Second row homonuclear diatomics

Table 11.1. Bond distances and De for some second row homonuclear diatomics.

O Albert Haim, "The Relative Energies of Molecular Orbitals for Second-Row Homonuclear Diatomic Molecules The Effect of s-p Mixing," /. Chem. Educ., Vol. 68, 1991,737-738. 

Table 3.3.2 summarizes the various properties of second-row homonuclear diatomic molecules. In the last column of the table, we list the bond order between atoms A and B in the molecule AB. Simply put, the bond order is a number that gives an indication of its strength relative to that of a two-electron single bond. Thus the bond order ofHf (cr ) is 1/2, while that of H2 (afs) is 1. For a system with antibonding electrons, we take the simplistic view that one antibonding electron cancels out one bonding electron. Thus the bond orders in lief (ofs o-j 1) and He2 (ofs aj s2) are 1 /2 and 0, respectively, and helium is not expected to form a diatomic molecule. 

Even though the one-electron H2+ Hamiltonian has been used to derive all 10 molecular orbitals (only one of which is even half-filled in H2+), a MO diagram built from the expanded basis will enable us to draw quite accurate boundary surfaces and relative energy separations for all the second row homonuclear diatomics. (This has been verified by the more rigorous approaches that are detailed later.) The figures sketched in Figure 9 show the MO diagrams that result from the 10 X 10 secular determinant. 

Table 1 Bond orders of second-row homonuclear diatomic molecules with bond lengths and energies...

A. HAIM, The relative energies of molecular orbitals for second-row homonuclear diatomic molecules. J. Chem. Educ., 68, Til (1991). 

The naive extension of the model to the bonds of the second-row homonuclear diatomics (Li2 +, Li2, Be2 +, Be2), mostly involving overlap... 

Figure 9.38 Energy-level diagram for MOs of second-row homonuclear diatomic molecules. The diagram assumes no interaction between the 2s atomic orbital on one atom and the 2p atomic orbitals on the other atom and experiment shows that it fits only for O2, F2, and Ne2-... 

SECOND-ROW HOMONUCLEAR DIATOMIC MOLECULES Let US proceed now to the atoms in the second row of the periodic table, namely, Li, Be, B, C, N, O, F, and Ne. These atoms have 2s, Ipx, 2py, and Ip valence orbitals. We first need to specify a coordinate system for the general homonuclear diatomic molecule A2, since the Ip orbitals have directional properties. The z axis is customarily assigned to be the unique molecular axis, as shown in Fig. 2-10. The molecular orbitals are obtained by adding and subtracting those atomic orbitals that overlap. 

It is seen that the long-range attraction makes the interaction softer the most hard interaction corresponds to A = He. For this case, the second and third terms in Eq. (10) provide a very small corrections to the leading first term. Besides, if 7g is close to Eq.(lO) yields a 2 4A . For the case of Ar interacting with second-row homonuclear diatoms or the second-row hydrides, Eq. (11) is more appropriate. Then the neglect of 5a will provide an upper estimate of a assuming 7fi 7jj and Ra 5 we get a < 3.5A". We therefore can infer an important conclusion on reasonable values of a or their upper limits for different collision partners. 

Give the term symbol for the ground-state electron configuration of each of the second-row homonuclear diatomic molecules (Li2 through F2). 

TABLE 6.1 Summary of the First- and Second-Row Homonuclear Diatomics ... 

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. 

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