Bond dissociation energy force constants

Electronic energy levels, bond dissociation energies, force constants of bonds, and interatomic distances... 

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 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 ... 

Here P°jj,)V is a constant (having energy units) characteristic of the bonding interaction between and %v its value is usually determined by forcing the molecular orbital energies obtained from such a qualitative orbital treatment to yield experimentally correct ionization potentials, bond dissociation energies, or electronic transition energies. 

TABLE 5. Calculated bond distances Re(A), total bond dissociation energies Do (kcalmol-1) which include ZPE corrections and force constants of the totally symmetric mode ke (Ncm-1) of the molecules EH4 and ECI4, using relativistic gradient-corrected DFT ... 

Two of its three adjustable parameters, De and re correspond to directly measurable molecular properties of bond dissociation energy and equilibrium interatomic distance respectively. The third parameter, a, is related to the force constants commonly used in spectroscopic analyses. A standard procedure [147] to obtain experimental potential energy curves is to calculate from spectroscopic data the constants ke, g, and j that appear in the expression... 

The constant of proportionality (k) in both cases is the stretching force constant and, in the same way that a force constant can measure the strength of a mechanical spring, k gives a measure of the strength of a chemical bond. Indeed, the force constant measures bond strength when the atoms are near their equilibrium positions, a far more subtle measure than the dissociation energy. 

In Section 1.3 three measures of bond strength were suggested, the bond dissociation energy, the bond energy term, and the bond stretching force constant. We now examine the definition and status of the third of these quantities. 

De Vos Burchart et al. have recently developed a force field for modeling zeolites.21 The model originally was intended for all-silica zeolites but was quickly extended to aluminum-containing zeolites. The parameters were derived from several sources. Standard bond dissociation energies were used, and the force constants were refined to fit the structure of ZSM-5, the structure and frequencies of a-quartz, in addition to unit cell dimensions of other zeolites. With the all-silica model, the authors were able to calculate heats of formation... 

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