Donor-acceptor interaction stabilization

The tendency of the halogens to form chain-like polyanions that are stabilized by delocalization of the negative charge [15,34] is a basic chemical principle. Donor-acceptor interactions between Lewis-acidic Br2 and halide anions, but also with polyhalides acting as Lewis bases, give rise to the formation of a variety of homo and heteroatomic adducts. The maximum number of atoms in these chains increases with the atomic weights... 

Since equatorial attack is roughly antiperiplanar to two C-C bonds of the cyclic ketone, an extended hypothesis of antiperiplanar attack was proposed39. Since the incipient bond is intrinsically electron deficient, the attack of a nucleophile occurs anti to the best electron-donor bond, with the electron-donor order C—S > C —H > C —C > C—N > C—O. The transition state-stabilizing donor- acceptor interactions are assumed to be more important for the stereochemical outcome of nucleophilic addition reactions than the torsional and steric effects suggested by Felkin. 

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... 

In the preceding section, the interaction energy between two reacting molecules has been discussed with the assumption of no nuclear configuration change. In the donor-acceptor interaction the delocalization stabilization is dominant. Eq. (3.25) indicates the importance of HO and LU in the donor-acceptor interaction. But the expression of Eq. (3.21) shows that in general cases the contribution of HO and LU to the quantity D is not so discriminative as those of the other MO s. 

Evidently, stable nitrile oxides can be investigated by spectral and X-ray methods using ordinary procedures. As examples, X-ray diffraction studies of o-sulfamoylbenzonitrile oxides (20), 5-methyl-2-(methylsulfonyl)-3-thiophene-carbonitrile oxide (21), (),( >-diphenylacrylonitrile oxide (22), and (dimorpholino-phosphoryl) carbonitrile oxide (23) can be cited. It should be underlined that structures of the latter compounds differ from those of classical stable 0,0 -disubstituted arylcarbonitrile oxides and tert-alkylcarbonitrile oxides. Therefore, not only purely steric shielding of the CNO group but also electrostatic or donor-acceptor interactions between the atoms of the latter and adjacent polar substituents (21, 23) and also electron delocalization in it-systems (20, 22) enhance the stability of nitrile oxide. 

Equation (2.18) establishes an important relationship connecting E, jJ2) (the stabilization energy) and Q, Jt (the charge transferred) in a general donor-acceptor interaction. Since Ac is typically a large energy separation (of order unity in a.u., 1 a.u. = 627.5 kcal mol-1), we can express this relationship in the approximate form... 

Sigma- and pi-type donor acceptor interactions Further details of the leading hp —hM donor-acceptor interactions are gathered in Table 2.5 and Figure 2.21. For each such interaction the table shows the hybrid form of the donor (hp)23 and acceptor (hM) orbitals, the occupancy of the acceptor, and the second-order estimate (cf. Eq. (1.35) or (2.7)) of the donor-acceptor stabilization energy. Let us now discuss some of the trends displayed in Table 2.5. 

We conclude that L2a-Qb donor-donor interactions are generally ineffective at lowering the total variational energy,28 whereas L2a-f2b donor-acceptor interactions are universally stabilizing. Comparison of Fig. 3.2 (or Fig. 1.3) with Fig. 3.13 shows clearly how this fundamental difference arises from the Pauli restriction on orbital occupancies. 

Although the (3-nc orbital is formally vacant in the cation, Table 3.7 shows that a small residual population (0.0346c) survives in this orbital. This occupancy can be attributed to a strong donor-acceptor interaction with the filled no orbital as depicted in Fig. 3.14. This n0 nc interaction is estimated by second-order perturbation theory (Eq. (1.24)) to stabilize the ion by 19.5 kcal mol-1, a significant delocalization that is primarily responsible for the slightly lower %p(L) value in this ion. 

The general Lewis-acid-base reaction (3.95) exemplifies the two-electron stabilizing donor-acceptor interaction of Fig. 1.3 (namely the nN->-nB interaction for (3.94)), which may be distinguished from the complementary bi-directional donor-acceptor interactions of covalent-bond formation (Section 3.2.1). However, this leaves open the question of whether (or how) the equilibrium bond reflects the formal difference between heterolytic (3.95) and homolytic (3.96) bond formation. 

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. 

The enhanced stabilizations of benzene are apparently due to the unique cyclic conjugative topology in which all sites are of closed-CT type. Each site thereby participates in complementary bi-directional donor-acceptor interactions,... 

Second-order perturbative estimates indicate that each trans-like donor-acceptor interaction (Fig. 3.55(a)) stabilizes the molecule by 2.58 kcal mol-1, compared with only 0.89 kcal mol-1 for the riv-likc interaction (Fig. 3.55(b)). The smaller gauche-like stabilizations (0.20 kcal mol-1 at 60° in the staggered conformer, 0.70 kcal mol-1 at 120° in the eclipsed conformer) diminish the difference somewhat, but still preserve a significant hyperconjugative advantage for the staggered conformer. 

According to second-order perturbative estimates, each och— o cf interaction of the cis isomer (lower left panel of Fig. 3.61) contributes 6.84 kcal mol-1 stabilization, about twice that of the ctch— o ch (3.75 kcal mol-1) 84 or (2.29 kcal mol-1) interactions of the trans isomer. Even though the remaining ocf och interaction of the cis isomer contributes only 1.17 kcal mol-1 stabilization, the sum of the four anti donor-acceptor interactions favors the cis isomer by almost 4 kcal mol-1. This hyperconjugative advantage of the cis isomer is sufficient to overcome the inherent steric and electrostatic advantage of the trans isomer. 

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. 

Figure 3.62 Leading n-

Figure 3.65 The prototypical no-uco anomeric donor-acceptor interaction in dihydroxymethane (estimated second-order stabilization energy 14.43 kcal...

Figure 3.73 Leading NBO donor-acceptor interactions (and second-order stabilization energies) in the aa conformer of the 1,2-diaminoethane cation 30+ (a) ccc cnn-6 and (b) cnn - o cc - Note that the two N lone-pair hybrids are in phase (+) in the acceptor cnn 6 (a), but out of phase ( ) in the donor ctnnX (b).

Figure 3.104 One of the four equivalent strong cbb Tbhb donor-acceptor interactions in B4Hio (estimated second-order stabilization 28.9 kcal mol-1). Note the resemblance to a canted cw-like vicinal ubiB, o b2h interaction.

Figure 3.119 Leading donor-acceptor interactions in AI2H6 (left) and Ga2H6 (right), showing overlap contours in the plane of three-center bridge-bonding (the pi plane, top row) and two-center skeletal bonding (the sigma plane, bottom two rows), with associated second-order stabilization energies in parentheses.

Thus, the ligand-stabilized d-orbital splitting pattern is qualitatively consistent with the expectation of crystal-field theory, but the physical origin of this splitting should be attributed to attractive donor-acceptor interactions such as (4.86b) rather than to any inherent electrostatic repulsions toward the incoming ligands. More accurate treatment of the spectroscopic 10Dq value should, of course, be based on separate consideration of the two spectroscopic states. 

However, significant stabilization is also contributed by nN—>-szn donor-acceptor interactions, each with estimated second-order interaction energy 49.4 kcal mol-1, as depicted in Fig. 4.52. Each ammine ligand thereby donates about 0.061 e to the zinc cation, primarily to the vacant 4s orbital which acquires about 0.371 e total occupancy. As before, the high formal hypervalency at the metal center is achieved within the limits of the duodectet rule, i.e., without significant involvement of extravalent metal p orbitals. 

Figure 4.62 Ti- H2 donor-acceptor interactions at R = 1.8 A (with estimated second-order stabilization energies in parentheses) (a) o-HH nTi and (b)...

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