Diatomic molecules bond dissociation

Strengths of Chemical Bonds, In CRC Handbook of Chemistry and Physics-, CRC Press Boca Raton. Tables with bond dissociation energies of diatomic molecules, bond dissociation enthalpies of polyatomic molecules, and standard enthalpies of formation of (mainly organic) radicals (at 298.15 K). The CRC Handbook is regularly updated, so the tables may vary according to the edition number. 

Studies of the spectra of Group I metal vapors at about the boiling points of the metals show the presence of 1 % of diatomic molecules whose dissociation energies decrease with increasing atomic number (Table 6.1). These molecules provide the most unambiguous cases of covalent bonding of the alkalis some s-p hybridization is considered to be involved. 

For diatomic molecules, bond enthalpies differ slightly from bond dissociation energies, and this difference can be determined using Equation 7.16 from Section 7.2 ... 

Morse potential and harmonic (parabolic) potential-energy surfaces for a diatomic molecule. The dissociation energy, Df, represents the energy required to sever the chemical bond. 

A simple example would be in a study of a diatomic molecule that in a Hartree-Fock calculation has a bonded cr orbital as the highest occupied MO (HOMO) and a a lowest unoccupied MO (LUMO). A CASSCF calculation would then use the two a electrons and set up four CSFs with single and double excitations from the HOMO into the a orbital. This allows the bond dissociation to be described correctly, with different amounts of the neutral atoms, ion pair, and bonded pair controlled by the Cl coefficients, with the optimal shapes of the orbitals also being found. For more complicated systems... 

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. 

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.

Hiroshima, 721 histidine, 443, 774 hole, 195 homeostasis, 386 HOMO, 126, 580 homogeneous alloy, 202 homogeneous catalyst, 565 homogeneous equilibria, 362 homogeneous mixture, F53 homolytic dissociation, 80 homonuclear diatomic molecule, 103 Hooke s law, 92 hormone, 670 horsepower, A4, 791 hour, A4 HPLC, 354 HRF products, 723 HTSC, 192 Humphreys series, 51 Hund, F 35 Hund s rule, 35, 37 Hurricane Rita, 144 hyaluronic acid, 344 hybrid orbital, 109 hybridization bond angle, 131 molecular shape, 111 hydrangea color, 463 hydrate, F32 hydrate isomer, 676 hydration, 178 hydrazine, 627... 

Within the framework of the Bom-Oppenheimer approximation, a diatomic molecule consists of two nuclei that are more-or-less attached by the surrounding electron cloud. Often the specific form of the resulting potential function is not known. However, if a chemical bond is formed between the two nuclei, the potential function displays a minimum at a distance that corresponds to the equilibrium bond length. Furthermore, the energy necessary to break the chemical bond, the dissociation energy, is often evaluated by spectroscopic measurements. It can be concluded, then, that the potential fiinction has the general form shown in Fig. 6. A simple derivation of the Born- Qppenheimer approximation is presented in Section 12.1. 

The relationships between bond length, stretching force constant, and bond dissociation energy are made clear by the potential energy curve for a diatomic molecule, the plot of the change in the internal energy AU of the molecule A2 as the internuclear separation is increased until the molecule dissociates into two A atoms ... 

We saw in Figure 2.1 that the energy needed to rupture the bond in a diatomic molecule, the bond dissociation energy is the energy Af/e. This is the energy that can be cal-... 

In a diatomic molecule, e.g. chlorine (CI2), the bond dissociation enthalpy and the average bond enthalpy will have the same value. This is because both enthalpy changes refer to the process Cl2(g)--> 2Cl(g). 

Table 3.13. Calculated spin multiplicity, bond length Re, and dissociation energy De of first-row homonuclear diatomic molecules, with comparison experimental valued1 in parentheses...

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