Beginning Organic Chemistry (BOC)
1. Basic Knowledge
e. Molecular Orbitals and Bonding.

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Molecular Orbitals and Bonding.

Although the Kekulé structures for molecules can reveal much about a molecule, sometimes it is necessary to use a more complex theory. According to quantum mechanics, sharing of electrons to form stable molecules occurs when electrons can move into wave forms with lower energies which are centred over more than one atom. Electrons in such waveforms do not have enough energy to move back into wave forms centred on one nucleus (atomic orbitals) and so the atoms remain together in a molecule.

In your first year course you meet the series of homonuclear diatomic molecules and the molecular wave forms (molecular orbitals, abbreviated mo) that can be calculated for those using a linear combination of atomic orbitals approach. The molecular orbitals for nitrogen are modeled at the University of Arizona site already visited. At this site look at the shapes of the s2p and p2p orbitals and the corresponding antibonding orbitals. (When the structure has downloaded into chime, switch the view to spacefilling.)

Some points that becomes evident when creating molecular orbitals are:

Some mo have lower energy than atomic orbitals they are derived from, and these are called bonding mo.
However, some mo have energies higher than the atomic orbitals they are derived from, and these are called antibonding mo.
Additionally, some mo have the same energy as the atomic orbitals they are derived from, and these are called non-bonding mo.

Some times it is important to use molecular orbitals to describe why certain events take place, or why a certain structure is more stable than might be expected. Most times in organic chemistry, though, models based on the simple Lewis or Kekulé structures have enough detail to be useful.

Date created: 2005 06 09.