Pair bonding energy

These predictions are borne out quantitatively in HL calculations of electron-pair bond energies and bond lengths for heteronuclear atomic pairs [114, 142]... 

Base pair Stacking energy Atom pair Bond energy ... 

In the general case of a Debye solid the temperature dependence of G is determined via the parameter 0d (see Chapter 3). Hence, the free enthalpy of a perfect sohd is determined for all temperatures by the parameters effective pair bonding energy, co-ordination number and Debye-temperature. While (e) is a meaningful parameter in the case of co radent crystals (for elemental crystals x-x> cf. Section 2.2.4), in the case of ionic crystals charge numbers, lattice constant and Madelung number are the appropriate parameters. Hence... 

The parameter redundancy is also the reason that care should be exercised when trying to decompose energy differences into individual terms. Although it may be possible to rationalize the preference of one conformation over another by for example increased steric repulsion between certain atom pairs, this is intimately related to the chosen functional form for the non-bonded energy, and the balance between this and the angle bend/torsional terms. The rotational banier in ethane, for example, may be reproduced solely by an HCCH torsional energy term, solely by an H-H van der Waals repulsion or solely by H-H electrostatic repulsion. Different force fields will have (slightly) different balances of these terms, and while one force field may contribute a conformational difference primarily to steric interactions, another may have the... 

For a given molecule and a given intemuclear separation a would have a definite value, such as to make the energy level for P+ lie as low as possible. If a happens to be nearly 1 for the equilibrium state of the molecule, it would be convenient to say that the bond is an electron-pair bond if a is nearly zero, it could be called an ionic bond. This definition is somewhat unsatisfactory in that it does not depend on easily observable quantities. For example, a compound which is ionic by the above definition might dissociate adiabatically into neutral atoms, the value of a changing from nearly zero to unity as the nuclei separate, and it would do this in case the electron affinity of X were less than the ionization potential of M. HF is an example of such a compound. There is evidence, given bdow, that the normal molecule approximates an ionic compound yet it would dissociate adiabatically into neutral F and H.13... 

It is shown that a stable shared-electron bond involving one eigenfunction for each of two atoms can be formed under certain circumstances with either one, two, or three electrons. An electron-pair bond can be formed by two arbitrary atoms. A one-electron bond and a three-electron bond, however, can be formed only when a certain criterion involving the nature of the atoms concerned is satisfied. Of these bonds the electron-pair bond is the most stable, with a dissociation energy of 2-4 v. e. The one-electron bond and the three-electron bond have a dissociation energy... 

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