Preferential-exclusion principle

Here, electrons 1 and 2 occupy the Is orbital with their spins paired. The third electron cannot enter this orbital because of the operation of the Pauli exclusion principle, and it goes into the 2s orbital. Lnlike the situation with hydrogen, the 2s orbital has a lower energy than the 2p orbital because of electron electron interaction (see Section 7.7), and it is occupied preferentially. The electronic structure of lithium can therefore be written as ls 2s. The electrons in the Is orbital are, on average, much closer to the nucleus than the electron in the 2s orbital, and they are said to occupy the K shell. 

In general, it is not possible to localise electrons at a certain point i. e., their position is not defined. It is only possible to know the probability that an electron is situated at a certain point if one tries to find it there. This probability varies in space, so there are some regions near the nucleus where the electron will be located preferentially, whereas it avoids others. The region where the electron can be found is called the orbital. Figure 1.1 shows some examples of such orbitals. As can be seen from the figure, orbitals can be spherically symmetric or directed. An electron shell usually comprises several orbitals. Each orbital can be occupied by no more than two electrons Pauli exclusion principle). 

While there are no extensive reports on the relative aromaticity of the heterocycles covered in this chapter, the general reactivity of these systems can be predicted based on first principles. By assuming that these fused systems are comprised of a five-membered rc-excessive heterocyclic system and a five-membered -deficient heterocyclic system, electrophilic agents are expected to react on the n-excessive subunit. Ab initio calculations on the thienothiazoles and furothiazoles predicted that electrophilic substitutions should occur exclusively on the furan or thiophene subunit with the regioselectivity being a function of the resonance-stabilization of the reactive intermediates <76KGS1202>. A priori, C-H deprotonation by a nonnucleophilic base should occur preferentially on the -deficient heterocyclic component. 

Aciclovir is an acyclic purine nucleoside. Its antiviral activity depends upon intracellular phosphorylation to its triphosphate derivative. Because of its higher affinity for viral thymidine kinase, aciclovir is phosphorylated at a much higher rate by the viral enzyme. Thus, it is almost exclusively active in infected cells, fulfilling one of the selectivity principles of antiviral drugs. In addition, aciclovir triphosphate serves as a better substrate for viral than for host cell DNA polymerase and thereby causes preferential termination of viral DNA sjmthesis (1). 

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