Molecular orbitals and electronic states

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. 

Note that these are the opposite of the electronic selection rules in Equation (7.70). 

The total electron density contributed by all the electrons in any molecule is a property that can be visualized and it is possible to imagine an experiment in which it could be observed. It is when we try to break down this electron density into a contribution from each electron that problems arise. The methods employing hybrid orbitals or equivalent orbitals are useful in certain circumsfances such as in rationalizing properties of a localized part of fhe molecule. Flowever, fhe promotion of an electron from one orbifal fo anofher, in an electronic transition, or the complete removal of it, in an ionization process, both obey symmetry selection mles. For this reason the orbitals used to describe the difference befween eifher fwo electronic states of the molecule or an electronic state of the molecule and an electronic state of the positive ion must be MOs which belong to symmetry species of the point group to which the molecule belongs. Such orbitals are called symmetry orbitals and are the only type we shall consider here. 

---

The progression of sections leads the reader from the principles of quantum mechanics and several model problems which illustrate these principles and relate to chemical phenomena, through atomic and molecular orbitals, N-electron configurations, states, and term symbols, vibrational and rotational energy levels, photon-induced transitions among various levels, and eventually to computational techniques for treating chemical bonding and reactivity. 

Figure 6.14. CO chemisorption on a transition metal. Molecular orbitals and density of states before (a,b) and after (c and d) adsorption. Effect of varying 0 and EF on electron backdonation (c) and donation (d). Based on Fig. 4 of ref. 98. See text for discussion. Reprinted with permission from Elsevier Science.

As holds for other cluster systems, certain magic cluster electron counts exist, which indicates for a certain cluster-halide ratio and interstitial present the filling of all bonding molecular orbitals and therefore the thermodynamically most stable situation. For main group interstitial atoms these are 14 cluster-based electrons whereas for transition-metal interstitials the magic number is 18 [1, 10-12]. All of these phases are synthesized by high-temperature solid-state chemical methods. A remarkable variety of different structure types has been... 

Elastic tunneling spectroscopy is discussed in the context of processes involving molecular ionization and electron affinity states, a technique we call orbital mediated tunneling spectroscopy, or OMTS. OMTS can be applied readily to M-I-A-M and M-I-A-I -M systems, but application to M-A-M junctions is problematic. Spectra can be obtained from single molecules. Ionization state results correlate well with UPS spectra obtained from the same systems in the same environment. Both ionization and affinity levels measured by OMTS can usually be correlated with one electron oxidation and reduction potentials for the molecular species in solution. OMTS can be identified by peaks in dl/dV vs bias voltage plots that do not occur at the same position in either bias polarity. Because of the intrinsic... 

Both the Si and Ti excited states arise from the promotion of an electron from the n molecular orbital to the Jt molecular orbital. They are referred to as (njji ) and 3(n,tt ) states, respectively. The S2 and T2 states arise from the promotion of an electron from the n molecular orbital to the Jt molecular orbital and are referred to as 1(n,nx ) and 3(jt,Jt ) states, respectively. The state diagram for methanal is shown in Figure 1.11. With regard to the different spin states in molecules, the following ideas are important ... 

Mulliken R. S. (1934). A new electroaffinity scale together with data on valence states and on valence ionization potentials and electron affinities. J. Chem. Phys., 2 782-793. Mulliken R. S. (1935). Electronic structure of molecules, XI Electroaffinity, molecular orbitals, and dipole moments. J. Chem. Phys., 3 573-591. 

Another type of process which may lead to a net gain or loss in the number of electron pairs is that involving change in electronic state. For example, the ground state of methylene, CH2, is a triplet with one electron in an in-plane molecular orbital and one electron in an out-of-plane molecular orbital. 

---