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Molecular Orbitals for CO2

Jmol models of wavefunctions calculated at the RHF/3-21G* level

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Notes

• This web page presents evidence from an ab initio modelling calculation, which may be useful to those learning or teaching about molecular orbitals for simple inorganic species
• It does not set out to teach
• If you would like a tutorial aimed at students following a beginners' course about atomic orbitals and their linear combination to make molecular orbitals, you could try Tutorial using the program Orbital

sp Mixing

• The σ orbitals (green or black) lie symmetrically across the π nodes of the π orbitals (red), so the two do not mix
• The node of the C(2p) orbital (green) coincides with an element of symmetry of the molecule, so C(2p) (green) does not mix with C(2s) (black): they stay orthogonal in the molecule and contribute to different σ MOs
• In contrast, the nodes of the O(2p) orbitals (black) do not coincide with an element of symmetry of the molecule, so O(2p) does mix with O(2s): they are not orthogonal in the molecule.   Both s and p orbitals of O are coloured black on the diagram.  Both contribute to some extent to all of the σ MOs, though only the more important correlation lines are marked in.   For a further exploration of sp mixing, see Molecular Orbitals for CO
• Non-mixing of s and p orbitals of the central atom is characteristic of the symmetric molecular geometries: symmetric linear triatomic, trigonal planar, tetrahedral and octahedral

MO Calculation

• These orbitals were calculated at a low ab initio level (rhf/3–21g*) which can, however, show bond polarisation and fully delocalised molecular orbitals
• At the much higher level df/6-311g(2df) the calculated molecular orbital models look very similar, but the weakly antibonding MO σC(2p)O(2p) appears below the bonding π MOs in the energy level diagram