Acceptor stability

Summarizing the results of many investigations, monosaccharides and such derivatives as D-mannitol and D-glucitol are rather weak acceptors. Disaccharides, including such acceptor products as isomaltose, are much better acceptors, except for certain molecules, for instance leucrose, which is not an acceptor.29,46,47 The decrease of enzyme activity with time has been described in terms of a first-order reaction. The inactivation parameters have been calculated for the immobilized enzyme. The inactivation constants kd were 0.0135 (1/d) when maltose was the acceptor (stabilizing), and 0.029 (1/d) when fructose was the acceptor.38... 

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. 

The possible donor-acceptor stabilizations for the two remaining structures in (3.101) are similarly... 

We now wish to describe qualitative trends in pi-conjugated systems in terms of the localized NBO donor-acceptor stabilizations (3.109) and NRT bond orders. For this purpose we consider the representative group of molecules 1-16 shown in Fig. 3.37. Numerical bond orders and conjugative stabilization energies pertaining to these molecules are gathered in Table 3.19. 

Table 3.23. Rotation barriers (Ecc — Estg) and leading a-cr donor acceptor stabilizations (—A Eaa ) in anti and syn orientations for methyl rotors (CH3—X, X = CH3, NH2, OH) and higher ethane-like congeners...

Figure 4.79 displays the optimized structures of secondary-Cp (IIsec) and primary-Cp(IIPri) complexes, and Table 4.43 includes geometrical and charge parameters of these propylene complexes for comparison with those of the corresponding ethylene complex in Table 4.42. The IIsec complex can be seen to have smaller Ti—Cp metal-alkene separation (by 0.1 A) and other evidence of tighter metal-alkene binding than that in the IIpri complex, in accordance with the donor-acceptor stabilizations discussed above. 

The relative strengths of conjugative and hyperconjugative interactions are sharply altered, and general delocalization phenomena become much more pronounced near transition metals, where the abundance of low-lying unfilled valence orbitals provides many new pathways for donor-acceptor stabilization. 

As in the molecular case (Dl), the definition (D2) allows the supramolecular unit(s) to be determined by an operational NBO search of a given electron distribution ir(fi, r2,..., Fjv ) 12. Given the NBO molecular units of the distribution, we can search the intermolecular interactions (e.g., the table of perturbative donor-acceptor stabilizations) to determine the connecting noncovalent bonds that satisfy the required thermal threshold, and thereby determine the contiguously bonded supramolecular unit(s) by (D2). 

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... 

The relative weakness of second-order estimate of donor-acceptor stabilization in the cr-7t case is... 

The smaller energy denominator, Ae = en — en, also leads to an altered proportionality between the donor-acceptor stabilization energy and the quantity of charge transferred (Qct), as discussed in Chapter 2 (see the discussion surrounding Eq. (2.18)). From the general proportionality... 

Donor-acceptor stabilizations of a TS A- B complex are intimately related to the general theory of catalysis. Reduction of the repulsive TS barrier between... 

Alternatively, we can also analyze the TS complex with respect to NBOs of the product-like Lewis structure (by using the standard SCHOOSE option of the NBO program). Figure 5.59(a) depicts an alternative view of the TS geometry, with bond sticks drawn to emphasize the relationship to the product cyclohexene molecule. Figures 5.59(b)-(d) depict the leading donor-acceptor stabilizations in the product NBO Lewis structure. 

As described in Section 3.4.2, hyperconjugative donor-acceptor stabilizations favor conformers in which one of the rotor C—H bonds eclipses an adjacent double bond. (This is equivalent to an ethane-like staggered preference if the double bond is pictured in terms of two bent banana bonds. ) Hence, in the case of a perfectly localized Lewis structure I, the methyl group would be expected to adopt the preferred pseudo-cA conformation la (with in-plane C—H syn to A=C),... 

These dipolar adducts are donor-acceptor stabilized azomethine imines and their structure was ascertained by x-ray crystallography <90HCA492>. 

An extreme case of the Cope rearrangement proceeds via dissociation to an ion-pair. [228], Complementary donor and acceptor stabilization of the ionic species is the cause for this abnormal reaction. 

The last entry in Table 3 is of particular interest because there is potential competition between two n-acceptors stabilizing the product. There is evidence indicating that resonance... 

Presumably, the latter is a case where biradical mechanisms occur due to the lack of strong donor-acceptor stabilization. 

Groups that are not pi acceptors stabilize an adjacent anion less effectively, and their CH3 ewg pAfaS are not yet available. Very electronegative groups, such as trimethylammonium cation, " N(CH3)3, and trifluoromethyl, CF3, can withdraw electron... 

Scheme 4.15 Intramolecular hydrogen bridges with fluorine as the acceptor stabilize two different conformations of 2-fluoro- and 6-fluoronore-pinephrine (2-F-NE and 6-F-NE, respectively), inducing different kinds of biological activity [21].

---