Theoretical calculations reactivity

Numerous m.o.-theoretical calculations have been made on quinoline and quinolinium. Comparisons of the experimental results with the theoretical predictions reveals that, as expected (see 7.2), localisation energies give the best correlation. jr-Electron densities are a poor criterion of reactivity in electrophilic substitution the most reactive sites for both the quinolinium ion and the neutral molecule are predicted to be the 3-, 6- and 8-positions. ... 

The first empirical and qualitative approach to the electronic structure of thiazole appeared in 1931 in a paper entitled Aspects of the chemistry of the thiazole group (115). In this historical review. Hunter showed the technical importance of the group, especially of the benzothiazole derivatives, and correlated the observed reactivity with the mobility of the electronic system. In 1943, Jensen et al. (116) explained the low value observed for the dipole moment of thiazole (1.64D in benzene) by the small contribution of the polar-limiting structures and thus by an essentially dienic character of the v system of thiazole. The first theoretical calculation of the electronic structure of thiazole. benzothiazole, and their methyl derivatives was performed by Pullman and Metzger using the Huckel method (5, 6, 8). 

D. Comparison of Free-Radical Reactivity with Theoretical Calculations... 

In this section, reactivity studies will be emphasized while in those devoted to synthesis (Section 4.04.3) theoretical calculations on reactions leading to the formation of pyrazoles (mainly 1,3-dipolar cycloadditions) will be discussed. It should be emphasized that the theoretical treatment of reactivity is a very complicated problem and for this reason, most of the calculations have been carried out on aromatic compounds, as they are the easiest to handle. In general, solvents are not taken into account thus, at the best, the situation described theoretically corresponds to reactions taking place in the gas phase. 

Somewhat surprisingly perhaps, it has been found that [l.l.l]propellane is considerably less reactive than [2.2.1]propellane. Use the theoretically calculated enthalpy data below to estimate the bond dissociation energy of the central bond in each of the three propellanes shown. How might this explain the relative reactivity of the [1-1.1]- and [2.2. Ijpropellanes ... 

Having considered how solvents can affect the reactivities of molecules in solution, let us consider some of the special features that arise in the gas phase, where solvation effects are totally eliminated. Although the majority of organic preparative reactions and mechanistic studies have been conducted in solution, some important reactions are carried out in the gas phase. Also, because most theoretical calculations do not treat solvent effects, experimental data from the gas phase are the most appropriate basis for comparison with theoretical results. Frequently, quite different trends in substituent effects are seen when systems in the gas phase are compared to similar systems in solution. 

In summary, theoretical calculations have not been a reliable guide to relative nucleophilic reactivity at different positions in the same azine or at comparable positions in different ring systems, but they may become so in the future. 

Theoretical calculations have predicted that imidazo[l,2-a]pyrimidine (160) should be attacked at C-3 by electrophiles, although reactivity will be lower than in the corresponding imidazo[l,2-a]pyridines (see D,l,e) (74JHC1013). The 3-bromo derivative of 160 was formed when the parent was treated with NBS in chloroform (66JOC809). The usual transformation of oxo to chloro was responsible for the preparation of 5-chloroimidazo[ 1,2-a]pyrimidine [66LA(699) 127]. 

X = CO2R or CN). Theoretical calculation at B3LYP/6-31G //HF/STO-3G level showed that the Si-H bond dissociation energies of H-Si(l 11) and (MesSifsSi-H are very similar, which further justifies the use of the well-established radical-based reactivity of (MesSifsSiH as a model for surface reactions. 

Selective oxidation of p-xylene to terephthaldehyde (TPAL) on W-Sb oxide catalysts was studied. While WO3 was active in p-xylene conversion but non-selective for TPAL formation, addition of Sb decreased the activity in p-xylene conversion but increased TPAL selectivity significantly. Structure change was also induced by Sb addition. Evidences from various characterization techniques and theoretical calculation suggest that Sb may exist as various forms, which have different p-xylene adsorption property, reactivity toward p-xylene and TPAL selectivity. Relative population of each species depends on Sb content. 

The results described in this review show that matrix stabilization of reactive organic intermediates at extremely low temperatures and their subsequent spectroscopic detection are convenient ways of structural investigation of these species. IR spectroscopy is the most useful technique for the identification of matrix-isolated molecules. Nevertheless, the complete study of the spectral properties and the structure of intermediates frozen in inert matrices is achieved when the IR spectroscopy is combined with UV and esr spectroscopic methods. At present theoretical calculations render considerable assistance for the explanation of the experimental spectra. Thus, along with the development of the experimental technique, matrix studies are becoming more and more complex. This fact allows one to expect further progress in the matrix spectroscopy of many more organic intermediates. 

The authors carried out theoretical calculations on this system as well as on the [ (PMej) ] system to compare their reactivity with hexafluorobenzene. They found that the barrier for [ (liPr) ] is approximately 10 kJ/mol lower in energy than the corresponding barrier for the phosphine-bearing system. This value was in agreement with the different reactivity of both complexes but could not fully explain the large difference in reaction times. Density functional Theory (DFT) calculations also showed that the trans product is more stable than the cis product (total energies are respectively -130.9 and 91.1 kJ/mol), which was in agreement with the experimental values. 

In specific applications, it is critically important to know which isomer is produced in a particular situation in order to ascertain its further reactivity. Indeed, further reactivity, in the form of rate coefficients and product ion distributions, both identifies which reactions generate the same isomeric forms and gives information to enable the isomeric forms to be identified (often by determining the energetics and comparing them with theoretical calculations). One such application is to molecular synthesis in interstellar gas clouds. In the synthesis of the >115 molecules (mainly neutral -85%) detected in these clouds,14 a major production route is via the radiatively stabilized analog of the collisional association discussed above,15 viz. ... 

Cobalt shows a dramatic size dependence(JJ cJ that resembles the behavior of iron more so than that of vanadium or niobium. The smallest cluster to react is the trimer and the 5-9 atom clusters are significantly more inert than any of the larger clusters. Cobalt also has a significant dip in reactivity between 19 and 22. A theoretical calculation rationalized the onset in reactivity at the trimer to be associated with energetic stability of the products(29). 

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