Homonuclear diatomic configurations

Some heteronuclear diatomic molecules, such as nitric oxide (NO), carbon monoxide (CO) and the short-lived CN molecule, contain atoms which are sufficiently similar that the MOs resemble quite closely those of homonuclear diatomics. In nitric oxide the 15 electrons can be fed into MOs, in the order relevant to O2 and F2, to give the ground configuration... 

Despite its very simple electronic configuration (Is ) hydrogen can, paradoxically, exist in over 50 different forms most of which have been well characterized. This multiplicity of forms arises firstly from the existence of atomic, molecular and ionized species in the gas phase H, H2, H+, H , H2" ", H3+. .., H11 + secondly, from the existence of three isotopes, jH, jH(D) and jH(T), and correspondingly of D, D2, HD, DT, etc. and, finally, from the existence of nuclear spin isomers for the homonuclear diatomic species. 

HOWTO DETERMINE THE ELECTRON CONFIGURATION AND BOND ORDER OF A HOMONUCLEAR DIATOMIC SPECIES... 

The molecular orbital energy-level diagrams of heteronuclear diatomic molecules are much harder to predict qualitatitvely and we have to calculate each one explicitly because the atomic orbitals contribute differently to each one. Figure 3.35 shows the calculated scheme typically found for CO and NO. We can use this diagram to state the electron configuration by using the same procedure as for homonuclear diatomic molecules. 

Textbook discussions of homonuclear diatomic molecules are commonly based on the familiar type of MO energy diagram shown in Fig. 3.28, which underlies the standard MO Aufbau procedure for constructing many-electron molecular configurations (which is analogous to the well-known procedure for atoms). Figure 3.28 purports to represent the energies and compositions of available MOs, which are... 

Although the most naive form of valence-bond and Lewis-structure theory would not predict the paramagnetism of O2, the VB-like NBO donor-acceptor perspective allows us to develop an alternative localized picture of general wavefunctions, including those of MO type. Let us therefore seek to develop a general NBO-based configurational picture of homonuclear diatomics to complement the usual MO description. 

These results for B2 are shown as the first row of Table 3.15, which summarizes the expected NBO configurations for the entire series of first-row homonuclear diatomics. 

Table 3.15. Valence-shell NBO configurations of first-row homonuclear diatomics... 

Figure 3.30 The NBO Aufbau diagram for first-row homonuclear diatomics, showing expected NHO configurations (right-hand panels) for each species.

From the quantum mechanical standpoint the appearance of the factor 1/2 = 1/s for the diatomic case means the configurations generated by a rotation of 180° are identical, so the number of distinguishable states is only one-half the classical total. Thus the classical value of the partition function must be divided by the symmetry number which is 1 for a heteronuclear diatomic and 2 for a homonuclear diatomic molecule. 

The electronic configurations of the homonuclear diatomic molecules of the elements of the second period, and some of their ions, are given in Table 4.1. 

Table 4.1 The electronic configurations of some homonuclear diatomic molecules and ions...

K. A. Peterson, R. A. Kendall, and T. H. Dunningjr.,/. Chem. Phys., 99, 9790 (1993). Benchmark Calculations with Correlated Molecular Wave Functions. III. Configuration Interaction Calculations on First Row Homonuclear Diatomics. 

Molecular oxygen (or dioxygen) O2 and related species are involved in many chemical reactions. The valence molecular orbitals and electronic configurations of the homonuclear diatomic species O2 and Oj are shown in Fig. 16.1.1. 

The latter mechanism is excluded for homonuclear diatomic molecules (dimers) owing to the absence of a permanent dipole moment in these molecules. In the case of heteronuclear molecules, on the other hand, such transitions are well known and are widely employed, in particular for the determination of configurational and relaxational parameters by methods of infrared spectroscopy. 

C. A4 Molecules.—There are two cases of interest here stable species such as P4 and interactions between homonuclear diatomic molecules. H4 is the simplest of the species, and it has been the subject of a great deal of theoretical study because of its relevance to the H2 + D2 - -2HD exchange reaction. Early work has been carefully discussed by Schaefer,1 and Bender and Schaefer have carried out more extensive calculations since then on the linear form.604 It is predicted that two H2 molecules may approach to within 1.6 bohr with an energy only 181 kJ above that of the separated molecules. A van der Waals attraction of 22 K is predicted at a separation of the centre of mass of H2-H2 of 7.1 bohr. The results of other studies have failed to find a transition state lying less than 458 kJ above H2 + D2. The calculations of Bender et al. used a DZ basis plus 2p-functions, larger than that used in earlier work by Wilson and Goddard, and by Rubinstein and Shavitt (see ref. 1). Full Cl with 2172 configurations was carried out, and the van der Waals minimum predicted was not found by the earlier workers. 

Table 6.3. Ground state electron configurations and states for homonuclear diatomic molecules in the first row of the periodic table...

It has been found useful to represent the interaction potential for a dimer of homonuclear diatomic molecules [4,5,46,58] as a spherical harmonic expansion, separating radial and angular dependencies. The radial coefficients include different types of contributions to the interaction potential (electrostatic, dispersion, repulsion due to overlap, induction, spin-spin coupling). For the three dimers of atmospheric relevance, we provided compact expansions, where the angular dependence is represented by spherical harmonics and truncating the series to a small number of physically motivated terms. The number of terms in the series are six for the N2-O2 systems, corresponding to the number of configurations of the dimer (for N2-N2 and O2-O2 this number of terms is reduced to five and four, respectively). 

In this section we consider homonuclear diatomic molecules (those composed of two identical atoms) formed by elements in Period 2 of the periodic table. The lithium atom has a 1 s22s electron configuration, and from our discussion in the previous section, it would seem logical to use the Li Is and 2s orbitals to form the MOs of the Li2 molecule. However, the Is orbitals on the lithium atoms are much smaller than the 2s orbitals and therefore do not overlap in space to any appreciable extent (see Fig. 14.33). Thus the two electrons... 

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