Topic+2+setB - TOPIC 2: COVALENT BONDING THEORIES (P-BLOCK...

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T OPIC 2: C OVALENT B ONDING T HEORIES ( P -B LOCK C HEMISTRY ) M OLECULAR O RBITAL T HEORY CHEM 1311, Spring 2010 1
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M OLECULAR O RBITAL (MO) T HEORY In principle, the electronic structure of molecules can be worked out in the same way as for atoms: solve the Schrödinger equation This gives molecular orbitals rather than atomic orbitals But: It is difficult to solve the Schrödinger equation for molecular species (only through approximation!) 2
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LCAO APPROXIMATION LCAO = “ L inear C ombination of A tomic O rbitals” The wavefunctions of molecular orbitals can be approximated by taking linear combinations of atomic orbitals Ψ σ = 1 2 Ψ 1 s (H a ) + Ψ 1 (H b ) [ ] linear combination (addition) of the wavefunction from two 1s orbitals 3
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LCAO APPROXIMATION A second MO (molecular orbital) can be obtained via subtraction of two AOs Ψ σ * = 1 2 Ψ 1s (H a ) − Ψ 1s (H b ) [ ] linear combination (subtraction) of the wavefunction from two 1s orbitals nodal plane the resulting wavefunction has a nodal plane perpendicular to the H–H bond axis (electron density = zero); the energy of an electron in this orbital is higher compared to the additive linear combination = antibonding orbital 4
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B ONDING IN H 2 Take two 1s orbitals as the basis Get two MO’s ψ + = φ 1s (A) + φ 1s (B) ψ - = φ 1s (A) - φ 1s (B) 5
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Occupying the bonding orbital(s) of a molecule makes the interaction between atoms attractive (they stick together) Occupying antibonding orbitals
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Topic+2+setB - TOPIC 2: COVALENT BONDING THEORIES (P-BLOCK...

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