Second row heteronuclear diatomics

The three polar molecules in the series are interesting because they all have anomalous directions to their dipole moments, i.e., the direction is different from that predicted by an elementary application of the idea of electronegativity, accepting the fact that there may be ambiguity in the definition of electronegativity for Ne. We will see how VB ideas interpret these anomalous dipole moments. 

We do the calculations with a 6-3IG basis in the same way as was done in Chapter 11 and for three arrangements of ST03G bases. This will allow us both to judge the stability of the qualitative predictions to the basis and to assess the ability of the calculations to obtain quantitative answers. 

We have already treated N2 in Chapter 11, but will look at it here from a somewhat different point of view. 

Results of calculations carried out with three different selection schemes and an ST03G AO will be described. The reader will recall that the scale factors for this basis are traditionally adjusted to give molecular geometries, and this must be remembered when interpreting the results. By now the reader should suspect that such a basis will not produce very accurate energies. Nevertheless, we see that the qualitative trends of the quantities match the experimental values. 

The three ways in which the structures are selected for the calculation follow, and in all cases the s orbitals of the atoms are doubly occupied. 

---

Second row heteronuclear diatomics 12.3 Dipole moments of CO, BF, and BeNe... 

A large number of strongly bound molecules have been studied with the correlation consistent basis sets, including the first- and second-row diatomic hydrides, the first-and second-row homonuclear diatomics, mixed first- and second-row heteronuclear diatomics, etc. As archetypes of strongly bound molecules, we will focus here on the CH, HF, N2, and CO molecules and the CH and C2H (n = 1-4) series. It is unlikely that these molecules will be fully representative of the class of strongly bound molecules. However, they display a range of behaviors that provide important insights into the selection of basis sets for molecular calculations. 

Before discussing heteronuclear diatomic XY, where both X and Y are second-row atoms, we first take HF as an example for detailed molecular orbital treatment. Since the H Is orbital (with energy -13.6 eV) and F 2p (-17.4 eV) have similar energies, while that of F 2s (-37.8 eV) is much lower, we only need to consider the interaction between FI Is and F 2p orbitals. 

Table 2-2 Examples of Heteronuclear Diatomics of the Second-Row Elements...

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. 

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... 

---