Calculated bond energies

Fournier and DePristo96 calculated bond energies in several small compounds containing disulfide bonds which are known to stabilize the tertiary structure of proteins. Bond dissociation energies are generally overestimated when LDA(SVWN) is used whereas the PW86/P86 functional brings them to within 5 kcal/mol of experimental values. 

To calculate bond energies or energies of molecule reduction during photosynthesis (E) the technique previously tested [6] for 68 binary and more complicated compounds following the equation was applied ... 

From Table 13.10 we see that the bond distances are reproduced better in this case than with the ST03G basis. We see that the break in the trend between CH and CH2 again appears, and we continue to attribute it to the change in the important atomic configuration at this juncture in the list. The calculated bond distances are about 4.2% high. The success in calculating bond energies is more difficult to assess, since there is considerably more imcertainty in the experimental results. 

Previous attempts to calculate bond energies in tin compounds employed levels of theory that were inadequate to provide accurate results. As discussed above, accurate bond energies require the use of either composite ah initio methods or methods employing a high level of electron correlation coupled with isogyric reactions to minimize basis set truncation and other systematic errors. Consequently, the results reported by Basch [46,96], which use a number of imcorrected ah initio methods or with very simple corrections (i.e., across-the-board energy corrections by finite amounts), are unhkely to be particularly accurate. 

Similarly, we can calculate bond energies for any type of bond we wish to create. Refer to Appendix 1 for bond energy values. 

This is borne out by the value of AFo, about 122 kcal./mole, which is approximately equal to the calculated bond energy of the system W + O, viz.,E = 116 kcal./mole. 

Fig. 4-3.—Square of bond strength (dashed curves) and calculated bond-energy values (full curves) for hybrid sp orbitals varying from pure p orbitals (a 0, left) to pure s orbitals (a = 10, right). The upper pair of curves are for L orbitals (2 and 2p), and the lower, with shifted vertical scale, for M orbitals (3s and 3p).

In Figure 4-3, (a) and (b) show the situation for Li2 with and without hybridisation. For the Li2 molecule a calculation indicates that a substantial increase in the calculated bond energy of the molecule is obtained by mixing about 10 per cent 2p character into the wave function. Thus (see Figure 4-3),... 

There is a difference between the two calculations. Bond energies are calculated from the energies of the specific bond found in many different compounds (an average). The calculations for heats of formation are for the particular molecule under consideration (the entire molecule). In reality, the energy required to break a bond is dependent on the location of that bond in a specific molecule because the bond energy for it is determined by the environment in which the bond is located. In other words, the bond energy depends not only on the specific bond, but on the influences of the bonds and atoms in the surroundings provided by the molecule in which that bond is found. 

Some BOVB-calculated bonding energies in double-zeta polarized basis sets... 

The bond energy of H to Ni surface calculated according to this procedure does not show a minimum as a function of H—Ni distance. This is a weakness of the noniterative EH procedure and requires calculated bond energies to be taken at a Ni—H distance equal to the sum of the atomic radii. The covalent bond energy calculated in this manner is 2 eV for most surfaces and compares well with the experimental value of 2.9 eV. 

Fig. 18 Interaction of a Pd monolayer with Ru(0001), Re(0001), W(llO) and Ta(110). Part (a) Calculated value for the density-of-states at the Fermi level and the measured desorption temperature as a function of the theoretical bonding energy. Part (b) Experimental and theoretical palladium Sdja core level shifts versus the calculated bonding energy. Reprinted from ref [34].

Figure 6.3 displays calculated bonding energies for CO, S, SO and thiophene on a series of carbide surfaces and metcars [16]. It is usually assumed that the chemical activity of the metal carbides decreases when the C concentration is increased [1]. 

The red-shift of the C=0 and Si=0 stretching vibration of 100 and 49 cm" , respectively, the calculated bond energy of complex 10 of more than 20 kcal/mol (RHF/6-31G(d,p)), and the calculated non-bonding Si-O distance of 1.98 A, being only slightly longer than the normal Si-0 bond distance, indicate the unusual electronic properties of 10. Evidently dioxasiletane 9 is formed as a thermally or photochemically unstable intermediate which rapidly decomposes to the silanone-formaldehyde complex. 

REF17 Tsuzuki, S., Uchimaru, T., Mikami, M. and Tanabe K. (1996) Basis set effects on the calculated bonding energies of neutral benzene dimers importance of diffuse polarization functions, Chem. Phys. Lett., 252, 206-210... 

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