Bonding considerations polyatomic molecules

For diatomic molecules, of course, A = u = Ojj. In polyatomic molecules, consideration of a stepwise cleavage of all the bonds gives which does not mean that the individual dissociation... 

Two basic methods, the valence-bond (VB) and the molecular orbital (MO) method, have been developed for the determination of approximate state functions. In practice, the MO method constitutes the simplest and most efficient approach for the treatment of polyatomic molecules. And, in fact, all the calculations for the systems under consideration have been carried out within the framework of the MO theory. 

Whatever the level of approximation used in the calculation, in general the molecular orbitals (m.o.s) for polyatomic molecules have contributions from the orbital functions of several atoms. They are non-localized, in the sense that they do not just involve each atom and a neighbour, as might be suggested by the traditional structural formulae. An exception is provided by the occurrence of localized m.o.s as non-bonding orbitals, namely for symmetry considerations, as is the case for the non-bonding electron pair of 7T symmetry in the H2O molecule studied in the previous chapter. 

Considerable difficulties arise when attempting to apply this expression to polyatomic molecules, owing to possible coupling between different modes of the same symmetry. Nevertheless, the data in Table 9 allow one to estimate the metal-metal bond dissociation energy in [M2X8]" ions (84) to be around 500kJmol, which is a very substantial value, exceeded among homonuclear units only by those of C C and N=N. 

Here the reaction process is one of spontaneous transformation, which requires sufficient energy to be present in the ether molecule to permit the rupture of a carbon-carbon bond. This energy is obviously internal to the molecule and cannot be represented as a translational-energy term. The pertinent question to ask is how a molecule acquires the required energy for the transformation to occur, and the answer lies in a consideration of the energy exchange from external (kinetic) to internal (rotational and vibrational) modes in polyatomic molecules. 

The effects of break-down of the Bom-Oppenhemer approximation have been considered in a series of papers by Bunker. r values for the bond lengths of light diatomic molecules are not quite constant when determined in the conventional way for different isotopic species e.g. HCl and DCl). The differences are of the order 10" A, and for this reason no further consideration will be given to the validity of the Bom-Oppenheimer approximation. Apart from the fact that other errors are invariably more important in the determination of molecular structure for polyatomic molecules, it does not seem possible to take account of the effect for polyatomic molecules at present. 

By the end of 1936, Coulson was already delineating an extensive program of postdoctoral work on organic molecules, which included the development of new calculational techniques the conceptual clarification of criteria used for comparing the molecular orbital method with the valence bond method the comparison of resonance energies of molecules by the molecular orbital and valence bond methods and the extension of these considerations to polyatomic molecules, that is, the extension of the molecular orbital method and the self-consistent field calculations to polyatomic molecules. 

A number of triatomic radicals can form dimers whose geometries have been well-characterized. A study of the electronic structures of these dimers can illustrate aspects of qualitative valence-bond and molecular orbital theory for electron-rich polyatomic molecules, and interconnections between these theories can be demonstrated. Dinitrogen tetroxide is a molecule par excellence that may be used for these purposes, and here we shall give primary consideration to its electronic structure and bond properties. 

The bonding in molecules containing more than two atoms can also be described in terms of molecular orbitals. We will not attempt to do this the energy level structure is considerably more complex than the one we considered. However, one point is worth mentioning. In polyatomic species, a pi molecular orbital can be spread over die entire molecule rather than being concentrated between two atoms. 

VSEPR theory can successfully account for many of the fine details in a structure, especially bond angles. However, we will be mainly concerned with the gross geometries of molecules and polyatomic ions. Structural minutiae are of considerable interest to most inorganic chemists, but they are important in the study of descriptive inorganic chemistry only to the extent that they may illuminate details of bonding which are relevant to the very existence of a substance, and to its reactions. 

---