Ground-state, involving bond

The simplest molecular orbital method to use, and the one involving the most drastic approximations and assumptions, is the Huckel method. One str ength of the Huckel method is that it provides a semiquantitative theoretical treatment of ground-state energies, bond orders, electron densities, and free valences that appeals to the pictorial sense of molecular structure and reactive affinity that most chemists use in their everyday work. Although one rarely sees Huckel calculations in the resear ch literature anymore, they introduce the reader to many of the concepts and much of the nomenclature used in more rigorous molecular orbital calculations. 

A second illustrative example of the utility of TRPES and TRCIS for studying complex molecular photodissociation dynamics that involve multiple electronic state is the case of the weakly bound cis-planar C2V nitric oxide dimer [174], The weak (Do = 710cm-1) 1 A ground-state covalent bond is formed by the pairing of two singly occupied ji orbitals, one from each NO(X2II) monomer. The very intense UV absorption spectrum of the NO dimer appears... 

In order to form bonds effectively in these directions, the nitrogen atom needs to use tetrahedral hybrid AOs (cf. methane. Section 1.10). We therefore need to promote the nitrogen atom into a valence state where its five valence electrons occupy tetrahedral hybrid AHs, each one-fourth s and three-fourths p (Fig. 3.27b). The effective occupation of the 2s and 2p AOs in such a state is then five-fourths 2s and fifteen-fourths 2p. To get there from the ground state involves in effect the promotion of three-fourths of an electron from the 2s AO to the 2p. The energy required to do this is supplied by the... 

As an example of this, a spin-coupled calculation was carried out of the dissociation of the HCN molecule in its ground state, involving the dissociation of the C-N triple bond ... 

The discussion above is of course a generalization of that described previously in Section 10-1 for N2O4. It is appropriate for all molecules that involve extended 6-electron 4-centre bonding units, i.e. for molecules that have one or more sets of 6 electrons distributed amongst 4 overlapping atomic orbitals (a, b, c and d located around four atomic centres). These orbitals may be either n, or a (Fig. 2-6) or a+ 71 (Fig. 1-5), or o + 5 (Fig. 8-2) or o (Fig. 2-6) in character. Each of NO and CIO has a Pauling 3-electron bond in its ground-state valence-bond structure (Sections 4-5 and 4-7), and dimers of these radicals are known to be... 

Two elasses of systems illustrate eases for whieh heterolytie bond dissoeiation lies lower than the homolytie produets. The first involves transition metal dimer eations, M2. Espeeially for metals to the right side of the periodie table, sueh eations ean be eonsidered to have ground-state eleetron eonfigurations with a d d +i eharaeter, where the d eleetrons are not heavily involved in the bonding and the a bond is formed primarily from the metal atom s orbitals. If the a bond is homolytieally broken, one forms X + Y = M (s d +i)... 

When the orbitals have been classified with respect to symmetry, they can be arranged according to energy and the correlation lines can be drawn as in Fig. 11.10. From the orbital correlation diagram, it can be concluded that the thermal concerted cycloadditon reaction between butadiene and ethylene is allowed. All bonding levels of the reactants correlate with product ground-state orbitals. Extension of orbital correlation analysis to cycloaddition reactions involving other numbers of n electrons leads to the conclusion that the suprafacial-suprafacial addition is allowed for systems with 4n + 2 n electrons but forbidden for systems with 4n 7t electrons. 

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