Lone-pair orbitals interactions

O. Williams, J. N. Scarsdale, L. Schafer, and H. J. Ceise, Ab initio studies of structural features not easily amenable to experiment. Part III. The influence of lone pair orbital interaction on molecular... 

The paucity of information on the mechanism of reactions, and on the structure of the transition state, and the role of the anomeric effect in its stabilization, constitutes the main reason why qualitative interpretation of reactivity as shown in the aforementioned examples is still very rare. An alternative, more-popular estimation of the relative reaction-rates of con-formers is based on the lone-pair orbital interactions, and their symmetry and energy in the ground state, and could be loosely associated with the perturbation theory of chemical reactivity. ... 

The O atom uses one of its sp or sp hybrids to form the CO a bond and antibond. When sp hybrids are used in conceptualizing the bonding, the other sp hybrid forms a lone pair orbital directed away from the CO bond axis one of the atomic p orbitals is involved in the CO n and 71 orbitals, while the other forms an in-plane non-bonding orbital. Alternatively, when sp hybrids are used, the two sp hybrids that do not interact with the C-atom sp2 orbital form the two non-bonding orbitals. Hence, the final picture of bonding, non-bonding, and antibonding orbitals does not depend on which hybrids one uses as intermediates. 

The synclinal conformation (sc) is appropriate for overlap of an oxygen nonbonded pair with the a C—Cl orbital. The preferred ap relationship, requires an antiperiplanar alignment of a lone-pair orbital with the bond to the electronegative substituent. Because of the donor-acceptor nature of the interaction it is enhanced in the order F < O < N for the donor (D) atom and N < O < F for the acceptor (A) atom. 

The simplest examples of this type of compound are enamines derived from the quinuclidine skeleton (67). The formulation of enamines of qflmuclidine in a inesomeric form would violate Bredt s rule. Actually, the ultraviolet spectrum of 2,3-benzoquinuclidine shows that there exists no interaction of aromatic ring tt electrons and the nitrogen-free electron pair (160,169). The overlap of the olefinic tt orbital and the lone pair orbital on nitrogen is precluded. 

The FMOs of acrolein to the left in Fig. 8.2 are basically slightly perturbed butadiene orbitals, while the FMOs of protonated acrolein resemble those of an allyl cation mixed in with a lone-pair orbital on the oxygen atom (Fig. 8.2, right). Based on the FMOs of protonated acrolein, Houk et al. [2] argued that the predominant interaction in a normal electron-demand carbo-Diels-Alder reaction is between the dienophile LUMO and diene HOMO (Fig. 8.1, left). This interaction is greatly... 

In several previous Sections (1.15, 1.19) we considered the interaction of lone-pair orbitals with orbitals localized in other regions of the molecules. In some cases an atom (ether oxygen or thioether sulfur, halogens) may carry several lone pairs. These lone pairs are generally described as localized orbitals pointing in tetrahedral directions on the atom to which they belong as in Fig. 47. The elee-... 

Gleiter and Ginsburg found that 4-substituted-l,2,4-triazoline-3,5-dione reacted with the propellanes 36 and 37 at the syn face of the cyclohexadiene with respect to the hetero-ring. They ascribed the selectivity to the secondary orbital interaction between the orbitels (LUMO) of 36 and 37 with antisymmetrical combination of lone pair orbitals (HOMO ) of the triazolinediones (Scheme 24) [29]. 

Electron density is transferred from a Lewis donor to a Lewis acceptor, here shown for an amine (donor) interacting with a phosphenium cation (acceptor). The lone pair orbital at the amine dives into the emptyp-orbital of the phosphenium cation, hollowing the ideas of Mulliken the donation towards an acceptor can be viewed as sketched in Scheme 2. 

It is due to the already significant surface atom-molecule interaction present in the case of ammonia and water before reaction, when the molecules adsorb strongly through their respective molecular lone-pair orbitals. 

To explain the stereochemistry of the photoaddition, Buchi proposed that the reaction of electron-rich olefins and excited ketone involves an interaction of the electron-deficient carbonyl lone-pair orbital with the electron-rich 7r-olefin orbitals to form a diradical intermediate which could subsequently close to give the observed products. Indeed, reaction to yield the most stable diradical intermediate usually does nicely rationalize the observed product distribution. Examples of this are as follows11005 ... 

The NBOs are an optimized set of localized bonding, antibonding, and lone pair orbitals. The bond orbitals typically have occupancies of 1.98e in molecules with one simple Lewis structure. In C2H6, Weinhold and coworkers36 have shown that the barrier height V3 = 1025 cm-1 can be understood in the NBO terms in the form of aCH — donor-acceptor interactions. When the corresponding off-diagonal... 

The spectra and calculations all led to the conclusion that there is an usually large interaction between both the 7T and lone-pair orbitals in the carbonyl portion of the molecule with the n and a orbitals of the olefin portion. The first ionization potential (9.57 eV) involves ionization of an electron from the oxygen lone pair, whereas the second (11.19 eV) involves ionization of an electron from the olefin rr-bond. The most vertical ionization is from the 7 ax MO (16.11 eV), the second lone-pair orbital on oxygen. 

Relief of the repulsion of coplanar np2 lone pair orbitals by np2 -> no interactions... 

This means that upon oxidation of neutral chalcogens giving the polychalcogen cations, electron density is removed from the occupied rap2 lone pair orbitals.15,16 From a molecular orbital (MO) view of such interactions... 

With an interaction as in Figure 42, the two 5a (Te-Te) orbitals that include the formally positively charged tricoordinate Te atoms accept electron density from the occupied 5p2 lone pair orbitals of the two central Te atoms. In this way the central Te-Te bond is shortened to about 264 pm through the two... 

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