Lone pair interactions

In 1,8-bis(dimethylamino)naphthalene steric and lone pair interactions are minimised in a structure (Einspahr et al., 1973) having the amino groups twisted and displaced above and below the plane of the naphthalene ring, as in [53] in which the almost planar naphthalene ring is shown as a dashed line. In l,8-bis(dimethylamino)-2,7-dimethoxynaphthalene, steric interactions involving the methoxy substituents bring about a further distortion as in [54] (Woolf, 1980). 

Focusing now on the sigma lone pair interactions, we can simplify the interaction diagram as shown in Fig. 11. The relative stabilization of the cis and trans geometries due to the interaction of the lone pairs with the central C-C bond can be assessed from consideration of all orbital interactions shown in Fig. 11. These interactions and their impact upon geometrical preference are discussed below. 

Fig. 14. Sigma fluorine lone pair interactions with the AO s of the carbon atom in F2CH2. A similar diagram can be drawn if the sigma MO s of the CH2 are considered...

Fig. 15. Sigma fluorine lone pair interactions with the sigma MO s of the C=C fragment...

The results of INDO calculations of planar and distorted difluorobenzene in which the C—F bonds have been displaced a few degrees above and below the mean plane, are shown above. As can be seen, the planar geometry maintains pi nonbonded attractive interaction. Sigma lone pair interactions will be similar to those which obtain in 1,2-difluoroethane and appear to be repulsive in nature. 

We now come to other aspects of Kollman s work. Specifically, in our original work, we commented upon the fact that greater nonbonded attraction between lone pairs is obtained in U or Y molecules. The relative stability of isomeric U and Y systems, obviously, depends on much more than lone pair interactions. Accordingly, our original work did not deal with this topic. Thus, the statement of Kollman,... 

The reasons for the reluctance of the diamines to undergo protonation is due to the inaccessibility of the basic sites. The high thermodynamic basicity is probably due to a combination of the formation of a strong intramolecular hydrogen bond and to unfavourable lone pair interactions in the diamines that cannot be relieved by solvation. 

MNDO. Despite its success, Dewar recognized certain weaknesses (6) in MINDO/3 due to the INDO approximation, such as the inability to model lone pair - lone pair interactions. Additionally, due to the use of diatomic parameters in MINDO/3, it was increasingly difficult to extend MINDO/3 to additional elements. Because of this, Dewar began working on a new model based on the better NDDO approximation. 

Lewis acid-base systems lone-pair interaction in, 16 100 PES of, 16 94... 

Analogously, shieldings of antiperiplanar carbons by nitrogen lone pairs have been reported. It was concluded that a considerable part of the 10 ppm difference of the C-2 chemical shifts in the isomers of 8-methyl-8-azabicyclo[3.2.1]octane must be due to lone-pair interaction, since a difference of only 6 ppm is expected from a y-ganche effect of the A-methyl group73. This is supported by the fact that the difference is reduced if the solvent is methanol in this case the lone pairs are involved in hydrogen bridging. 

Most of the (W,W)-carbenes are predicted to be linear and this substitution pattern results in a polarized two-electron three-center n system. Here also, the C—W bonds have some multiple bond character these (W,W)-carbenes are best described by the superposition of two ylidic structures featuring a positive charge at the carbene carbon atom. The most studied carbenes of this type are the transient dicarbomethoxycarbenes and the masked diborylcarbenes. Since no carbenes of the latter type have yet been isolated, they are not included in this chapter. Lastly, the quasilinear (D,W)-carbenes combine both types of electronic interaction. The D substituent lone pair interacts with the py orbital, while the W substituent vacant orbital interacts with the px orbital. These two interactions result in a polarized allene-type system with DC and CW multiple bonds. Good examples of this type of carbene are given by the transient halogenocarboethoxycarbenes and by the stable (phosphino)(silyl)- and (phosphino)(phosphonio)carbenes (see below). 

The electronic structure of the surface chemical bond is discussed in depth in the present chapter for a number of example systems taken from the five categories of bonding types (i) atomic radical, (ii) diatomics with unsaturated -systems (Blyholder model), (iii) unsaturated hydrocarbons (Dewar-Chatt-Duncanson model), (iv) lone pair interactions, and (v) saturated hydrocarbons (physisorption). 

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