Covalent bonds quantum mechanical calculation

It should be added that our inability, so far, to obtain reliable weights for the structures (i) to (v) does not prevent us from using empirical, or semi-empirical methods. As early as 1949 Pauling [24] had used an empirical relation between bond order and bond length to estimate the covalent character of H 02 as a function of the lengths of 01—H and H 02. The value which he found for the Ox 02 distance 2 70 A, was similar to that subsequently obtained in the more detailed calculations of Tsubomtjra [21]. Pauling s work was considerably extended by Coulson and Danielsson [25] who used alternative empirical relations, and found substantially the same values. However, in all this work, and in the quantum-mechanical calculations which followed [23], the only structures considered were (i), (ii) and (iv). Tsubomura seems to be the only person to have included all five structures. The essential role played by (iv) was noticed some time ago by Sokolov [26]. 

Within the last 5 years a number of quantum mechanical calculations have been carried out, in particular for hydrogen-bonded systems such as HgO, NHg, and HF as solvents. Contrary to electrostatic models, MO calculations allow for the possibility of covalent bond formation and, consequently, constitute a fundamentally better approach to ion-molecule interactions. Some interesting results of recent model calculations, although qualitative in nature, are discussed in Section V. 

The postulate of the geometric mean has been derived from some quantum-mechanical calculations and from the empirical fact that the differences A between the actual bond energy Da-b of a single bond A—B and the energy Da-b of the corresponding hypothetical purely covalent single bond. 

Jensen [9] indicated that there is no evidence that Drago s parameters reflect the relative electrostatic and covalent contributions to the bonding in resulting adducts. They were not correlated with either a physical property (dipole moment, ionization potential) or with a quantum-mechanically calculated index. Drago s approach is a purely empirical method of calculating enthalpy of formation for molecular adducts. Fowkes applied the Lewis E C equation [19] and has attempted to determine E and C parameters for both polymers and surfaces. However, Jensen [9] indicated the potential problem connected... 

N2 Complexing. LCAO-MO calculations have been made on the electronic structures of the complexes M,N2 (M = H, Li, Be, or B" ), N2,BX3 (X = F or H), and M,N2M (M = H, Li", B", or BH3). The calculations were used to discuss models of nitrogen fixation by enzymes in biological systems. Quantum mechanical calculations by the CNDQ/2 method confirm the possibility of N2 complex formation with Li and F" ions. The contribution of covalent bonds to the energy of the complexes has been analysed and their lability discussed. ... 

This chapter walks you through the evolution of covalent bond theory, starting with the Lewis dot structures you likely covered in General Chemistry and then advancing to valence bond and molecular orbital theories that stem from quantum mechanics calculations. As with most chemistry, this chapter is purely about following the bouncing balls (electrons). 

MoFe-Protein contains a second cluster, the M-cluster. [33] This is covalently bonded to cysteine and histidine of the a-suhunit. A homocitrate anion R)-2-Hydroxyhutane-l,2,4-tricarhoxylic acid) is complexed as a bidentate ligand with molybdenum (Fig. 4.5). On the basis of better resolved X-ray structural analysis and quantum mechanical calculations, initially it was thought, a nitrogen atom is located in the centre of the Fe/Mo complex, [34] but more recent results gave evidence for an interstitial carbido ligand [35,36], which is unprecedent in bioinorganic chemistry. 

This discovery was to be the beginning of the use of exchange terms in the quantum mechanics of atoms and molecules. It became the key factor that shortly afterward allowed Walter Heitler and Fritz London to obtain the first successful quantum mechanical calculation of the covalent bond in the simplest case of a diatomic hydrogen molecule. Exchange terms would also pave the way for the notion of quantum mechanical resonance and the development of the quantum mechanical theories of bonding by Linus Pauling and many others. ... 

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