zero of the energy scale in this figure is the energy of widely separated hydrogen atoms.

From a quantitative point of view, the high abundance of undissociated HeH+ ions measured in the mass spectrometer shows conclusively that the excitation of the daughter ions through the orbital electron shaking is a relatively inefficient mechanism in the decay of this simple molecule. The high stability of the HeH" molecular ion is not surprising, since it is electronically almost equivalent to the HeH+ ion, observed in the mass spectrometer since the early 1920 s, and which has been the subject of many theoretical calculations as the most simple two-electron heteronuclear molecule. Its dissociation energy, calculated by several... 

Tao, F.-M., The counterpoise method and bond functions in molecular dissociation energy calculations, Chem. Phys. Lett. 206, 560-564 (1993). 

From bond dissociation energy calculated by Schroeder et Ab initio calculations by Lee et al. ... 

This reaction is analyzed along with limited equilibrium data for three other reactions. The latter are in satisfactory agreement with reaction A. Dissociation energies calculated from our adopted values ( ) are 13.9 kcal mol" larger than those of the corresponding diatomic molecules l.e., (FAl-0) = 133.9 4, (OAl-P) = 173.2 4, or A H = 13.9 5 kcal mol" for the... 

Problem 6.12 (a) What would the allyl radical have to do to return to the starting material (b) From bond dissociation energies, calculate the minimum for this reaction. 

Given the following bond-dissociation energies, calculate the average bond enthalpy for the Ti Cl hond. 

Table 5.1. Gaseous monomeric alkalimetal halides, MX(g) experimental electric dipole moments, /Tgi electric dipole moments predicted by the spherical ion model, /rel(calc) equihbiium bond distances, Rg vibrational wavenumbers, ox, dissociation energies at zero K, Dq reduced masses, /xm force constants,/r dissociation energies calculated from the spherical ion model according to equation (5.16a), Do(calc).

The dissociation energies calculated from equations (5.16) or (5.16a) depend on the value chosen for the exponent , but the reader may convince herself that a change of n from 10 to 12 has little effect. 

Problem 5.4 LiH is a colorless solid at room temperature. The melting point is about 700 and the melt conducts electricity. Both the solid and the hquid are therefore believed to consist of Li" " and H (hydride) ions. The electric dipole moment of the gaseous LiH is 5.88 D, the dissociation energy at zero K is 234 kJ moP, the bond distance is 160 pm, and the vibrational wavenumber 1406 cm. Calculate the electric dipole moment from the spherical ion model. Show that the dissociation energy calculated from the spherical ion model is in good agreement with the experimental if the exponent in the Born repulsion term is reduced from n = 10 to 5. Is there any reason... 

The dissociation energies calculated in this manner are listed in the last column in Table 14.1. Comparison with the electronic dissociation energies listed in Table 14.2 shows that the E-E bonds formed by combination of the fragments in their triplet state are significantly stronger than the single bonds in the alkane analogs. We conclude, therefore, that the E-E bonds... 

The dissociation energies calculated with the exponent n = 10 have been listed in reference [15]. The calculated dissociation energy of LiF is higher than the experimental value all other calculated dissociation energies are too low. The largest deviations, about 15%, are found for the two iodides Lil and Csl. The average deviation for the 20 alkali metal halides is 8%, as compared to the 2.5% reported for the crystalline phase. 

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