Discussion Fr 12.06.2009 at 12:15, PK 11.2
Home work 8
MOs: homonuclear molecules
Exercise 1: Electron configuration
a) Excited carbon molecule C2 has an electron configuration 1σ2g 2σ*2u 1p3u 2p1g. Determine the total molecular spin (S), the projection of the electronic orbital angular momentum (Λ), and the parity (g,u) of all possible molecular states.
b) Which of these states has the lowest energy?.
Exercise 2: Dissociation energies and bond lengths
a) Why the dissociation energy of N2 is larger than that of N2+, while the dissociation energy of O2 is smaller than that of O2+ ?b) Explain qualitatively why the vapor of lithium metal at room temperature is primary monoatomic, while the nitrogen and oxygen gases are diatomic even at high temperatures.
c) Why the equilibrium internuclear distance in O2 molecule is smaller than that in F2 molecule?
Hint: use the bond order conception for this discussion. The bond
order
is given by b = (n - n*)/2, where
n is the number of electrons in the bonding orbitals and n* is the number of electrons in the antibonding orbital
d) Why the equilibrium internuclear distance in Li2 is larger than that in F2 ?
Exercise 3: Excited states
a) What is the ground state electron configuration of Ne2 molecule? Is this molecule stable? Promote one electron from the highest filled MO (HOMO) to the lowest unfilled MO (LUMO). The new configuration corresponds to an excited molecular state. Is the excited molecule stable? Draw (qualitatively) the potential curves of the Ne2 ground and excited states. Assume that two Ne atoms undergo collisions which can result in both ground and excited state molecules and consider the electromagnetic transitions occurring between these two states.
The answer will explain you the main operation mechanism of an excimer laser.
b) You know that the bond in the H2 molecule can be viewed as being formed by the overlapping of the 1s orbitals of two H atoms. In the same way the bond in the F2 molecule is formed by overlap of 2pz orbitals of two F atoms. In both cases each atom contributes one electron to the bond and therefore two electrons share the bond as a pair. This type of bonding is called electron pair bonding. Can you explain why the carbon atom C can easily provide up to four electron pair bonds forming such molecules as C4, CH4 , and others, while the nitrogen atom N can form three electron pair bonds (NH3), but cannot form four bonds?
Hint: consider the electron structure diagram of each atoms and promote paired electrons to higher energy levels.
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