Orbital overlap stabilized molecules

Orbital Overlap Stabilizes Molecules This principle is part of our explanation for covalent bonds. When orbitals of the same phase from different nuclei overlap, the electrons in these orbitals can be shared by both nuclei, resulting in stabilization. The result is a covalent bond. 

These opposite signs can be explained by considering a twofold orbital interaction between the two parts of an arenediazonium ion, namely between the jr-HOMO of the diazonio group and the cr-LUMO of the aryl residue, and between the jr-HOMO of the aryl residue and the jr-LUMO of the diazonio group. These two overlaps stabilize the C—N bond and reduce the rate of dediazoniation into a phenyl cation and a nitrogen molecule. The two opposing HOMO-LUMO interactions are shown in Figure 1. Thus... 

As is known,<1968,108) there is some evidence for the overlap of the empty d orbital of phosphorus with the methylene p orbital this stabilizes the molecule as a partly ionic species. It would be worthwhile investigating the magnitude and signs of couplings in various ylides having a range of stabilisation. 

Keeping in mind that all three x-rayed flavodoxins (7, 8, 9, 10, 11) exhibit C(7,8) as the only possible point of direct outer contact while all the rest of the flavin molecule is buried in the protein, we must finally admit that chemically C(8) is the reasonable site of orbital overlap between flavin and secondary heteroaromatic le -donor-acceptor molecules such as flavin itself and, above all, cytochrome. Flavodoxins represent the case of flavoproteins scheduled for le -only transfer by a hydrogen bond directed from the apoprotein towards N(5), stabilizing the radical Hp. The bond is strong enough to maintain the active protein reduced in the lower le -shuttle between Flred and Fl. 

In cycloaddition, two new a bonds are formed by use of n electrons of the reactants. The concerted reaction results from overlap of orbitals of one molecule with orbitals of the other. As before, it is on electrons in the HOMO that we focus attention. But which orbital does the HOMO overlap Each new orbital in the product can contain only two electrons. The HOMO of each reactant already contains two electrons, so it must overlap an empty orbital of the other reactant it picks the most stable of these, the lowest unoccupied molecular orbital (LUMO). In the transition state of cycloaddition, then, stabilization comes chiefly from overlap between the HOMO of one reactant and the LUMO of the other. 

The terms aromatic and antiaromatic have been extended to describe the stabilization or destabilization of TRANSITION STATES of PERICYCLIC REACTIONS. The hypothetical reference structure is here less clearly defined, and use of the term is based on application of the Huckel (4n+2) rule and on consideration of the topology of orbital overlap in the transition state. Reactions of molecules in the ground state involving antiaromatic transition states proceed, if at all, much less easily than those involving aromatic transition states. 

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