Electron, delocalization lone-pair

The determinant (= total molecular wavefunction T) just described will lead to (remainder of Section 5.2) n occupied, and a number of unoccupied, component spatial molecular orbitals i//. These orbitals i// from the straightforward Slater determinant are called canonical (in mathematics the word means in simplest or standard form ) molecular orbitals. Since each occupied spatial ip can be thought of as a region of space which accommodates a pair of electrons, we might expect that when the shapes of these orbitals are displayed ( visualized Section 5.5.6) each one would look like a bond or a lone pair. However, this is often not the case for example, we do not find that one of the canonical MOs of water connects the O with one H, and another canonical MO connects the O with another H. Instead most of these MOs are spread over much of a molecule, i.e. delocalized (lone pairs, unlike conventional bonds, do tend to stand out). However, it is possible to combine the canonical MOs to get localized MOs which look like our conventional bonds and lone pairs. This is done by using the columns (or rows) of the Slater T to create a T with modified columns (or rows) if a column/row of a determinant is multiplied by k and added to another column/row, the determinant remains kD (Section 4.3.3). We see that if this is applied to the Slater determinant with k = 1, we will get a new determinant corresponding to exactly the same total wavefunction, i.e. to the same molecule, but built up from different component occupied MOs i//. The new T and the new i// s are no less or more correct than the previous ones, but by appropriate manipulation of the columns/rows the i// s can be made to correspond to our ideas of bonds and lone pairs. These localized MOs are sometimes useful. 

Histamine has a five-membered ring with two ji bonds and two nitrogen atoms, each of which contains a lone pair of electrons. The heterocycle has four n electrons from the two double bonds. The lone pair on N1 also occupies a p orbital, making the heterocycle completely conjugated, and giving it a total of six n electrons. The lone pair on N1 is thus delocalized over the five-membered ring and the heterocycle is aromatic. The lone pair on N2 occupies an sp hybrid orbital perpendicular to the delocalized n electrons. 

Aromatic heterocycles with a localized electron pair on N are more basic than those with a delocalized lone pair from the N atom (25.1 OD). 

Aniline is a much weaker base because its delocalized lone pair is more strongly held than the nitrogen lone pair in cyclohexylamine. The more strongly held the electron pair, the less able it is to abstract a proton. 

To confirm the structures of NaBH4 reduction products of 1,2,4-triazines, Japanese workers recently published an X-ray study, showing that the product has the 2,5-dihydro structure (25). The dihydrotriazine ring was found to take the boat conformation, which, because of the delocalization of the n electrons and lone-pair electrons of N-2, was significantly squashed.33... 

Figure 12-1 Generation of increased-valenee structures from standard Lewis structures by delocalizing lone-pair electrons into vacant bonding orbitals.

Figure 14-1 Generation of increased-valence structures for FNO2, NO2, F2SO and O3 from Lewis structures by delocalizing lone-pair electrons into vacant antibonding molecular orbitals. (In structure (9), an oxygen electron of the NO is delocalized into an O-N bonding molecular orbital.)...

By delocalizing lone-pair electrons from the Fe and 0 of structures (35) and (36) into vacant bonding Fe-O or 0-0 orbitals, we reduce the magnitude of the atomic formal charges, and obtain increased-valence structures (37) arrd (38)16-20... 

The corresponding resonance description shows the delocalization of the nitrogen lone pair electrons m terms of contributions from dipolar structures... 

The orbital and resonance models for bonding in arylamines are simply alternative ways of describing the same phenomenon Delocalization of the nitrogen lone pair decreases the electron density at nitrogen while increasing it m the rr system of the aro matic ring We ve already seen one chemical consequence of this m the high level of reactivity of aniline m electrophilic aromatic substitution reactions (Section 12 12) Other ways m which electron delocalization affects the properties of arylamines are described m later sections of this chapter... 

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