Lone pairs, nonequivalence

With a steric number of 5, chlorine has trigonal bipyramidal electron group geomehy. This means the inner atom requires five directional orbitals, which are provided hymsp d hybrid set. Fluorine uses its valence 2 p orbitals to form bonds by overlapping with the hybrid orbitals on the chlorine atom. Remember that the trigonal bipyramid has nonequivalent axial and equatorial sites. As we describe in Chapter 9, lone pairs always occupy equatorial positions. See the orbital overlap view on the next page. 

It follows that the two seemingly different representations of the lone pairs of water, in terms of equivalent sp3 hybrids or nonequivalent canonical MOs, are both correct in that they correspond to the same unique polyelectronic wave functions. 

Table 1-1. Nitrogen lone pair natural population (NP(lp)), nuclear shielding (with its paramagnetic and diamagnetic contributions) (ppm) and absorption energies (eV) for pyridazine and water—pyridazine clusters in vacuo (VAC) and in the presence of an external continuum (PCM). For 2 w clusters two values corresponding to the two nonequivalent nitrogens are reported. Calculations are at B3LYP/6-311++G(2d,2p) level for populations and nuclear shielding and at TDB3LYP/6-31+G(d,p) level for absorption energies...

The geometries found for the complexes of formaldehyde with Brst and second row cations in theoretical studies were analyzed in terms of molecular orbitals. Based on the results of photoelectron spectroscopy, it was argued that the carbonyl group contains two nonequivalent lone pairs an sp-hybridized orbital contains one pair of electrons along the C—O axis and a second, higher energy lone pair in a p-like orbital lies perpendicular to the C=0 axis (Figure 7). ... 

If the symmetry of the system is broken, as in glycosyl derivatives, it is best to adopt the nonequivalent (p-type and n-type) representation of the lone pairs on oxygen. This is illustrated with a-D-xylopyranosyl fluoride in Fig. 2. With this representation it is important to note that the p-type lone pair is tilted at a dihedral angle of ca. 30% to the axial C—F bond. 

Soon, it was pointed out by Fuchs et al. (89) that the equivalence of oxygen lone electron pairs is not consistent with X-ray structures of carbohydrates. They scrutinized 111 structures and found torsion angles O in Oendo—C—O—R fragments appreciably larger than 60°. If lone pairs were equivalent, one might expect 0 = 60° to be preferred whereas nonequivalence implies 0 > 60°. Indeed, Cosse-Barbi et al. (69,257) have recently shown that the endo anomeric effect is stronger in furanoses than in pyranoses. They... 

The similar valence-bond idea, particularly appealing, is that the bonding pairs and the lone pair are in four tetrahedral sp orbitals. This structure places the four electron pairs as far away from each other as possible. The tetrahedral structure of NHa is shown in Fig. 6-9. The slight deviation of the H—N—H bond angle from the tetrahedral angle of 109° is considered a result of the nonequivalence of the bonding and non-bonding pairs of electrons. 

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