Lone-pair orientation

The steric number identifies how many groups of electrons must be widely separated in three-dimensional space. In ammonia, for example, the nitrogen atom bonds to three hydrogen atoms, and it has one lone pair of electrons. How are the three hydrogen atoms and the lone pair oriented in space Just as in methane, the four groups of electrons are positioned as far apart as possible, thus minimizing electron-electron repulsion. 

Coupling comparable with that found in awft -oximes is also observed in cyclic systems such as pyridine, quinoline, 5-phenylisothiazole and isoxazole in which the nitrogen lone-pair lies E(cis) to the adjacent hydrogen. Confirmation of the importance of the lone-pair orientation is further derived from the fact that removal of the nitrogen lone-pair by protonation or quatenization results in the reduction of H) to the value found in... 

The latter is in relation with those proposed by Deeth [30] and Berne [155]. Both involve the d-shell energy as an additional contribution to that of the MM scheme and use the AOM model with interpolated parameters to estimate the latter. In the case of the approach [30] there are two main problems. First is that the AOM parameters involved are assumed to depend only on the separation between the metal and donor atoms. This is obviously an oversimplification since from the formulae Eq. (25) it is clear that the lone pair orientation is of crucial importance. This is taken into account in the EHCF/MM method. Second important flaw is the absence of any correlation in describing the d-shell in the model [30]. This precludes correct description of the switch between different spin states of the open d-shell, although in some situations different spin states can be described uniformly. 

Stereochemically dependent upheld protonation shifts (trifluoroacetic acid) in piperidine derivatives have been claimed.76 The similarities between shifts of the / carbon in piperidine (51) and quinuclidine (54) and in 52 and 53 were taken to indicate similar lone-pair orientations, but in the light of work described in Section III,A this interpretation must be dropped. 

Photoelectron spectra and STO-3G calculations on methylpiperidines show that equatorial methyl substituents at the 2-, 3-, and 4-positions lower the ionization potential of the nitrogen lone pair by <0.1 eV. An axial methyl at C-2, however, lowers the ionization potential by 0.26 eV. These influences on ionization potentials are nearly identical in both piperidines and A-methylpiperidines, indicating that the lone-pair orientations in both systems are similar. The STO-3G calculations indicate a favoring of the equatorial A-H conformation of the piperidines by 1.0—1.9 kcal mol-1.174... 

The xenon atom in XeF4 is bonded to four atoms and has two lone pairs. As you might expect, the lone pairs orient as far away from each other as possible to minimize electronic repulsions, giving the molecule a square planar shape ... 

Here, M is taken to be a member of the 3d series. The 3d, 4s and 4p orbitals are deemed to constitute its valence orbitals. L is a ligand which has available for bonding to M only a lone pair, oriented such that o overlap is possible with M orbitals having non-zero electron density along the M-L axes. Fig. 8.1 shows the labelling of the Cartesian axes, with respect to which the M atomic orbitals are labelled. The L lone pairs are... 

Another significant deviation from known rules has been observed in the rather low affinity of K+ with the 1,10-diaza crown 18-C-6 with only AG=10 kJ/mol (in methanol). It has been shown that the free energy of binding AG in crown ether and ciyptand complexes usually is an additive function of number and electron donicity of the host donor atoms which are in contact with the metal ion.[17] Molecular mechanics calculations suggest the reduced affinity with the diazacrown to be due to the N-lone pairs in pseudoaxial position, pointing away from the metal ion (Figure 4). This has led to experiments with the N-methyl substituted crown here the N-alkyl substituents would clash which each other inside the macrocycle, therefore a pseudoequatorial lone pair orientation towards the cation is enforced, and the stability of the complex indeed returns to the normal scale with an increase to 29.5 kJ/mol.[ 18]... 

In his final section Nichols discusses the concept of the importance of the nitrogen lone pair orientation. Although he correctly points out that this idea was first suggested for DA analogues by Goldberg et al. (21) it has, in fact, been raised some time ago in connection with a similar problem in another area, i.e., the binding of opiates to their receptor sites (22, 23). 

From studies with quaternary compounds it has been concluded, however, that it is probably the positively charged form of the opiates that binds to the receptor(s) (24). Bearing in mind the interesting results with the previously mentioned sulfonium analogue of DA (14) the significance of the nitrogen lone-pair orientation theory is open to question. 

The bridged aminotetralin 44 (dezocine, p. 397) is a potent agonist in animals (reports of its agonist actions in the GPI conflict) but has antagonist properties in morphine-dependent monkeys, as shown by its precipitation of a withdrawal syndrome at a potency close to that of nalorphine/100) Dezocine appears to be the only example of an antagonist with a primary amino function and particular aspects of its pharmacological profile may be related to the nitrogen lone-pair orientation as proposed by Belleau (p.464). 

Figure 1 The Bi(l) coordination in the red pigment Bi14Cr024 the arrow indicates the lone-pair orientation...

N and C nuclei in [22] and [24] have the same relative orientation. However, the very different values observed for 3/(N-C) suggest that factors other than lone-pair orientation are important in these systems. 

The barriers to permutational isomerizations in five-membered spiro-phosphoranes have been rationalized in terms of BPR processes involving diequatorial five-membered rings and preferred lone-pair orientations. The energy difference between (22) and (23) is composed of a general strain term due to the increased bond angle at phosphorus, the energy required to rotate the lone pairs on both X and Y from the preferred equatorial orientation to an... 

Carbon-13 NMR spectroscopic properties of quinolizidines have been reviewed , generally with special emphasis on the influence of the cis or trans arrangement of the ring fusion and of the nitrogen lone-pair orientation on the C NMR spectra. 

The stereospecific influence of the N+—0 group on /(C,C) couplings in a series of nonaromatic amine oxides was discussed in detail by Potmischil et who compared it with the N lone pair orientational effect on that type of coupling. ... 

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