Through-space interaction, characteristics

As briefly mentioned in Section IV.A.1, the main peak of CO on MgO(OOl) gradually shifts from 2157 cm 1 (at 6 = 0) to 2148 cm-1 (at 6 = 1), and this behavior has been ascribed to changes in lateral interactions. As discussed by many authors (22, 23, 73,136-140), in overlayers formed by very weakly adsorbed diatomic species the interactions among the oscillators are essentially of the through space type. These interactions occur among the static and dynamic dipoles of the diatomic species. For this reason, the shift AD is the sum of two contributions, AD = ADstat + ADdyn. When the interaction energies become larger than those characteristic of simple physisorption... 

There is a second type of stereoselectivity that is characteristic of the Diels-Alder reaction. The addition of a dienophile such as maleic anhydride to a cyclic diene like 1,3-cyclopentadiene could provide two products, the endo-adduct 2 and the exoadduct 3 (Eq. 12.5). However, only the e do-cycloadduct 2, in which the two boldfaced hydrogens are syn to the one-carbon bridge, is observed experimentally, and its preferential formation follows what is now commonly termed the Alder rule. The basis for this result is believed to be stabilization of the transition state 4 by secondary orbital interactions that occur through space between the p-orbitals on the internal carbons of the diene and the carbonyl carbon atoms of the dienophile, as shown by the dashed lines in 4. Analogous stabilization is not possible in transition state 5. Structure 4 is thus characterized as the one being stabilized by maximum orbital overlap. It should be noted that not all Diels-Alder reactions are as stereoselective as the one between l,3maleic anhydride mixtures of endo-and cxo-products are sometimes obtained. 

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