Ideas of Quantum Chemistry P87 shows how quantum mechanics is applied to chemistry to give it a theoretical foundation. The structure of the book (a TREE-form) emphasizes the logical relationships between various topics, facts and methods. It shows the reader which parts of the text are needed for understanding specific aspects of the subject matter. Interspersed throughout the text are short biographies of key scientists and their contributions to the development of the field. | 826 14. Intermolecular Motion of Electrons and Nuclei Chemical Reactions sp3 hybrids Fig. oriented towards the other ethylene Therefore we may form the symmetry orbitals once again recognize their bonding and antibonding character and hence the order of their orbital energies without any calculations just by inspection Fig. . The lowest energy corresponds of course to SS because the newly formed a chemical bonds correspond to the bonding combination and the lateral overlap of the hybrids is also of the bonding character the next in energy however is the AS because of the bonding interactions in the newly formed a bonds while the lateral interaction is weakly antibonding then follows the SA-symmetry orbital antibonding interaction along the bonds that is only slightly compensated by the lateral bonding overlap of the hybrids and finally the highest-energy corresponds to the totally antibonding orbital of the AA-symmetry. According to the Woodward-Hoffmann rules the four n electrons on which we focus occupy the SS and SA orbitals from the beginning to the end of the reaction. This corresponds to low energy at the beginning of the reaction R but is very unfavourable at its end P because the unoccupied AS orbital is lower in the energy scale. And what if we were smart and excited the reactants by laser This would allow double occupation of the AS orbital right at the beginning of the reaction and end up with a low energy configuration. To excite an electron per molecule means to put one on orbital n while the second electron stays on orbital n. Of two possible spin states singlet and triplet the triplet state is lower in energy see Chapter 8 p. 391 . This situation was described by eq. and the result is that when one electron sits on nucleus a the other sits on b. These electrons have parallel spins - everything is prepared for the reaction. Therefore the two ethylene molecules when excited to the triplet state open their closed-shells
