Electronic configurations of some molecules

For practical implementations it is necessary to represent the molecular electronic wave functions as a linear combination of some convenient set of basis functions. In principle any choice of basis set is permissible although the basis set must span any electronic configuration of the molecule. This implies that the basis must form a complete set. 

Table 4.1 The electronic configurations of some homonuclear diatomic molecules and ions...

In general, the octet rule works for representative metals (Groups lA, IIA) and nonmetals, but not for the transition, inner-transition or post-transition elements. These elements seek additional stability by having filled half-filled or filled orbitals d or/subshell orbitals. The octet rale does not, however, accurately predict the electron configurations of all molecules and compounds. Not all nonmetals, nor metals, can form compounds that satisfy the octet rale. As a result, the octet rale must be used with caution when predicting the electron configurations of molecules and compounds. For example, some atoms violate the octet rale and are surrounded with more than four electron pairs. 

Ground-state RhC molecules, produced by the vaporization of a mixture of rhodium and carbon from a graphite cell, have been trapped in Ne and Ar matrices at 4 K. The electronic configuration of the molecule as determined from its e.s.r. spectra is largely AdafiAd-nfi AddfiSscsfy with some mixing of Ada with 5sa. 

The previous section looked at the electron configurations of some simple molecules H2, Hc2, Li2, and Bc2. These are homonuclear diatomic moiecuies—that is, molecules composed of two like nuclei. (Heteronuciear diatomic moiecuies are molecules composed of two different nuclei—for example, CO and NO.) To find the electron configurations of other homonuclear diatomic molecules, we need to have additional molecular orbitals. 

Polyatomic molecules cover such a wide range of different types that it is not possible here to discuss the MOs and electron configurations of more than a very few. The molecules that we shall discuss are those of the general type AFI2, where A is a first-row element, formaldehyde (FI2CO), benzene and some regular octahedral transition metal complexes. 

A molecule exhibits a great difference in the speeds of electronic transitions and vibrational atomic motions. The absorbtion of photon and a change in the electronic state of a molecule occurs in 10 15—10—18 s. The vibrational motion of atoms in a molecule takes place in 10 1 s. Therefore, an electronically excited molecule has the interatomic configuration of the nonexited state during some period of time. Different situations for the exited molecule can exist. Each situation is governed by the Franck-Condon principle [203,204],... 

The electronic configurations of the homonuclear diatomic molecules of the elements of the second period, and some of their ions, are given in Table 4.1. 

It was then discovered that the configurations of the molecules correspond to an increase in ligancy of the boron atoms to 5 or 6 and of some of the hydrogen atoms to 2. These configurations provide strong support of the suggestion made by Lewis77 that the electron pairs reso-... 

In the MO formalism it is quite straightforward to deal with the excited states of a molecule. An adequate wavefunction of an excited state can be constructed according to the resultant configuration and its symmetry arising from electron promotion among MO series. Compared with numerous MO-based methods, VB approaches are far less employed to study excited states due to the difficulty in VB computations. Recently, by observing the correlation between MO theory and resonance theory, as well as the symmetry-adapted VB wavefunction described in the last section, we performed VB calculations on low-lying states of some molecules [71, 72],... 

In discussing the method of linear combinations of atomic orbitals, it was pointed out that covalent bonds could be formed between atoms only if the valence electron clouds of the two atoms could overlap. In particular, an atom having unpaired valence electrons would be likely to form covalent linkages if overlap with the orbital of an unpaired electron of another atom were possible such a process would take place with the pairing of spins. Knowing the electron configurations of the atoms and the shapes of their clouds, we should, in many cases, predict the bonds that will form, the approximate shape of the resulting molecule, and, with some skill in wave mechanics, the approximate stability of the molecule. 

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