Dissociation energy determination from

The nature of the molecular ionization and the final state of the dissociated cation must be understood for quantitative applications of these principles. In Table I, the bond dissociation energies for several homonuclear diatomic molecules are calculated using Equation 4 and are compared to the dissociation energies found by spectroscopic means. The dissociation energy determined from ionization energies decreases from... 

Fig. 2). Using these results a kinetics model based on the Arrhenius equation, in which the activation energy was determined from a fitting a Morse potential using the bond dissociation energy determined from the calculations ... 

Interestingly, the potential is in general rather isotropic terms of only 10-20 cm" in anisotropy are evident, in contrast with an approximately 130 cm" dissociation energy determined from psuedodiatomic calculations for a one-dimensional radial well. What anisotropy exists in-... 

Table I. Bond Dissociation Energies Determined from HF Infrared Chemiluminescence. 

Figure 4.9 Morse potential energy curves for chloromethane and its ions. The curves are calculated using the activation energy determined from data in Figure 4.8. The high-temperature data is for unimolecular dissociation via the curve crossing on the approach side of the molecule. Only the VEa is negative and dissociation occurs in the Franck Condon transition. The thermal energy dissociation occurs through the thermal activation of the molecule, as is the case for all DEC(l) molecules.

The N B bond distances in the four strongest complexes in Table 16.1, with dissociation energies ranging from 109 to 143 kJ mol , fall into a relatively narrow range from 164 to 167 pm. We note, however, that the N B bond distances in the weaker complex, (CH3)3NB(CH3)3, T)(N B) = 71 kJ mol , is 170 pm. The N B distance in H3NB(CH3)3 may be even longer, but has not been determined. 

CI2, Br2,12, and some interhalogen compounds XX form complexes with ammonia that are sufficiently stable to allow the structures to be determined in the gas phase. The structure of the complex H3NICI is shown in Fig. 18.6 [4]. The ICl molecule is situated on the threefold symmetry axis of the NH3 molecule, i.e. in the direction of the elecfron lone pair on the N atom. The N-I distance is 271 pm, 71 pm longer than the estimated single, covalent bond distance obtained from the MSS rule, but still 82 pm shorter than the sum of the van der Waals radii of N and 1. The experimental information needed for calculation of the dissociation energy of the complex is unfortunately not available, quantum chemical calculations have yielded dissociation energies ranging from 42 to 52 kJ mol [5,6]. 

Table 11 Zn-C bond distances in dimethylzinc, diethylzinc, di-iso-propylzinc, and di-tert-butylzinc determined by gas electron diffraction and quantum chemical DFT calculations mean Zn-C bond dissociation energies derived from experimental thermochemical data and from calculated energies of isodesmic reactions combined with the experimental bond dissociation energy of Mc2Zn and net atomic charges of Zn and a-C atoms obtained by Mulliken population analysis...

In this section you have seen how heats of com bustion can be used to determine relative stabilities of isomeric alkanes In later sections we shall expand our scope to include the experimentally determined heats of certain other reactions such as bond dissociation energies (Section 4 16) and heats of hydrogenation (Section 6 2) to see how AH° values from various sources can aid our understanding of structure and reactivity... 

The parameters D and a can be obtained by fitting 1E to the actual ground state of the given molecule (D is determined by the observed bond dissociation energy and a is determined by the vibrational force constant). This allows one to express J and K in terms of available experimental information. That is, from eqs. (1.47), (1.49), and (1.52) we obtain... 

The initial kinetic energy of 0 ions produced by dissociative attachment in 02 at an electron energy of 6.9 e.v. may be determined from Equation 4 to be 1.64 e.v. using values of 1.465 e.v. (1) for A(0) and 5.09 e.v. (7) for D(O—O). The residence time for 0 ions calculated from Equation 1 is 6.0 X 10 7 sec. at 10 volts repeller potential. Rate constants for Reaction 6 determined from data at varying Vr are shown in Table I and are seen to increase sharply with increasing repeller potential, as expected for an endothermic process. 

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. 

Figure 3.6. Detailed energy diagram for determining the dissociation energy Dgof H2, from thermodynamic data.

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