Deprotonation relative energies

Figure 15 gives a summary of the calculated deprotonation energies (DPE) at four different sites, and the relative energies of the corresponding anions. It is clear that the amine nitrogen is by far the most preferred deprotonation site. The DPE(N—H) amounts to... 

FIGURE 15. Deprotonation energies (DPE) of aniline at different sites and relative energies (AE) of the anions. Values given in kJ mol-1 were obtained from B3LYP/6-311 + +G(d,p) + ZPE calculations... 

Besides geometric and vibrational properties, identification of the relative energies of compounds, or the energy differences between points on the potential energy surface of a particular compound have also been undertaken.[70-73] Calculations of the bond dissociation energy, reaction energy, electron affinity, heat of formation, and enthalpy of deprotonization are practical examples of the type of properties that have been determined for salts by using quantum chemistry methods. 

The experimentally determined (S)/(R)-ratio of 18/82 was compared with the relative stabilities of the two diastereomeric products ([Co((S),(S)-ppm)((R)-ala)] / [Co((S),(S)-ppm)((S)-ala)] ), calculated by strain-energy minimization. The reported strain energies, based on a single conformer for each of the two diastereomeric products (identical to the crystal structure of the complex with coordinated (R)-alanine [328]), are in good agreement with the experimentally determined data (23/77 versus 18/82). A full conformational analysis led to a ratio of 30/70 when only conformational flexibility is allowed, or 33/67 when other isomers were also included in the analysis [294]. The assumption in the original report was that the enantio-selectivity is based on the relative energies of the diastereomeric forms of the cobalt(III) products [327]. Fortunately, a qualitatively similar result is expected if the stereoselectivity is controlled by the deprotonated intermediates. However, a quantitatively accurate prediction of the product ratio is not expected in this case. 

Although these deprotonations are kinetically controlled we were surprised to learn that simple semiempirical (PM3) calculations on the diastereomeric lithium intermediates are consistent with the observed selectivities [80]. Apparently, similar structural features determine the relative energies of the diastereomeric transition states and diastereomeric ground states in these internal chelate-directed lithiations. A further conclusion can be drawn (with more uncertainty) due to the favorable complexation of the lithium cation by four donor ligands, most intermediates (if not all) are monomeric and have a very low tendency for oligomerization. 

Fig. 3. Relative-energy profile of stationary points MlN-1, TS-1 and MIN-2 for the deprotonation reaction towards the (/f,S)-epimer, Deprotonation 1, B3LYP/6-31+G(d).

The loss of a proton from suitably substituted carbocations can provide both the trans- and ds-isomers of the resulting alkene, but the frans-isomer normally predominates. For example, deprotonation at C3 of the 2-pentyl carbocation produces mainly trans-peniene. By analyzing the relative energies of the conformational isomers of the carbocation that lead to the two isomeric 2-pentenes, explain why the fr s-isomer is formed preferentially. This analysis is aided by the use of Newman projections based on the partial structure shown. 

Is the most delocalized enolate also the most easily formed enolate Calculate relative deprotonation energies from the enolate precursors using the deprotonation energy of acetone as a standard. 

In the gas phase all sulfane molecules are relatively strong Bronsted acids. Their acidity is defined by the enthalpy or, alternatively, by the Gibbs energy of the following deprotonation reaction ... 

Ab initio calculation of Diels-Alder reactions of a series of 5-heteroatom substituted cyclopentadienes Cp-X (65 X = NH, 50 X = NH, 64 X = NH3, 67 X = O", 54 X = OH, 68 X = OH3% 69 X = PH, 51 X = PH, 70 X = PH3% 71 X = S, 55 X = SH, 72 X = SH/) with ethylene at HF/6-31++G(d)//HF/6-31-i i-G(d) level by BumeU and coworkers [37] provided counterexamples of the Cieplak effect. The calculation showed that ionization of substituents has a profound effect on the n facial selectivity deprotonation enhances syn addition and protonation enhances anti addition. The transition states for syn addition to the deprotonated dienes are stabilized relative to those of the neutral dienes, while those for anti addition are destabilized relative to those of the neutral dienes. On the other hand, activation energies for syn addition to the protonated dienes are similar to those of the neutral dienes, but those for anti addition are very much lowered relative to neutral dienes (Table 6). 

The titration coordinates evolve along with the dynamics of the conformational degrees of freedom, r, in simulations with GB implicit solvent models [37, 57], An extended Hamiltonian formalism, in analogy to the A dynamics technique developed for free energy calculations [50], is used to propagate the titration coordinates. The deprotonated and protonated states are those, for which the A value is approximately 1 or 0 (end-point states), respectively. Thus, in contrast to the acidostat method, where A represents the extent of deprotonation, is estimated from the relative occupancy of the states with A 1 (see later discussions). The extended Hamiltonian in the CPHMD method is a sum of the following terms [42],... 

Fig. 4 Free energy reaction coordinate profiles that illustrate a change in the relative kinetic barriers for partitioning of carbocations between nucleophilic addition of solvent and deprotonation resulting from a change in the curvature of the potential energy surface for the nucleophile addition reaction. This would correspond to an increase in the intrinsic barrier for the thermoneutral carbocation-nucleophile addition reaction.

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