Density functional theory substituent effects

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). 

Phenylperoxy radical has similarly been a topic of experimental and theoretical interest. Tokmakov et al. " calculated a potential energy surface for phenyl radical and O2 using ab initio G2(MP2) calculations. Weisman and Head-Gordon used time-dependent density functional theory (TD-DFT) calculations to examine the effect of substituents on the phenylperoxy radical s UV-vis absorption spectrum. " Lin and Mebel used ab initio methods to study the phenoxy radical -f O-atom reaction. "... 

The homolytic bond dissociation energies (BDEs) of phenohc O—H bonds has been the subject of a computational study focusing on substituent effects by ab initio and density functional theory (DFT) methods.6 Consistent overestimation of the BDEs by MP2 and MP4 calculations was associated with spin contamination in the reference UHF wave functions, whilst the DFT calculations (particularly the B3LYP/6-31G level of theory) were relatively unaffected. Ab initio calculations of the photosensitized C—C BDEs of /f-phenethyl ethers has revealed a significant configurational... 

Also in 2-substituted ethanesulphonates,35 the 33S chemical shift has a reverse substituent effect and correlates with both Taft substituent constants and the chemical shift of the carboxylic carbon in related carboxylic acids. It seems that the substituent effect does not depend on conformation, but prevailingly on intramolecular electronic effects. Density functional theory (DFT) calculations of 33S nuclear shielding constants and natural bond orbital (NBO) analysis made it possible to conclude that substituents cause a variation in the polarization of the S-C and S-O bonds and of the oxygen lone pairs of the C — S03 moiety. This affects the electron density in the surroundings of the sulphur nucleus and consequently the expansion or contraction of 3p sulphur orbitals. 

Sperling, D., Fabian, J. Substituent effects on the vinylcyclopropane-cyclopentene rearrangement. A theoretical study by restricted and unrestricted density functional theory. Eur. J. Org. Chem. 1999, 215-220. 

Similar substituent effects have been determined in the reaction of complexes 10 to form palladacycles 11 (Scheme 11.3) [31]. The opposite process-the intramolecular palladium-catalyzed arylation of alkanes to form dihydrobenzofuranes-has also been examined [32]. For this transformation, a mechanism based on a C—H bond-activation process by the aryl-Pd(II) involving a three-center transition state was found to be more consistent with the experimental kinetic isotope effect (3.6 at 115 °C), as well as with density functional theory (DFT) calculations. 

Ab initio MO and density functional theory study of substituent effects on electron attachment to benzyl chlorides ... 

G. Palermo, R. Riccio, G. Bifulco, Effect of electronegative substituents and angular dependence on the heteronuclear spin spin coupling constant h- empirical prediction equation derived by density functional theory calculations, J. Org. Chem. 75 (2010) 1982-1991. 

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