Substituent effects theoretical studies

Recently, with the advent of efficient programs for carrying out ab initio molecular orbital calculations (5), it has become possible to examine substituent effects theoretically. In one such study, Hehre et al. (6) have conducted STO-3G calculations on a large range of monosubstituted benzenes. In an extension of that work, we present here an account of similar calculations on both disubsti-tuted and polysubstituted benzenes. 

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. 

Analysis of substituent effects on tautomeric equilibria by the method of perturbed molecular orbitals Theoretical studies of heteroaromatic compounds (MNDO, 4-31G)... 

Propagation in cyclopolymerization may be substantially faster than for analogous monoene monomers.15" The various theories put forward to account for this observation are summarized in Butler s review.98 A recent theoretical study by Ttiziin et al.u3 looks at the effects of substituents on the rate of the cyclization step. 

Very recently there has been an experimental and theoretical study of electronic substituent effects in 4-aminoaryl (4-substituted aryl) sulfones146. PMR, 13C NMR and infrared measurements were involved and semi-empirical all-valence CNDO/2 calculations, with and without sulfur d orbitals, were carried out. Various correlations between spectral results and substituent constants are presented. There is good agreement between experimental and theoretical data, which does not depend on the inclusion or exclusion of the sulfur d orbitals from the calculations. 

Kistler KA, Matsika S (2007) Cytosine in context a theoretical study of substituent effects on the excitation energies of 2-pyrimidinone derivatives. J Phys Chem A 111 8708-8716... 

The most recent theoretical study, by Alhrichs and co-workers, deals with the di(phosphino)carbene Id and (phosphino) (phosphonio)carbenes Ie,f.16 The optimized geometry of the di(phosphino)carbene Id is weakly bent (PCP angle 160.5°) and highly unsymmetrical Only one of the phosphorus centers (P1) is in a planar environment, and it is much more closely bonded to the carbenic center than the other one (P1C 1.533 and P2C 1.765 A). The atomic charges (P1 +1.0, C -0.8, P2 +0.6) indicate that the short P bond is a double bond reinforced by Coulombic attraction, while the nature of the molecular orbitals revealed a slight delocalization of the carbene lone pair into the low-lying a (P-N) orbitals of the two phosphino substituents. The distortion from the symmetrical structure can be viewed as a second-order Jahn-Teller effect. 

A theoretical study of substituent effects in the thio-Claisen rearrangement (91) -> (92) has been carried out. The study has shown that 2,5-disubstimtion leads to tighter transition states and to a substantial lowering of the enthalpy of... 

In addition to these probes, theoretical calculations are being made in efforts to decide upon the inherently most stable stmcture of the ion. These will be discussed along with the conclusions which have been derived from the study of isotope effects in solvolysis and substituent effects in stabilizing media. 

The addition of O2 (whether or A ) to 138 or 4 has apparently not been studied theoretically although selection rules for singlet oxygen reactions/ interpretation of substituent effects and orbital correlations for homolytic fission and re-formation of the peroxide bond in 9,10-diphenyl-anthracene peroxide and rubrene peroxide have been published. 

Most of the fairly extensive photochemistry of aromatic compounds has not been studied in sufficient detail to permit disentanglement of singlet and triplet mechanisms. Theoretical calculations indicate that the pattern of substituent effects on side-chain reactions of excited disubstituted benzenes should be quite different from that observed in the ground states of the molecules. One problem associated with these predictions is the question of whether or not they are appropriate for triplets as well as for corresponding singlet excited states. Consider the following system ... 

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