Donor-acceptor orbital interactions

A prominent feature of this mechanism is that the growing polymer chain alternately swings between two r/.v-disposed coordination sites during each monomer insertion. General mechanistic outlines of this reaction have been extensively examined by large-scale computations and confirmed by experimental means.59 Our present goal is to clarify the localized donor-acceptor-orbital interactions that underlie (4.106), particularly the nature of the alkyl-alkene complex II. 

This chapter outlined several fundamental factors that control the magnitude and importance of stereoelectronic effects. In the following chapters, we will discuss what the consequences of donor-acceptor orbital interactions are. We will focus on the two questions of general importance for organic chemists stability and reactivity. In the very next chapter, we will provide examples of conformational effects controlled by vicinal stereoelectronic interactions. We will illustrate that such effects often provide the key electronic stabilization that is responsible for the preferred conformational profiles and explains the shapes of many key organic functional groups. 

There are at least three different definitions and vantage points that chemists use to expand upon the electrostatic paradigm. They are nucleophile-electrophile combinations, Lewis acid-base reactions, and donor-acceptor orbital interactions. We have used these terms repeatedly throughout this book because they are presented in introductory organic chemistry classes, but here we give them strict definitions. Each of these definitions is a subtle variation on the other, and often it is essentially a case of semantics to decide which best describes a particular reaction. 

C) Synergistic H2 coordination. When the metal has both donor and acceptor orbitals, interaction with the cthh and ohh orbitals of H2 can act synergistically (cooperatively) in the Dewar-Chatt-Duncanson mode. Such self-reinforcing charge flow enables a significantly stronger interaction than does simple dative coordination of H2. 

To produce an interaction diagram we need the frontier orbitals of Cr and CO. The former are 3d, 4s and 4p whereas the latter are 5a and 2ir and Cr possesses six valence electrons whereas each CO has two in the HOMO. For simplicity we will first consider only the CO donor orbitals and then add the acceptor orbital interactions later. For the first problem then we have nine metal functions and six ligand functions plus 18 electrons total. Thus, we must generate 15 MOs of which 9 will be filled. Is this a simplification The full problem requires nine metal functions and 6 x (4 + 4) ligand functions for a total of 57 MOs Of course, in a real calculation, the fragment analysis is done by the computer on the MO solutions of the complete... 

Donor properties of lone pairs Due to the high energy and polarizibility of lone pairs, they are the best donors in orbital interactions. As a result, their stereoelectronic properties often play a key role in inter- and intramolecular donor-acceptor interactions. 

The trimer can be described by a Au2 dimeric unit to which an additional atom with an unpaired electron is added. Two donor-acceptor-type interactions are then possible between the two partners. On the one hand, the unpaired 6 electron of the additional atom acts as a donor toward the anti-bonding a orbital of the dimeric unit (acceptor). On the other hand, the bonding a orbital of the dimeric unit acts as a donor toward the partially filled 6 orbital of the additional atom (acceptor). Following the model description presented above, the AU3 trimer is expected to adopt a linear or triangular configuration according to the predominance of one of the two mentioned interactions. These two extreme situations are illustrated below ... 

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. 

Alkyl substituents accelerate electrophilic addition reactions of alkenes and retard nucleophilic additions to carbonyl compounds. The bonding orbital of the alkyl groups interacts with the n bonding orbital, i.e., the HOMO of alkenes and raises the energy (Scheme 22). The reactivity increases toward electron acceptors. The orbital interacts with jt (LUMO) of carbonyl compounds and raises the energy (Scheme 23). The reactivity decreases toward electron donors. 

Keywords Cycloadditions, Chemical orbital theory. Donor-acceptor interaction. Electron delocalization band. Electron transfer band, Erontier orbital. Mechanistic spectrum, NAD(P)H reactions. Orbital amplitude. Orbital interaction. Orbital phase. Pseudoexcitation band. Quasi-intermediate, Reactivity, Selectivity, Singlet oxygen. Surface reactions... 

The pseudoexcitation in donors occurs through the d-a -d interaction. The a-d-a orbital interaction causes the pseudoexcitation in the acceptors. The simultaneous pseudoexcitations in the donors and acceptors are caused by the a-d-a -d interaction (Scheme 2). 

Scheme 2 Change of the frontier orbital interactions with the power of donors and acceptors...

With the power of the donors and acceptors, changes occur in the important frontier orbital interactions (Scheme 2) and in the mechanism of chemical reactions. The continuous change forms a mechanistic spectrum composed of the delocalization band to pseudoexcitation band to the electron transfer band. 

The introduction of heteroatoms into the hydrocarbon diradicals is a frequently applied strategy to tune the spin preference and relative stabilities of diradicals. The heteroatoms may change the energies of donor or acceptor orbitals, and consequently affect the donor-acceptor interaction involved in the cyclic orbital interaction. Take 2-oxopropane-l,3-diyl, or so-called oxyallyl (OXA, 18) as an example [29]. It is a hetero analog of TMM, as shown in Fig. 14. The replacement of CH with oxygen in the central fl unit leads to a decrease in energies of Jt and k orbitals. This may enhance the orbital interaction through one path (denoted by bold lines) and weaken that via the other (denoted by wavy lines) relative to the continuous cyclic orbital interaction in the parent species 1 (Fig. 14). As a result, the p-Jt -q... 

The substituents and heteroatoms can be used to tune the spin preference of the acyclic diradicals by changing the energy levels of electron-donating and -accepting orbitals and hence the donor-acceptor interaction. 

The donor-acceptor formation can be considered by transfer of electrons from the donor to the acceptor. In principle one can assume donor-acceptor interaction from A (donor) to B (acceptor) or alternatively, since B (A) has also occupied (unoccupied) orbitals, the opposite charge transfer, from B to A. Such a view refers to mutual electron transfer and has been commonly estabUshed for the analysis of charge transfer spectra of n-complexes [12]. A classical example for a donor-acceptor complex, 2, involving a cationic phosphorus species has been reported by Parry et al. [13]. It is considered that the triaminophosphines act as donor as well as an acceptor towards the phosphenium cation. While 2 refers to a P-donor, M-donors are in general more common, as for example amines, 3a, pyridines, 3b, or the very nucleophilic dimethylaminopyridine (DMAP) [ 14], 3c. It is even a strong donor towards phosphorus trichloride [15]. 

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