Three-Membered Theory

Like Rh and Ir, all three members of this triad have the fee structure predicted by band theory calculations for elements with nearly filled d shells. Also in this region of the periodic table, densities and mps are decreasing with increase in Z across the table thus, although by comparison... 

The regioselectivity in radical addition reactions to alkenes in general has successfully been interpreted by a combination of steric and electronic effects1815,47. In the absence of steric effects, regiochemical preferences can readily be explained with FMO theory. The most relevant polyene orbital for the addition of nucleophilic radicals to polyenes will be the LUMO for the addition of electrophilic orbitals it will be the HOMO. Table 10 lists the HOMO and LUMO coefficients (without the phase sign) for the first three members of the polyene family together with those for ethylene as calculated from Hiickel theory and with the AMI semiempirical method48. 

Lathan, W. A. Radom, L., Hariharan, P. C., Hehre, W. J., and Pople, J. A. Structures and Stabilities of Three-Membered Rings from ab initm Molecular Orbital Theory. 40, 1 -45... 

Carbonium Ions as Reaction Intermediates.—The properties of electron-deficient substances may be expected to be of great importance in the theory of chemical reactions. For example, a positively charged (and hence electron-deficient) carbon atom in a complex carbonium ion would be expected to cause adjacent atoms to increase their ligancies, as by the formation of a three-membered ring and by the use of bridging hydrogen atoms. The analysis of the mechanisms of chemical reactions may in the course of time permit much more precise principles to be formulated than are now at hand. 

According to valence bond theory, any such system in which the number of carbon atoms is even would be expected to have resonance stabilization because of the existence of canonical forms of the type illustrated below for the first three members of the homologous series ... 

Ethylene oxides, like other three-membered ring systems, possess many singular features that invite a basis in theory. To satisfy Hub demand, much effort has boon devoted to the task of determining with precision such fundamental properties of the molecule os bond lengths, bond angles, and bond energies- With the advent of modern instrumental methods it has been possible to develop a dependable physical basis fin- theoretical speculations on the electronic structure of ethylene oxide. The present section is concerned with this aspect of epoxide chemistry. 

These discrepancies result (a) from the harmonic approximation used in all calculations [to,- (theory) > v, (exp)], (b) the known deficiencies of minimal and DZ basis sets to describe three-membered rings [polarization functions are needed to describe small CCC bond angles a>,(DZ + P) > w,(DZ) > to,(minimal basis)] and (c) the need of electron correlated wave functions to correctly describe the curvature of the potential energy surface at a minimum energy point [ [Pg.102]

The effect of different heteroatoms on the ring opening of substituted three-membered rings was investigated at the B3LYP/TZV+P level of theory.115 Two specific water molecules were included in the calculations and the COSMO method was used to include the bulk solvent. With the methyl thiolate nucleophile, thiirane was... 

The reactions between silylene (H2Si ) and the three-membered ring compounds, oxirane, thiirane, and selenirane, which provide possible routes to Si=X (X = O, S, Se) double bonds were studied by ab initio calculations at the MP2 and QCISD correlated levels of theory <2000CJC1496>. Three possible mechanisms have been examined (Scheme 4). Based on the results, the authors have concluded that the silatane route , via the corresponding oxa-, thia-, and selenasilacyclobutanes, is not a likely mechanism for the production of H2Si=X. 

Density functional theory (DFT) studies on [A]chalcogena[iV]pericyclynes (n = 0-3, 5) demonstrate their relative stability and hence their possible existence as stable species. By minimizing repulsive interactions between the chalcogens lone pairs, the molecules adopt structures that resemble, in shape, cycloalkanes or elemental chalcogens. [3]Chalcogena[3]pericyclynes may be interconverted with their valence tautomers, benzene derivatives with three fused three-membered rings <20040L589>. 

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