Diatomic molecules qualitative molecular orbital

In addition to the homonudear molecules, the elements of the second period form numerous important and interesting heteronudear species, both neutral molecules and diatomic ions. The molecular orbital diagrams for several of these species are shown in Figure 3.9. Keep in mind that the energies of the molecular orbitals having the same designations are not equal for these species. The diagrams are only qualitatively correct. 

The same principles that we have used for the description of diatomic molecules will now be used in a description of the electronic structures of triatomic molecules. However, let it be clear that in using molecular-orbital theory with any hope of success, we first have to know the molecular geometry. Only in very rare cases is it possible from qualitative molecular-orbital considerations to predict the geometry of a given molecule. Usually this can only be discussed after a thorough calculation has been carried out. 

In the last section, we used the approximate functions (13.57) and (13.58) for the two lowest HI electronic states. Now we construct approximate functions for further excited states so as to build up a supply of Hl-like molecular orbitals. We shall then use these MOs to discuss many-electron diatomic molecules qualitatively, just as we used hydrogenlike AOs to discuss many-electron atoms. 

Give a qualitative description of the bonding of the BN molecule using molecular orbitals. Compare it with diatomic carbon. 

The molecular orbital (MO) approach to the electronic structure of diatomic, and also polyatomic, molecules is not the only one which is used but it lends itself to a fairly qualitative description, which we require here. 

The se the orie s are inevitablj based upem analyse s of the interactions and transformations of molecular orbitals, and consequently the accurate construction and re presentation of molecular orbitals has become essential, furthermore, although the forms of molecular orbitals in diatomics and of delocalized tt orbitals in conjugated systems are familiar, a general, non-computational method for determining the qualitative nature of or and t orbitals in arbitrary molecules has been lacking. 

Values of the MacLaurin coefficients computed from good, self-consistent-field wavefunctions have been reported [355] for 125 linear molecules and molecular ions. Only type I and II momentum densities were found for these molecules, and they corresponded to negative and positive values of IIq(O), respectively. An analysis in terms of molecular orbital contributions was made, and periodic trends were examined [355]. The qualitative results of that work [355] are correct but recent, purely numerical, Hartree-Fock calculations [356] for 78 diatomic molecules have demonstrated that the highly regarded wavefunctions of Cade, Huo, and Wahl [357-359] are not accurate for IIo(O) and especially IIo(O). These problems can be traced to a lack of sufficiently diffuse functions in their large basis sets of Slater-type functions. 

There are a number of different approaches to the description of molecular electronic states. In this section we describe molecular orbital theory, which has been by far the most significant and popular approach to both the qualitative and quantitative description of molecular electronic structure. In subsequent sections we will describe the theory of the correlation of molecular states to the Russell Saunders states of the separated atoms we will also discuss what is known as the united atom approach to the description of molecular electronic states, an approach which is confined to diatomic molecules. 

There are substantial numbers of small molecular species that are important species at high temperatures that are not normally discussed in standard inorganic courses. For example, the vapor over solid NaCl contains diatomic NaCl molecules as well as NaCl dimers. Does the stoichiometry NaCl fit with the simple ideas of valence derived from molecules like SiCU Explain. Construct a qualitative MO diagram with orbital drawings for the NaCl diatomic molecule and compare it with that for a homonuclear, isoelectronic diatomic molecule. Does this help in answering the first question ... 

The molecular orbital model as a linear combination of atomic orbitals introduced in Chapter 4 was extended in Chapter 6 to diatomic molecules and in Chapter 7 to small polyatomic molecules where advantage was taken of symmetry considerations. At the end of Chapter 7, a brief outline was presented of how to proceed quantitatively to apply the theory to any molecule, based on the variational principle and the solution of a secular determinant. In Chapter 9, this basic procedure was applied to molecules whose geometries allow their classification as conjugated tt systems. We now proceed to three additional types of systems, briefly developing firm qualitative or semiquantitative conclusions, once more strongly related to geometric considerations. They are the recently discovered spheroidal carbon cluster molecule, Cgo (ref. 137), the octahedral complexes of transition metals, and the broad class of metals and semi-metals. 

With molecular orbital diagrams such as those for H2 and Hc2, we can begin to see the power of molecular orbital theory. For a diatomic molecule described using molecular orbital theory, we can calculate the bond order. The value of the bond order indicates, qualitatively, how stable a... 

In diatomic molecules that do not have an inversion center, there is a qualitative similarity of the available molecular orbitals to those of a homonuclear diatomic molecule. There are always orbitals with the same number of nodal surfaces the difference is that the nodal surfaces need not be planes and may be offset from the middle of the molecule. Though we cannot label such orbitals g or u, there are bonding and antibonding combinations qualitatively similar to those of homonuclear diatomic molecules. 

For this reason, let us consider another paper from the early years that offers a more qualitative analysis of molecular bonding. [Lennard-Jones, 1929] presents a molecular orbital analysis applicable not only to the ground state of the hydrogen molecule but also to a whole range of small (n < 10) diatomics. While this analysis does not yield quantitative expressions for system wavefunctions (they are not written down at all), it does allow the prediction of a variety of molecular properties including bonding and multiple spectroscopic characteristics. 

Two different bonding indiees will be used in this book, namely bond-number and bond-order. The bond number refers to the number of pairs of electrons that form a covalent bond. It may be calculated from the weights of the valence-bond structures that are used to describe the electronic structure of the molecule, as is demonstrated above for N2O. The bond-order is a molecular orbital index of bonding. For the purpose of qualitative discussion of diatomic bonding, we shall define the bond-order to be /4 (No. of bonding electrons) - (No. of antibonding electrons). Another definition of bond-order will be introduced in Chapter 14. 

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