Diatomic molecules of second-row elements

Representing the pz orbitals on atoms 1 and 2 by z, andz2, the combinations of atomic wave functions 

The two bonding 7r orbitals represented by these wave functions are degenerate. The wave functions for the antibonding states are identical in form except that negative signs are used in the combination of atomic wave functions and in the normalization constants. 

The combination of three p orbitals on one atom with three p orbitals on another leads to the formation of one a and two tt bonding molecular orbitals. The order in which the orbitals are populated is 

The fact that the B2 molecule is paramagnetic shows that the highest occupied molecular orbitals (usually abbreviated as HOMO) are the degenerate tt orbitals, each of which is occupied by one electron. 

Further evidence that this is the correct energy level scheme to be used for C2 comes from the fact that the molecule is diamagnetic. The molecular orbital configurations for these molecules can be written as 

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FIGURE 3.7 Energy level diagrams for diatomic molecules of second-row elements. Early members of the series follow the diagram shown in (b), whereas later members follow (a). 

In writing these configurations for diatomic molecules of second-row elements, we have omitted the electrons from the Is orbitals because they are not part of the valence shells of the atoms. When considering the oxygen molecule, for which the a orbital arising from the combinations of the 2pz orbitals lies lower in energy than the 7r orbitals, we find that the electron configuration is... 

All the molecules considered so far have had H as one of the participants in the bonding scheme. In this section we will start to consider the MOs produced for interactions between heavier atoms, beginning with homonuclear diatomics, A2, of second-row elements. General MO diagrams for these diatomics will be constructed and then used to discuss the relative stability of those that are observed experimentally N2, O2 and F2 and those that are not found under normal circumstances, Li2, Be2, B2 and C2. In common with H2, these molecules all belong to the Dooh point group and so the 2s valence orbitals will be linked together as <7g+ and SALCs ... 

The next homonuclear molecule composed of second row elements is B2, which has six total valence electrons to accommodate. We can approximate the next higher energy molecular orbitals for B2 and the rest of the period 2 diatomic molecules as linear... 

The basic principles dealing with the molecular orbital description of the bonding in diatomic molecules have been presented in the previous section. However, somewhat different considerations are involved when second-row elements are involved in the bonding because of the differences between s and p orbitals. When the orbitals being combined are p orbitals, the lobes can combine in such a way that the overlap is symmetric around the intemuclear axis. Overlap in this way gives rise to a a bond. This type of overlap involves p orbitals for which the overlap is essentially "end on" as shown in Figure 3.5. For reasons that will become clear later, it will be assumed that the pz orbital is the one used in this type of combination. 

Calculated vibrational frequencies for main-group hydrides containing one first or second-row element are provided in Appendix A7 (Tables A7-1 to A7-8), and compared both with experimentally measured values and, where available, with harmonic experimental frequencies. The same theoretical models considered for diatomic molecules are also examined here. A summary of mean absolute errors for symmetric stretching frequencies (only) is provided in Table 7-2. 

Oxygen, fluorine, and man. These three molecules can be treated with the same energy diagram that we have been using for other diatomic molecules of the second-row elements. As we shall see shortly, the intervening molecules, B, C-. and N2. require additional considerations, which lead to an alteration in (he relative energies of the molecular orbitals. 

Table 6.1 shows the bond energies, the bond lengths and the magnetic features of homonuclear diatomic molecules and ions involving second row elements. 

In the last fifteen years, concerning the first and second row elements of the periodic table, the following diatomic molecules containing the Sc atom have been examined by ab initio post-HF methods ScH [4], ScH [5], ScHe [6], ScLi... 

The second-row elements including carbon, oxygen and nitrogen involve p atomic orbitals as well as 2s orbitals. An example of a heteronuclear diatomic molecule involving these elements is carbon monoxide, C=0. The carbon monoxide molecule has 14 electrons, and the orbitals for each atom are Is, 2s, 2p, and... 

Figure 7,28 A general MO diagram for diatomic molecules A2 for second-row elements. This diagram assumes no hybridization of the or o-u+ combinations of s and of p AOs.

In this chapter, we shall generate valence-bond structures for some diatomic molecules and ions that involve atoms of first-row and second-row elements. To do this, initially we shall nse their molecular orbital configurations together with the prototype valenee-bond structures of Table 3-2. Green and Linnett adopted this approach to the construction of valence-bond stractures, and they have described many of the valenee-bond structures that we shall consider here. We shall restrict our attention to the moleeular orbitals that are constructed from the valence-shell 2s and 2p or 3s and 3p atomie orbitals, and for simplicity in the molecular orbital notation, neglect any hybridization that may occur between s and po (= Pz) atomic orbitals. 

There are only a few heteronuclear diatomic molecules that are formed from elements of the first and second rows of the Periodic Table and are stable as diatomic molecules in the gas phase at normal temperatures and pressures. These are HF, CO and NO. Others have been observed at high temperatures, in discharge lamps, in flames or in space. Examples are LiH, LiF, OH, BeH, BeO, BF, BH, CH, CN and NH. Some of the molecules in this second list will be stable with respect to the two separate atoms but not at normal temperatures and pressures with respect to other forms of the compound. LiH, LiF and BeO are normally found as ionic solids. The other molecules are unstable with respect to covalent compounds in which the atoms have their normal valencies H20, BeH2, BF3, B2H6, CH4, (CN)2 and NH3. 

The bond order in N2, O2, and F2 is the same whether or not mixing is taken into account, but the order of the (Tg(2p) and TT (2p) orbitals is different in N2 than in O2 and F2. As stated previously and further described in Section 5.3.1, the energy difference between the 2s and 2p orbitals of the second row main group elements increases with increasing Z, from 5.7 eV in boron to 21.5 eV in fluorine. As this difference increases, the s-p interaction (mixing) decreases, and the normal order of molecular orbitals returns in O2 and F2. The higher a-g(2p) orbital (relative to 7T (2p)) occurs in many heteronuclear diatomic molecules, such as CO, described in Section 5.3.1. 

Just as we treated the bonding in H2 by using molecular orbital theory, we can consider the MO description of otiier diatomic molecules. Initially we will restrict our discussion to homonuclear diatomic molecules (those composed of two identical atoms) of elements in flie second row of the periodic table. As we will see, die procedure for determining the distribution of electrons in these molecules closely follows the one we used for H2. 

Figure 10.34 shows the relative energies of the molecular orbitals obtained from 2s and 2p atomic orbitals. This order of molecular orbitals reproduces the known electron configurations of homonuclear diatomic molecules composed of elements in the second row of the periodic table. The order of filling is... 

It has been outlined earlier how s and p orbitals on adjacent atoms can overlap to form a and n molecular orbitals. These types of atomic orbital overlap can occur at the same time, to form multiple bonds. This can he illustrated by the diatomic molecules, X2, of the elements of the second row of the periodic table. 

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