For example, you should have no trouble drawing the VB pictures for CO, NH 3, and benzene, but we will find that these are increasingly challenging with MO theory. While MO theory is more correct than VB theory and can be very accurate in predicting the properties of molecules, it is also rather complicated even for fairly simple molecules. We can get more accurate energies from MO theory by computational "number crunching." Many commercial and open-source programs have been developed for accurately calculating the electronic structure of molecules and extended solids. These simple models do not give very accurate orbital and bond energies, but they do explain concepts such as resonance (e.g., in the ferrocene molecule) that are hard to represent otherwise. This can help us understand patterns of bonding and reactivity that are otherwise difficult to explain.Īlthough MO theory in principle gives us a way to calculate the energies and wavefunctions of electrons in molecules very precisely, usually we settle for simplified models here too. With it we can also get a picture of where the electrons are in the molecule, as shown in the image at the right. Molecular orbital (MO) theory has the potential to be more quantitative.
It fails to describe some bonding situations accurately because it ignores the wave nature of the electrons. Valence bond (VB) theory gave us a qualitative picture of chemical bonding, which was useful for predicting the shapes of molecules, bond strengths, etc. 11 2.10 Chains and rings of π-conjugated systemsĬhapter 2: Molecular Orbital Theory The lowest unoccupied molecular orbital of the carbon monoxide molecule is a π antibonding orbital that derives from the 2p orbitals of carbon (left) and oxygen (right).10 2.9 Homology of σ and π orbitals in MO diagrams.
9 2.8 Building up the MOs of more complex molecules: NH 3, P 4.2 2.1 Constructing molecular orbitals from atomic orbitals.
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