Orbital distortions

This complexation accentuates both the energy and orbital distortion effects of the substituent and enhances both the reactivity and selectivity of the dienophile, relative to the uncomplexed compound.The effects are well modeled by 3-21G-level computations on the transition-state stmctures. ... 

Compared to the overlap of the undistorted atomic orbitals used in the HL wave function, which is just 5ab. it is seen that the overlap is increased (c is positive), i.e. the orbitals distort so that they overlap better in order to make a bond. Although the distortion is fairly small (a few %) this effectively eliminates the need for including ionic VB terms. When c is variationally optimized, the MO-CI, VB-HL and VB-CF wave functions (eqs. (7.4), (7.7) and (7.8)) are all completely equivalent. The MO approach incorporates the flexibility in terms of an excited determinant, the VB-FIL in terms of ionic structures, and the VB-CF in terms of distorted atomic orbitals. 

The endo pyramidization results in the high exo reactivity and, in fact, this pyramidi-zation was confmned by Wipff and Morokuma [37], The orbital distortion also implies large negative electrostatic potential in the exo face. Very recently, Abbasoglu and Yilmaz [38] calculated a derivative of norbomene, i.e., endo tricyclo[3.2.1.0 ] oct-6-ene and confirmed the exo extension of the HOMO accompanied by the endo pyramidization, the large negative electrostatic potential on the exo face, and the preferential exo addition of Br. ... 

Steric repulsions come from two orbital-four electron interactions between two occupied orbitals. Facially selective reactions do occur in sterically unbiased systems, and these facial selectivities can be interpreted in terms of unsymmetrical K faces. Particular emphasis has been placed on the dissymmetrization of the orbital extension, i.e., orbital distortions [1, 2]. The orbital distortions are described in (Chapter Orbital Mixing Rules by Inagaki in this volume). Here, we review the effects of unsymmetrization of the orbitals due to phase environment in the vicinity of the reaction centers [3]. 

The orbital distortions due to relativity can be pictured in a qualitative way as follows the relativistic potential appears as a short-range attractive term that, above all, stabilizes s-orbitals and makes them more compact as well as steep close to the nucleus. The same happens to a lesser extent for the p-orbitals. Since the low-lying s- and /7-orbitals are now closer to the nucleus they do shield the nuclear charge better than in the nonrelativistic case. Consequently, d- and /-orbitals are... 

Recently, we analyzed the role of electron repulsion relative to bond breaking and antiaromaticity effects on a quantitative basis using Natural Bond Orbital (NBO) analysis.24 Two other destabilizing factors were considered at the initial stage of the cyclization in addition to four-electron repulsion between the filled in-plane acetylenic re-orbitals - distortion/breaking of the acetylenic bonds as a result of their bending, and the fact that, at a distance of ca. 3 A, the in-plane re-orbitals become parallel and reach a geometry that resembles the antiaromatic TS of the symmetry forbidden [2S + 2S] cycloaddition (vide infra). 

There is one report that showed how torsional, involving allylic CH bonds and steric effects but not orbital distortions, provide an explanation for the stereoselectivity of pyrrolidinone enolate alkylations. A prediction was... 

Orbital distortion index. A positive sign indicates distortion toward the axial direction. 

Other factors seem to have little effect. The concept of orbital distortion has appealed to many chemists, including the present author.101 It is based on the idea that an asymmetric environment can induce a pyramidalization of an sp2 carbon atom. This causes the 71 orbital to become larger on one face of the molecule than the other,... 

If spin-orbit coupling is present, there must be an ordering of the electron orbits associated with any collinear ordering of the spins. Therefore spin-orbit distortions are usually associated with a... 

FeF2, CoF2, and NiF2 also have body-centered ordering of the first kind, as is predicted since the t2o orbitals are more than half filled. Because of the low Neel points, no crystallographic studies of the spin-orbit distortions about the octahedral Fe2+ and Co2+ ions have been reported. 

The fourth and last terms in (11.49) are spin-orbit distortion corrections to the spin-rotation and Fermi contact interactions. The hyperfine and quadrupole terms in this Hamiltonian refer to the 14N nucleus. 

The atoms should be positioned in the molecule in such a way that effective overlap can occur with a minimum of orbital distortion. 

In fact, this ambiguity between polarization and secondary orbital interactions is akin to a similar ambiguity in orbital distortion explanations of stereoselectivity. Liotta suggested that the direction of attack on alkenes lacking a plane of symmetry was determined by orbital distortion due to a — ir mixing64. Fukui has proposed a similar explantion for the direction of attack of electrophiles on norbornene and related bicyclic systems65. It is believed that most stereoselectivity phenomena have been explained in this way by Liotta and Burgess66. ... 

At this point it is worth rephrasing some of the issues of the above discussions. The UPS spectra are a measure of the single-particle excitation spectrum of the molecule, in so far as removal of an electron is concerned, while UAS data are a measure of the particle-hole excitation spectrum. In other terms, UPS measures the molecular-ion states while UAS measures excited states of the neutral molecule. For a molecule in isolation, in a one-electron picture the valence electron molecular cation states are comprised of the set of one-electron molecular orbitals (mo s) containing one half-filled (usually non-degenerate) molecular orbital and the totality of other fully occupied orbitals, distorted from their situation in the neutral molecule due to the removal of an electron from the molecule in a photoelectron... 

These findings lend support to Deverell s electronic distortion theory (22) which conveys the idea that orbital distortion affects both the electric field gradient and the paramagnetic shielding term according to ... 

The wide applicability of (10.30) justifies showing its computational form. Formally (10.30) is a potential matrix element (6.88) in the distorted-wave representation for a three-body collision with the bound orbital i) replaced by the continuum orbital (distorted wave) lx (ks))- The direct matrix element is written in a form analogous to (7.62) using the distorted-wave form (4.58) of (7.45) for the continuum orbitals, (7.49) for the bound orbital a) and (7.60) for the electron—target potential. Note that the term l/ro in (7.60) vanishes if we require >f (ks)) to be orthogonal to a). This requirement is implicit in (10.10,10.11) and is normally imposed in implementing (10.30). 

Compared to the overlap of the undistorted atomic orbitals used in the HL wave function, which is just 5ab> it is seen that the overlap is increased (c is positive), i.e. the orbitals distort so that they overlap better in order to make a bond. Although the distortion is fairly small (a few %) this effectively eliminates the need for including ... 

The geometric deformation of molecules can also induce orbital distortions. Seebach and coworkers [IS] have shown that dioxanones 1 are distorted. In the ground state, carbon-4 is pyramidalized through the mean plane of the ring toward carbon-2. This distortion is enhanced in the transition state for a nucleophilic attack, so such reactions are stereoselective because of better overlap control (Figure L7). 

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