13. Molecular Orbital Theory

The script begins with an introduction to MIT OpenCourseWare, highlighting its mission to provide free educational resources, with a prompt for donations to support the initiative. The lecture then delves into molecular geometry, discussing the importance of understanding the Lewis structure for determining molecular shapes. The focus is on a molecule with phosphorus, where the valence electrons and bonding patterns lead to a tetrahedral geometry influenced by the presence of a lone pair, resulting in bond angles slightly less than the typical 109.5 degrees.

The lecture continues with an introduction to molecular orbital (MO) theory, which describes the behavior of electrons in molecules as delocalized over the entire molecule rather than being confined to individual atoms. The concept of atomic orbitals combining to form molecular orbitals through a linear combination of atomic orbitals (LCAO) is introduced. The theory is applied to s orbitals, explaining the formation of bonding and antibonding molecular orbitals, and how these relate to the stability and energy levels within a molecule.

This section explores how atomic orbitals interact to form molecular orbitals, focusing on the principles of constructive and destructive interference. Constructive interference leads to the formation of bonding orbitals with increased electron probability density between nuclei, resulting in lower energy and molecular stability. Conversely, destructive interference results in antibonding orbitals with nodal planes that exclude electron density between nuclei, leading to higher energy and reduced stability.

The script discusses the stability of electrons in bonding and antibonding orbitals, explaining that electrons in bonding orbitals are more stably bound due to the attractive force of both nuclei. This results in lower energy states and increased stability. The lecture also covers the energy levels of molecular orbitals, illustrating that bonding orbitals are lower in energy compared to atomic orbitals, while antibonding orbitals are higher in energy.

The lecture uses molecular orbital theory to predict the existence and stability of molecules like dihelium (He2), which is theorized to have a bond order of zero and thus no bond, making it a weak or non-existent molecule. The theory's predictions are compared with experimental findings, validating the theory's accuracy in predicting molecular stability based on electron configurations and bond orders.

The script provides an in-depth explanation of how to calculate bond order using molecular orbital diagrams, with examples including lithium and beryllium. It emphasizes the importance of filling the lowest energy orbitals first and the impact of electron configurations on molecular stability. The lecture also discusses the significance of bond dissociation energy and how it relates to the strength of chemical bonds.

This section delves into the behavior of p orbitals in molecular orbital theory, explaining the formation of pi (π) bonding and antibonding orbitals. The lecture illustrates how constructive interference leads to the creation of pi bonding orbitals with a nodal plane along the bond axis, while destructive interference results in pi antibonding orbitals with additional nodal planes. The stability of molecules with p orbitals, such as boron and carbon, is discussed in relation to their electron configurations and bond orders.

The script compares the molecular orbital diagrams and stabilities of different molecules, focusing on the distribution of electrons in bonding and antibonding orbitals. It explains how a higher number of electrons in bonding orbitals contribute to greater molecular stability, as seen in the comparison between B2 and C2 molecules. The lecture also emphasizes the importance of including all relevant molecular orbitals in diagrams for accurate analysis.

The lecture concludes with a discussion on molecular nitrogen (N2), highlighting its strong triple bond and the molecule's stability. The script explains the electron configuration of N2, its bond order, and its implications for the molecule's reactivity and importance in biological processes. The difficulty of breaking the nitrogen triple bond and its industrial significance are also touched upon.