Computational studies stabilizing effects, carbocation

Model computational studies aimed at understanding structure-reactivity relationships and substituent effects on carbocation stability for aza-PAHs derivatives were performed by density functional theory (DFT). Comparisons were made with the biological activity data when available. Protonation of the epoxides and diol epoxides, and subsequent epoxide ring opening reactions were analyzed for several families of compounds. Bay-region carbocations were formed via the O-protonated epoxides in barrierless processes. Relative carbocation stabilities were determined in the gas phase and in water as solvent (by the PCM method). 

Generation and NMR studies of the carbocations from various classes of PAHs under stable ion conditions, in combination with computational studies, provide a powerful means to model their biological electrophiles. These approaches allow the determination of their structures, relative stabilities, charge delocalization modes, and substituent effects, as a way to understand structure/reactivity relationships. 

The expanding application of computational chemistry is reflected by amplified discussion of this area, especially density function theory (DFT) calculations in Chapter 1. Examples of computational studies are included in subsequent chapters that deal with specific structures, reactions and properties. Chapter 2 discusses the principles of both configuration and conformation, which were previously treated in two separate chapters. The current emphasis on enantioselectivity, including development of many enantioselective catalysts, prompted the expansion of the section on stereoselective reactions to include examples of enantioselective reactions. Chapter 3, which covers the application of thermodynamics and kinetics to organic chemistry, has been reorganized to place emphasis on structural effects on stability and reactivity. This chapter lays the groundwork for later chapters by considering stability effects on carbocations, carbanions, radicals, and carbonyl compounds. 

Computational studies have compared substituent effects on the stability of ketenes, allenes, diazomethanes, diazirines, and cyclopropenes. Ketenes belong to the first generation of reactive intermediates along with carbocations, carbanions, radicals, and carbenes, and are intensively studied members of the cumulene family, with many useful synthetic applications. Ketenes were first recognized in 1905, when diphenylketene, a stable and isolable example, was obtained from the dehalogenation of the a-bromodiphenylacetyl bromide (Scheme 7.37). The most characteristic reaction of ketene is cycloaddition, as in the formation of p-laclams. 

A computational study was undertaken to evaluate the stabilizing effects of a-sulfur and a-fluorine groups at carbocation centers. This was done by calculating the relative energies for protonation, methylation, and bromination of HFC = CH(SMe) - comparing the regioisomers of electrophilic attack. In general, the a-SMe carbocations were found to be considerably more stable. 

Developments in the study of carbocations in superacid media over the past 30 years have been reviewed.1 The thermodynamics [AG(g)] of the reaction R+(g) + Rref OH(g) -> ROH(g) + R+ref(g) involving Rref = f-butyl and 21 R+ has been studied by high-level computation.2 A plot of AG(g) versus AG(solution) shows an excellent correlation, except for phenyl-substituted R+, which form a separate correlation family. The magnitude of the most positive surface electrostatic potential was proposed as an effective measure of the stability of gas-phase carbocations, with results presented for a number of structurally diverse cations.3 The electrostatic potential directly... 

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