Lone-pair repulsion and

In addition to the effects of a cyclic transition state, of lone-pair repulsions, and of rate of removal of hydride ion mentioned above, the position of nucleophilic substitution can be altered by a) hydrogen... 

The solid-state structures of orthorhombic S864 and monoclinic Se866 are shown in Figure 15. As may be seen from the average E-E-E bond and EE EE torsion angles, this structure is almost completely relaxed and consequently lone pair repulsion and ring strain are lowest in this arrangement. Other S8 structures were assessed by quantum chemical calculations.69... 

At the MP2 level of theory, the half-chair conformer of 4//-l,3-dithiin 55 is 2.0kcalmoP more stable than the boat conformer (Equation 4) <1998JCC1064> and has a calculated twist angle of 33.2° the relative stability of the halfchair conformer was attributed to absent lone-pair-lone-pair repulsions and a decrease of torsional strain owing to an... 

Interaction is a two-orbital, four-electron one. It is destabilizing, repulsive, as 55a shows. In one-electron theories, this is where steric effects, lone pair repulsions, and the like are to be found.11 41 These interactions may be important. They may prevent bonding interactions and from being realized. There is a special variant of this interaction that may occur in the solid but is unlikely to occur in discrete molecules. This is sketched in 55b—the antibonding component of a four-electron, two-... 

Another point of view, based on calculations at the MP2/6-31G //HF/6-31G level141, says that the main factor (ca 50%) for the increase in the basicity of proton sponges is the destabilization of the base due to the nitrogen lone-pair repulsion, and the hydrogen bond contribution is at most 30%. An even smaller value of the stabilization effect of the hydrogen bond results from experimental work of Perrin and Ohta50,99 and Lloyd-Jones and coworkers109. In our opinion, in solution, the truth lies between these extreme standpoints. 

Neither of these techniques has been much used to study the heterocycles falling within the remit of this chapter. In a rare example, the photoelectron spectrum of (13a), in which no bands exhibited any splitting, showed a lowest ionization potential of 8.55 eV (i.e. virtually identical to 1,3,5-trithianes) <89JA6745>. The absence of splitting means that the nitrogen lone pair does not interact with those on the sulfur atoms. This constitutes additional evidence that the A-methyl group adopts an axial orientation in order to avoid lone pair-lone pair repulsion and possibly allow favorable interactions. 

Of these, the actual structure will be the one with a lowest eneigy, where the repulsion between the five orbitals is at its minimum. Since the bond pair - bond pair repulsion is the least, followed by bond pair - lone pair repulsion and the strongest being between lone pair-lone pair, and also groups as at 90 to each other repel strongly while the groups at 120 to each other repel much less, structure 3 is most... 

When two of the six electron groups around the central atom are lone pairs, as in Xep4, the lone pairs occupy positions across from one another (to minimize lone pair-lone pair repulsions), and the resulting molecular geometry is square planar. 

The situation is more complex for triazoles and tetrazoles where qther effects such as lone-pair repulsions intervene see discussion in (76AHC(SD296). 

The method has also been applied to partially saturated systems. For instance, the dipole moments of a series of 1-acylpyrazolines (42) with R = H, Me, Et and Ph have been measured (72CHE445) they range from 3.46 to 4.81 D. When compared with values computed by the fragmentary calculation method, the conclusion was reached that here also the E form predominates. In all these examples the lone pair-lone pair repulsions determine the most stable conformation. 

Assuming that aromatic stabilization of 24a and 24b is of the same magnitude, and this is also true for the lone-pair repulsion in the pairs 24a/25a and 24b/25b, the energy, A , of the isodesmic reaction (1) corresponds to the contribution from greater aromatic stabilization of 25a with respect to 25b (Scheme 26). At the MP2/6-31G approximation, AE = 10.5 kJ mol (94JOC2799). Similar arguments applied to the isodesmic reaction (2) allow estimation of the energy contribution due to the repulsion of adjacent lone pairs in 25a. In MP2/6-31G approximation, A = -25.9 kJ mol ... 

JCS(P2)57]. It follows, therefore, that the destabilizing lone-pair repulsion effeet appeared in the H tautomer 25a overeomes positive eontribu-tion of its greater aromatie stabilization relative to 25b and serves as a major faetor defining the relative stability of the tautomers. 

The rule is applicable to all but S3 in Table 1 [7], The most stable is thiozone (C J, whereas Sej has symmetry [14], The lone pair repulsion may destabilize the S3 ring and is weaker in the Se3 ring due to the smaller overlap between the nonbonding orbitals. 

It is important to point out that recent results on density based overlap integrals [16] confirm the interest of the formulation of Erep as a sum of bond-bond, bond-lone pair and lone pair-lone pair repulsion indeed, core electrons do not contribute to the value of the overlap integrals. 

Figure 34 Solid-state structure of grey Se. (see ref. 96) The structure ofpolymeric S (see refs. 100 and 101) and grey Te (see ref. 95) is similar (Se and Te are trigonal space group P3j21, that of S is monoclinic and slightly distorted). E-E distances 206.6pm (S), 237.5pm (Te). (a) A view that shows the orientation of the chains with respect to the np2-np2 lone pair repulsion. The E-E-E-E torsion angles are 85.3° (S), 100.6° (Se), 100.7° (Te) (b, c) views showing the helical arrangements of the chains. The E-E-E bond angles are 106.0° (S), 103.1° (Se) and 103.1° (Te)...

In this chapter, we demonstrated that the restriction of building a compound with only one type of an element is not a restriction at all and a multitude of neutral, cationic as well as anionic polychalcogen structures is currently known. As expected for the more electronegative nonmetal (S) and meta metals (Se, Te), the bonding within these moieties is covalent and a small number of interactions, namely, p2-rap2 lone pair repulsion, n- and n -n bonding as well as p2- cr interactions, are sufficient to rationalize the structures and account for the bond lengths alternations or weak transannular interactions that are often found. 

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