Читать книгу Organic Mechanisms - Xiaoping Sun - Страница 33

In a molecule, AOs of the constituent atoms overlap (LCAOs) giving rise to the formation of a set of MOs. As a result, all the electrons will move in the entire molecule. Some MOs with lower energies will be filled (occupied) by the electrons. Other MOs with higher energies will remain empty (unoccupied). The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) in a molecular are particularly important, and they are referred to as frontier molecular orbitals (FMOs). In H₂, there are only two MOs (Fig. 1.8). Both of them, σ_1s (HOMO) and σ_1s^* (LUMO), are FMOs. In the C=C double bond, there are one σ‐bond and one π‐bond (Fig. 1.10a). The FMOs are π_p (HOMO) and π_p^* (LUMO). In 1,3‐butadiene (CH₂=CHCH=CH₂), the FMOs are ψ₂ (HOMO) and ψ₃ (LUMO) [Fig. 1.11b]. In general, only the FMOs (HOMO and LUMO) participate in reactions and the other orbitals (lower‐lying or upper‐lying MOs) remain approximately intact during a chemical reaction [6]. Many concerted reactions are effected by interaction of the HOMO of one reactant with the LUMO of the other. This requires that the reacting FMOs must have symmetry‐match [1]. This case will be further addressed in the individual chapters, particularly in Chapter 4. In addition, when the energy of a photon (hν) at certain wavelength (frequency) matches the HOMO–LUMO energy gap, the photon can be absorbed by the molecule resulting in electronic transition from HOMO to LUMO. This type of absorption is common to many compounds whose molecules contain π‐bonds. π (ΗΟMΟ)–π* (LUMO) transition occurs when the light with matching energy is absorbed. The transition gives rise to certain colors for the compounds containing π‐bonds. In addition, the transition makes both HOMO and LUMO singly occupied. As a result, photochemical reactions may become possible via the singly occupied molecular orbitals (SOMOs) (Chapter 4).