Exam 2 lecture 3

Consider skeletal structures: carbon atoms are at each vertex or corner, and at each end of a line, but are not explicitly labeled with "C." Hydrogen atoms bonded to carbon are also not shown. However, all heteroatoms like oxygen, nitrogen, and chlorine are shown explicitly, along with their associated hydrogens. Multiple bonds are shown as multiple lines, for instance, a double bond is represented by two lines and a triple bond by three lines. This method provides a clear visual representation of molecular architecture. Unlike earlier theories such as Lewis theory or Valence bond theory, molecular orbitals extend over the entire molecule, contributing to its overall binding rather than just individual pairs of atoms. This means that molecular orbitals encompass the whole molecule, creating a more complex yet comprehensive bonding model. Viewing molecular orbitals as spread out across the entire molecule gives us a deeper insight into the intricate nature of chemical bonds and molecular stability. We'll also examine conjugated polyenes to understand how to construct molecular orbital diagrams for polyatomic molecules, using examples like 1,3-butadiene and 1,3,5-hexatriene. We can describe what are known as frontier orbitals, which include the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), using pi molecular orbitals built from atomic p-orbitals. These frontier orbitals play a critical role in determining a molecule's reactivity and properties, especially for molecules with conjugated (delocalized) pi systems. In conjugated dienes, for instance, we typically find four carbon atoms in the chain, providing four p-orbitals to form pi bonds. This results in four pi orbitals in the frontier orbitals, occupied by four electrons. The lowest energy orbitals, where n=1, will have zero nodes, and we observe that energy increases as the number of nodes increases. This behavior starts to resemble a particle in a box model, offering a quantum mechanical perspective on electron confinement within these molecular systems. We will also practice applying these concepts, for example, by considering octatetraene to determine how many levels would be in its molecular orbital diagram and how many nodes are present in its HOMO. Understanding these details is key to predicting molecular behavior and properties.