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In
the approximation that each electron
in a molecule
has a distinct, independent wave function, the spatial distribution of
an electron
wave function corresponds to a molecular orbital. These, in turn, can be
approximated as sums of contributions from the orbitals characteristic of the
isolated atoms.
An electron
added to a molecule--or,
similarly, one excited to a higher-energy
state within a molecule--would
occupy a state
with a different wave function from the rest; an unoccupied state of this kind corresponds to an unoccupied molecular orbital. Orbital-symmetry effects on reaction
rates arise when a reaction
requires overlap
between two lobes of the orbitals on each of two reagents:
if the algebraic signs of the wave functions in the facing lobes do not
match, bond
formation between those orbitals is prohibited.
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