Donor-acceptor energetic interaction

For the present case of the interactions shown in (3.101), the 7ta— 7tb donor-acceptor interaction, for example, leads to energetic stabilization... 

Orientational and energetic factors in donor acceptor interactions Let us next examine the dependence of Eqs. (3.109) on the factors in the numerator and denominator that affect conjugative stabilization, taking Eq. (3.109a) as an example. 

Numerous small geometrical and energetic differences contribute to the cis-trans net energy difference. However, it is evident that ordinary hyperconjugative donor-acceptor interactions (akin to those in ethane-like molecules) can qualitatively account for the surprising stability of the cis configuration, without invocation of steric attraction or other ad-hoc mechanisms. 

Further evidence of the importance of intermolecular donor-acceptor interactions can be obtained by deleting these interactions from the variational calculation, and recalculating the optimized geometries with charge transfer (CT) omitted. The structures resulting from such CT-deleted species are shown in Fig. 5.2. Energetic and structural properties of CT-deleted species are summarized in Table 5.4 for direct comparison with the actual H-bonded species in Table 5.1. 

The H-bonded species in Fig. 5.5 exhibit smooth variations of H-bond length and strength with respect to position in the periodic table, in a manner consistent with the expected ordering (5.36b) of Lewis-acid strengths. In each case the inter-molecular interaction is dominated by the no->aAH donor-acceptor stabilization shown in the right-hand panel. Energetic, geometrical, and vibrational properties of... 

Figure 5.6 Correlated variations of energetic (AE0...h), geometrical (Ro-e and Ai AH), and vibrational (Avah) properties of H-bonded water complexes (H20- -HA, A = F [crosses], OF [circles], NH2 [triangles], and CH3 [squares] with respect to donor-acceptor interaction AEn(Jd2) (abscissa, 1 kcalmol 1 tick marks). Vertical tick marks in each panel correspond to the quantity being plotted (a) AAo-h, 1 kcalmol 1 (b) Ro-e, 0.1 A (c) A Ah, 0.001 A and (d) Avah, 100 cm 1. (See Table 5.7 for numerical values.)...

In summary, we can say that, because of the unique absence of angular and radial nodes in the H-atom valence shell, the hydride oah orbital is uniquely suited to strong n-a donor-acceptor interactions with Lewis bases. In turn, the unique energetic and angular features of nB-aAH interactions (or equivalently, of B H—A <—> B—H+ A covalent-ionic resonance) can be directly associated with the distinctive structural and spectroscopic properties of B - H—A hydrogen bonding. 

Evidence for the reverse process, donation of electron density from the nucleophilic dimer atom to an electron-deficient molecule, also exists. Konecny and Doren theoretically found that borane (BH3) will dissociatively adsorb on Si(100)-2x1 [293]. While much of the reaction is barrierless, they note an interaction between the boron atom and the nucleophilic atom of the Si dimer during the dissociation process. Cao and Hamers have demonstrated experimentally that the electron density of the nucleophilic dimer atom can be donated to the empty orbital of boron trifluoride (BF3) [278]. XPS on a clean Si(100)-2 x 1 surface at 190 indicates that BF3 dissociates into BF2(a) and F(a) species. However, when BF3 is exposed on a Si(100)-2 x 1 surface previously covered with a saturation dose of trimethylamine, little B-F dissociation occurs, as evidenced by the photoelectron spectrum. They conclude that BF3 molecularly adsorbs to the nucleophilic dimer atom and DFT calculations indicate that the most energetically favorable product is a surface-mediated donor-acceptor complex (trimethylamine-Si-Si-BF3) as shown in Figure 5.19. 

It is called trivial because it does not require any energetic interaction between the donor and the acceptor. It is merely reabsorption of fluorescence radiation in accordance with Beer s Law and shows r 2 dependence on donor-acceptor distance. Although called trivial, it causes radiation imprisonment and can be important factor to be considered in fluorescence measurements. It may introduce error and distort emission spectrum by absorbing only that portion which overlaps its absorption spectrum. It is specially troublesome in studies on concentration quenching. 

There are other reactions, also easily describable by electron movement (curved arrows), in which n electrons are donated. In such reactions the n electrons are bonded electrons and hence die n-donor nucleophile is a weak electron donor. Consequently, a much stronger electron acceptor (stronger electrophile) is required for die electronic pull for electron donation to occur successfully. However, such descriptions are simply a matter of semantics because curved-arrow notation only shows changes in electrons, it does not indicate driving force. For a donor-acceptor interaction to occur productively, there has to be an energetic chiving force for the process, and die energy levels of die donor and acceptor must be matched so diat election movement from die donor to the acceptor can occur. 

According to this sequence, formation of cis- and tnms-stilbenes is preceded by formation of a magnetosensitive ion radical by a singlet-triplet conversion. This means that spin polarization must be observed in cis- and fram-stilbenes, and the isomerization rate must depend on the intensity of the magnetic field. These predictions were confirmed experimentally (Lyoshina et al. 1980). Hence, the ion radical route for trans/cis conversion is the main one under photoirradiation conditions. Until now, the mechanisms assumed for such processes have involved energy transfer and did not take into account donor-acceptor interaction. This interaction makes the process energetically more favorable. 

Donor/acceptor interactions in charge-transfer (CT) complexes are determined primarily by the symmetry and the energetics of the frontier orbitals (HOMO and LUMO) [20]. A... 

In octahedral silicon complexes, two AO (d z, d,2 y2, e symmetry) participate simultaneously in the a-bonding (sp d -hybridization). The d,y, d and d, orbitals with tjq symmetry can be used for ii-bonding with the appropriate orbitals of the substituents. It was put forward that an additional donor-acceptor interaction which increases the coordination number of the second row Main Group element involves continuum of the energetic states lying above the ionization potential... 

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