Lone pair delocalized

Of course, there is intrinsically no sharp physical distinction between a strongly delocalized lone pair and a highly polarized dative bond. As shown by Eq. (1.31a), a general NBO allows smooth variations of polarization coefficients between the single-center ionic (ca = 0, cb = 1) and two-center covalent (ca = cb) limits. 

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. 

If the negative charge on the anion can actually be delocalized round the ring, as with substituted phenols, we should expect the size of p to increase. Both the phenol and the anion are delocalized but it is more important for the anion. The effect is larger for the ionization of anilinium salts as the acid (ArNH ) does not have a delocalized lone pair but the conjugate base (ArNH2) does. 

Alkylamines (RNH2) are more basic than arylamines (C6H5NH2), which have a delocalized lone pair from the N atom (25.1 OB). 

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. 

The topological method also addresses other chemical questions. Connected atom pairs are located by the bond path. Bader has defined the existence of the chemical bond as the existence of a bond path in a molecule at its local energy minimum geometry.33,34 ggnt bonds are indicated by bent bond paths. Ring strain, surface delocalization, lone pairs, electrophilic and nucleophilic sites, bond order, bond strength, molecular structure, topological instability. 

Several isomeric forms can be written for group 13 triple bonds, shown in Figure 26. Bending of the C-Ga-Ga angle away from 180° is evidence for the presence of lone pairs, and calculations on a variety of model compounds are consistent with a Ga-Ga bond order of 2. The isomeric form with one tt bond and one delocalized lone pair (as in the group 14 alkene analogs) is possibly the best description of this molecule. ... 

Fig. 20.3. Schematic representation of some non-bonding and tt-bonding molecular orbitals in SO2 or NO2. Left the non-bonding sp atomic orbital on the central atom and two delocalized lone pair orbitals formed by linear combinations of oxygen p orbitals in the molecular plane. Right the n molecular orbitals formed by linear combination of the valence shell pjc AOs of the three atoms. Comparison with the n orbitals of CO2 (Fig. 20.1) shows that the change from a linear to an angular structure has little effect on the n orbitals perpendicular to the molecular plane.

Recall that one of our five patterns was a lone pair that is allylic to a it bond. Such a lone pair will participate in resonance and is said to be delocalized. When an atom possesses a delocalized lone pair, the geometry of that atom is affected by the presence of the lone pair. As an example, consider the strucmre of an amide ... 

Let s get some practice identifying localized and delocalized lone pairs and using that information to determine geometry. 

There are two lone pairs in the structure of nicotine. In general, localized lone pairs are much more reactive than delocalized lone pairs. With this information in mind, do you expect both lone pairs in nicotine to be reactive Justify your answer. 

A delocalized lone pair participates in resonance and occupies a p orbitai. 

Resonance Structures (Sections 2.7-2.11) Delocalized Lone Pairs (Section 2.12)... 

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