Lone Pair Repulsion

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. 

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... 

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. 

Where we place the first lone pair makes no difference, inasmuch as all six positions are identical. If we place the second lone pair an rwhere except opposite the first, there is a 90° angle between the two lone pairs, generating large lone-pair repulsion. 

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. 

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... 

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. 

The aromaticity of triazole tautomers was assessed by the Bird indices 122 2H-1,2,3-triazole (91B, 1= 88) was found to be slightly more aromatic than its 1H-isomer (91A, 1= 73). The small difference in Bird indices supports only a weak influence of the aromaticity, and the lower stability of the 1 //-isomer was explained by the nitrogen lone-pair repulsion that destabilizes cyclic azo derivatives. 

One further point is worthy of brief mention. While we have focused on lone pair/lone pair repulsive interactions that destabilize transition state C, it is conceivable that A is actually stabilized relative to C by a favorable charge-charge interaction between the ester carbonyl (5 ) and the aldehydic carbonyl carbon (5+) owing to the proximity of these groups in A. While it is not yet possible to resolve the relative contributions of these distinct stereoelectronic effects, it is clear that our mechanistic proposal e)mlains the experimental results only if the dioxaborolane and the C-COaiPr bonds exist in the conformations indicated in B. Any conformational infidelity at either site would be expected to lead to diminished enantioselectivity. 

Originally, it was proposed that lone pair repulsions between one of the tartrate ester carbonyl oxygens and the aldehyde oxygen in transition structure 60 were responsible for the preference for transition structure 59 and the consequent enantiofacial selectivity (Scheme 5). Recent theoretical calculations. 

This section was intended only to demonstrate the importance of minimizing lone pair repulsions. A more representative hydroxylamine will now be discussed in greater detail. 

We have approached these multi-faceted systems by looking in particular at two local molecular properties the electrostatic potential, P(r) and Vs(r). and the local ionization energy, /s(r). In terms of these, we have addressed hydrogen bonding, lone pair-lone pair repulsion, conformer and isomer stability, acidity/basicity and local polarizability. We have sought to show how theoretical and computational analyses can complement experimental studies in characterizing and predicting molecular behavior. ... 

In Table 2 are listed the hydroxylamines, oximes and hydroxamic acids for which we have determined the gas phase structures. We tried to select a representative group in each category. There are two types of oximes, as indicated, aldoximes and ketoximes. Due to restricted rotation around the C=N double bond, these can exist in two isomeric forms (except when R = H for an aldoxime and R = R" for a ketoxime). We have investigated both isomers in nearly every instance. For aldoximes, they are generally labeled syn when the H and OH are on the same side of the double bond and anti when on opposite sides. Note that the ketoximes in Table 2 contain one pair of isomers in which the >C=NOH group is not bonded to two carbons instead one bond is to a chlorine. One of these isomers wiU be of interest in Section B.D in the context of hydrogen bonding vi lone pair—lone pair repulsion. 

The structures of these molecules show the effects of intramolecular electrostatic interactions. Two examples are the lone pair—lone pair repulsion that is an important determinant of hydroxylamine and oxime conformations, and the intramolecular hydrogen bonding in hydroxamic acids that promotes the near-planarities of their —C(=0)—NO frameworks. 

Formaldoxime (2) has been shown to have a planar structure with Cs symmetry (Figures 1 and 4). Experimental and theoretical studies found the anti conformer (anti-2) to have the lower energy (with Asyn-mtiE = ca 24.2 kJmol ), which may reflect the lone-pair-lone-pair repulsion between oxygen and nitrogen atoms. The anti-syn internal rotational barrier is about 38-42 kJmol depending on the level of theory applied " . 

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