Chemistry 181
Professor S. Alex Kandel
Fall 2014
Due 9/23/2015
Problem Set 4
Graded problem:
This should be written up and handed in before class begins on Wednesday the 24th.
This problem may also be discussed in class.
1. Like dinitrogen (N2 ), carbon monoxide (CO) has 10 valence electrons, and the
two molecules are quite similar when it comes to their bonding.
(a) Sketch and fill a molecular orbital diagram for CO; use a homonuclear
diagram similar to that used for N2 .
(b) Given the above diagram, what is the bond order of CO? Should it be
paramagnetic or diamagnetic? Would it have a dipole moment?
(c) Do any of the above answers change if the σ2pz orbital is lower in energy
than the π2px,y orbitals (as it is in O2 )? If so, how and why?
(d) The 2s and 2p energy levels for an oxygen atom are lower than those for
a carbon atom (why?). Redraw the molecular orbital diagram taking this
into account. What does the redrawn diagram say about a molecular dipole
moment?
(e) The molecule BF and the hypothetical molecule BeNe both also have 10
valence electrons, but do not have bonding similar to CO or N2 . Use
molecular orbital diagrams to explain what is different, and why.
CO, BF, and BeNe will all have asymmetric molecular orbital diagrams.
The eventual effect of shifting the molecular orbitals of one atom with respect to the other will be to make all orbitals non-bonding; conversely, if
the shift in energy is very small, the orbitals will by and large keep their
character and function. Cartoon diagrams showing this trend are on the
last page. Check them out and see if you can assign bonding, anti-bonding,
and non-bonding orbitals for each, as well as approximate bond order.
It’s a good guess that BeNe is represented by the totally non-bonding case,
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but figuring out where CO and BF are on this continuum ultimately requires either knowledge of the relative orbital energies or other information
about the nature of bonding. For CO, experimental data suggests a C≡O
triple bond, shorter and stronger than C–O bonds in virtually any other
compound, as well as a very small dipole moment.
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