Calculation of bond energies

Ziegler, T, Baerends, E.J., Snijders, J.G., Ravenek, W. and Tschinke, V. (1989) Calculation of bond energies in compounds of heavy elements by quasi-relativistic approach. The Journal of Physical Chemistry, 93, 3050-3056. 

The calculations of bond energy based on the same technique but on covalence distances of atoms for free molecule P...0 (sesquialteral bond) and for molecule P=0 in P4O10 (double bond) are given in table 4 for comparison. 

Present quantum chemistry is mainly concerned with the calculation of bond energies. Accordingly, for the rest of this chapter we shall concentrate mainly on solvation enthalpies. The latter, being magnitudes composed of a lot of strongly interwoven effects, do not allow a simple separation of bulk and outer-shell influences. 

T. Ziegler and A. Rauk, Theor. Chim. Acta, 46, 1 (1977). On the Calculation of Bonding Energies by the Hartree-Fock-Slater Method. 

M. S. Gordon and D. G. Truhlar, /. Am. Chem. Soc., 108, 5412 (1986). Scaling All Correlation Energy in Perturbation Theory Calculations of Bond Energies and Barrier Heights. 

The first point to be stressed in any account of metal ion bonding is the great success of simple ionic calculations of bond energies for the IA and IIA metal halides. (At the same time we should note that no covalent model has ever achieved quantitative reliability.) Thus Fig. 2 shows that... 

There is an uncertainty of about 44 kcal per gram-atom in the heat of atomization of graphite. Show that if this heat of atomization is used in the calculation of bond energies, the following uncertainties should arise ... 

Calculations based on equations (3 and 4) are given in Tables 3 and 4. At the same time, in some cases the results of calculations of bond energy for fragments of NH2, N02 and N20 that are introduced into other complex structures are given. The deviations of computational data from the experimental ones [8] do not exceed 10% for complex structures. 

Calculation of bond energies is useful in itself and also as an independent check on the thermochemical arguments used. Equations (8)... 

For calculation of bond energies it is normally assumed that the... 

In Chapter 3 the covalent bond has been discussed and the question now arises whether this is the only possible type of bond between atoms. Let us consider the gaseous molecule of sodium chloride. The sodium and chlorine atoms each have one unpaired electron, 35 in sodium and 3/> in chlorine, so that in principle the formation of a covalent bond is possible. The calculation of bond energies presents considerable difficulties even in a simple molecule such as hydrogen and the calculation for more complicated molecules is impossible except by an approximate method such as that introduced by Pauling In this method, it is assumed that the energy of the covalent bond A—B, is equal to one half of the sum of the bond energies of the homopolar molecules A—A and B—B, i.e. 

The standard enthalpy of formation of gaseous atoms from the element is known as the heat of atomization. Heats of atomization are always positive, and are important in the calculation of bond energies. 

Of course, using the additive scheme rules out the possibility of calculating of the contribution of electron correlation component in bond energy. In fact, the difference between estimated from the Morse potential and experimental bond energies [40] decreases in HI, HBr, HC1, HF, H2 92.8 (31.4%), 81.8 (22.5 %), 59.5 (13.8 %), -25.8 (4.5 %), 30.4 (7.0 %) kJ/mol, respectively. Apparently, the accuracy of the calculation of bond energy in small molecules is usually low-level. Exceptions can be found with molecules containing atoms of inert gases. The enforced dissociation of NeOH was shown to be described by Morse-like potential... 

Clearly, the second of these is of little value unless the first is satisfied within some well-defined and well-understood hierarchy of approximation we do not wish to have explanations of the strength of a chemical bond using theories which are not capable of giving a good quantitative calculation of bond energies, for example. 

In this contribution we shall present several applications of the new method, which we shall refer to as LSD/NL, to the calculation of bond energies of transition metal complexes. We shall focus on trends along a transition period and/or down a transition triad. The following subjects will be discussed a) metal-metal bonds in dimers of the group 6 transition metals b) metal-ligand bonds in early and late transition metal complexes c) the relative strength of metal-hydrogen and metal-methyl bond in transition metal complexes d) the metal-carbonyl bond in hexa- penta-and tetra-carbonyl complexes. 

The effect of fluorine substituents on the strength of a metal-aryl bond is graphically illustrated by theoretical calculations of bond energy correlations. Fluorine substituents in the two positions ortho to the metal-fluorine bond are shown to have far more effect on the M-C bond energy than substituents in the meta- or para- positions in the case of Re-G bonds this difference is calculated to be ca. 50kJmoP These conclusions are borne out by experiment. 

Reliable data for the calculation of bond energies are only available for the series SF2, SF4 and SFg the MBE of SF2 defined as half the energy of the reaction... 

To improve the situation with the average reference bond energies, Dewar and co-workers developed a new scheme for the calculation of bond energies. The Dewar resonance energy (DRE) was defined for hydrocarbons as follows ... 

Ziegler T, Rauk A (1977) On the calculation of bonding energies by the Hartree Fock Slater method. Theor Chim Acta 46 1-10... 

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