Dissociation energy diatomic molecule

For more on the determination of diatomic-molecule dissociation energies, see A. G. Gaydon, Dissociation Energies and Spectra of Diatomic Molecules, Third Edition, Chapman and Hall, London, 1968. 

So far we have summarized some basic reactions starting with the cosmic ray ionization of Hj. However cosmic ray ionization of He, which is considerably less abundant in dense clouds than H2 (about V4) seems to be important for two reasons firstly, an activation energy barrier (Johnsen and Biondi, 1974) is likely to keep the reaction rates of H with H and Hj anomalously small (reaction rate 8 x 10 cm sec Sando et al. 1972), and therefore He remains available for the ionization of neutral molecules. Secondly, in most cases, the charge transfer from He to diatomic molecules dissociates them, producing essentially ionized heavy elements, such as C, N , O". The reaction sequence has the general form (see note added in proof). 

For the simplest case of a diatomic molecule dissociating instantaneously to products of high translational energy, the factor c — 2, but if the molecule has a finite average lifetime, r, then... 

Fig. 66. Influence of rotation on an energy curve V (p) of a diatomic molecule. Dissociation by rotation.

The direct dissociation of diatomic molecules is the most well studied process in gas-surface dynamics, the one for which the combination of surface science and molecular beam teclmiques allied to the computation of total energies and detailed and painstaking solution of the molecular dynamics has been most successful. The result is a substantial body of knowledge concerning the importance of the various degrees of freedom (e.g. molecular rotation) to the reaction dynamics, the details of which are contained in a number of review articles [2, 36, 37, 38, 39, 40 and 41]. 

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. 

In a diatomic molecule one of the main effects of mechanical anharmonicity, the only type that concerns us in detail, is to cause the vibrational energy levels to close up smoothly with increasing v, as shown in Figure 6.4. The separation of the levels becomes zero at the limit of dissociation. 

All heteronuclear diatomic molecules, in their ground electronic state, dissociate into neutral atoms, however strongly polar they may be. The simple explanation for this is that dissociation into a positive and a negative ion is much less likely because of the attractive force between the ions even at a relatively large separation. The highly polar Nal molecule is no exception. The lowest energy dissociation process is... 

D is the chemical energy of dissociation which cair be obtained from thermodynamic data, aird is the reduced mass of the diatomic molecule... 

If we were to calculate the potential energy V of the diatomic molecule AB as a function of the distance tab between the centers of the atoms, the result would be a curve having a shape like that seen in Fig. 5-1. This is a bond dissociation curve, the path from the minimum (the equilibrium intemuclear distance in the diatomic molecule) to increasing values of tab describing the dissociation of the molecule. It is conventional to take as the zero of energy the infinitely separated species. 

Figure S-1. Form of a potential energy curve for diatomic molecule AB. VfrAa) is the potential energy, Tab is the intemuclear distance, is the equilibrium intemuclear distance, and D is the bond dissociation energy. (The zero point energy is neglected in the figure.)...

Figure 13.18 Bond dissociation energies for gaseous, homonuclear diatomic molecules (from J. A. Kerr in Handbook of Chemistry and Physics, 73rd edn., 1992-3, CRC Press, Boca Raton, Florida), pp. 9.129-9.137.

The same ideas may be applied to the other processes of Fig. 1. The work required to dissociate a diatomic molecule into two electricallt/ neutral atoms may he quite small the dissociation energy of the bromine molecule Br2 in a vacuum, for example, is only 1.915 electron-volts. On the other hand, the work to dissociate a molecule into two atomic ions in a vacuum cannot be as small as this, since work must be done to set up the full electrostatic field of the positive ion, and the full electrostatic field of the negative ion and this must amount to at least a few electron-volts.1 In addition, the non-electrostatic forces may make a small or large contribution. 

The name dissociation energy is given to the work required to break up a diatomic molecule which is in its lowest rotation-vibrational state, and to leave the two particles (either atoms or ions) at rest in a vacuum. This quantity, which will be denoted by D , corresponds to the length of the arrow in Fig. 7 or Fig. 8a, where the length is the vertical distance between the lowest level of the molecule and the horizontal line which... 

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