Localization energy ortho

Pankratov in 2000 computed relative a-complex energies for nitration, which he referred to as cationic localization energies, for a large number of monosubstituted benzenes by the use of the semiempirical PM3 method [91]. Good linear relationships were found for predicting the positional selectivity from the a-complex energies. However, different scaling factors were needed for the different positions, that is, the ortho, meta, and para positions. 

Instead of using repeated solution of a suitable eigenvalue equation to optimize the orbitals, as in conventional forms of SCF theory, we have found it more convenient to optimize by a gradient method based on direct evaluation of the ener functional (4), ortho normalization being restored after every parameter variation. Although many iterations are required, the energy evaluation is extremely rapid, the process is very stable, and any constraints on the parameters (e.g. due to spatial symmetry or choice of some type of localization) are very easily imposed. It is also a simple matter to optimize with respect to non-linear parameters such as orbital exponents. 

The energy of the localized transition state for the ortho route (uncatalyzed reaction) is 14 kcal moL higher than that of the meta channel. Therefore, the ortho channel can be excluded. Unlike the uncatalyzed transformation, the TADDOL-catalyzed HDA... 

The energy of the localized transition state for the ortho route (uncatalyzed reaction) is 14kcal/mol higher than that of the meta channel. Therefore, the ortho channel can be excluded. Unlike the uncatalyzed transformation, the TADDOL-catalyzed HDA reaction exhibited a clear energetic preference for the endo- over the exo-approach. Thus, only endo transition states were considered. The number of possible reaction paths/transition states is thus reduced from eight to two, namely endo-approach with re- or si-face attack of the model diene to the activated benzaldehyde. 

If the electrophile attacks the benzene ring at a position ortho or para to a + / substituent (i.e., to one electron-donating by inductive effect), the activated complex will be similar to 81 or 82, respectively. Resonance structures 81c and 82c are of particularly low energy because in these the positive charge is localized on the carbon that bears the + / group. Attack at the meta position does not allow such a resonance structure to be drawn. 

Recently, we investigated the associative alkylation reaction of toluene with methanol catalyzed by an acidic Mordenite (see Figures 13 and 14) by means of periodic ab initio calculations." We observed that for this reaction some transition selectivity occurred, and induced sufficiently large differences in activation energies to explain the small changes in the para/meta/ortho distribution experimentally observed on large pore zeolites. Thepara isomer is the more valuable product as it is an important intermediate for terphthalic acid, an important polymer monomer." The steric constraints obtained for the transition state structures could be estimated from local intermediates for which the orientations of the toluene molecule were similar as the ones observed for the transition states (see Figure 14). 

Hamiltonian Equation 4 for potentials characteristic of each of the surface regions. For simplicity we will assume the parameter, y, of the potential energy Equation 8 is the same in each surface region but the parameter, D, giving the depth of the potential well varies. A minor modification of the theory of localized unimolecular adsorption by Hill 14) can then be used to calculate the distribution of ortho-para or isotopic species on a surface in equilibrium with a gaseous mixture of the same species. 

All azoaromatics, be they of the azobenzene, arainoazobenzene, or donor/acceptor pseudo-stilbene type, experience considerable spectral changes on protonation or complexation. Ortho metallation has the same effect.Usually the ic— ic band is red-shifted, possibly due to the localized charge at the N-atom. By the same token, the (n,Jt ) state is shifted to higher energies. Minor band shifts and intensity changes indicate double protonation of azobenzene. 

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