Closed shell repulsion

Kim Y S, Kim S K and Lee W D 1981 Dependence of the closed-shell repulsive interaction on the overlap of the electron densities Chem. Phys. Lett. 80 574... 

In seeking to rationalize the repulsive structure found in the He (2 S) + Ar,Kr,Xe,D2 interactions, we examine the model for noble-gas partners proposed by Siska et al.108,109 1,2 and the rationale presented by Isaacson et al.121 for He (l S) + D2. In the model potential function of Siska et al.108- 109 tjje iow energy repulsion is represented by a switchover from alkalilike—closed-shell repulsive behavior to ion core (He+)-—closed-shell Rydberg-like behavior with decreasing internuclear distance in other words,... 

An alternative two-step mechanism involving a spin-paired diradical intermediate has also been considered for 1,3-cycloadditions.18,68,69 However, ab initio calculations70-72 on a wide variety of 1,3-dipoles and dipolarophiles are found to coincide essentially with a synchronous 1,3-cycloaddition mechanism.15,17 On the other hand, a two-step mechanism passing through two transition states separated by an intermediate has been derived using the MINDO/3 method, and found to be compatible with substituent and solvent effects as well as stereospecificity observed in 1,3-cycloadditions.73 However, several factors beyond FMO interactions, such as closed shell repulsions, geometrical distortions, polarization, and secondary orbital interactions, all influence mechanisms, rates, and regioselectivities in cycloaddition reactions.74... 

When two molecules approach each other, the filled orbitals of these molecules begin to overlap, leading to a destabilization referred to as exchange, or closed-shell, repulsion. For the interaction of filled orbital 0 with filled orbital 0j, the destabilization, AE, is given by the expression below7) ... 

Returning to the Diels-Alder reaction, the symmetrical transition state will have no closed-shell repulsion between the HOMO s, due to the opposite symmetries, and resulting zero overlap, of these orbitals (Fig. 4)s In an asynchronous transition state, the overlap of all it orbitals occurs, and closed-shell repulsion will increase. As Epiotis has pointed out for aromatic molecules8), it is possible to gener-... 

Bicyclo[ 1.1.1 ]pent-l-yl anion 16. The basicity and electrophilicity of 16 relative to ordinary tertiary alkyl anions are not easy to predict without a computation. On the one hand, the high percent of s character in the hybrid holding the lone pair on the bridgehead carbon atom suggests that this anion will have quite low basicity. On the other hand, the transannular interaction between the two bridgehead carbons can be expected to increase the basicity, due to a relief in transannular closed-shell repulsions when a lone pair is replaced by a bond at the bridgehead. Indeed, the computed (MP2/6-31G ) interbridgehead distance decreases from 1.974 A in the anion to 1.872 A in bicyclo[l.l.l] pentane ... 

The Closed-Shell Repulsion Term. A number of approaches to the calculation of closed-shell repulsion energies may be found in the literature. Ladd and Lee have successfully used the expression Vr = B exp(-ro/p)... 

Closed shell repulsions are known to be the main determinants of the packing patterns of aromatic hydrocarbons in crystals, and play a part also in fixing the... 

On the other hand, Houk et al. proposed that the relationship between non-planar distortions and diastereofacial selection arises because both ground-state geometries and the relative transition-state stabilities are controlled by closed-shell repulsion effects [210]. Thus, pyramidalization per se is not the cause of stereoselectivity but is indicative of repulsive interactions that can be expected to assume even greater importance in the transition state [211]. 

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