CCL: question on molecular orbitals in CO and NO
- From: "Wayne Steinmetz" <WES04747*pomona.edu>
- Subject: CCL: question on molecular orbitals in CO and NO
- Date: Fri, 11 Nov 2005 11:02:20 -0800
Sent to CCL by: "Wayne Steinmetz" [WES04747:pomona.edu]
You have asked a host of questions around one theme, qualitative MO
An excellent summary of experimental data can be found in P. Krupenie,
The Band spectrum of Carbon Monoxide. This was published by the US
Government Printing Office as NSRDS-NBS 5. The government periodicals
desk at the UCLA library may help you track down this document.
Krupenie also published a critical review of spectroscopic data for N2
and O2 in the Journal of Physical Chemical Reference Data.
What is your audience for the qualitative use of MO theory? I have been
convinced that MO theory provides a good reason for bringing back the
index (Index censorum librorum). The index was not established as an
absolute prohibition to reading materials but as a warning. Only those
who are mature enough to understand the materials should be exposed to
it. When I teach General Chemistry, I ask the students to burn the
section on the MO treatment of diatomic molecules. It is usually a
swindle. I do use MO theory in the examination of the properties of
transition-metal complexes. In this case, crystal field theory is a
swindle and VB doesn't explain much.
I do use MO theory to discuss H2 as the treatment provides insights into
the nature of bonding. Period. I found that the extension to
homonuclear and heteronuclear diatomic molecules to be
counterproductive. One has to be well versed in quantum mechanics to
use MO theory correctly. Alexander Pope was right in stating "A little
knowledge is a dangerous thing; drink deeplyu or not at all of the
Pyrean (sic) spring".
In the right context, qualitative MO theory can be used as a framework
for interpreting experimental data. The simplistic approach in most
textbooks cannot be used in most cases to predict the results. Given
the low cost of programs such as Spartan that are based on good physics,
why should we bother with the old, flawed approaches at all? In this
case, the availability of good software should change the way we teach!
Molecular orbitals are a means to an end. They are not observables.
The electron density and the total energy are measurable. This is one
of the reasons why DFT is so successful. I have found that the spacing
between the orbital energies and in some cases the order depend
critically on the basis set. Hence, if very sophisticated methods yield
different results for these intermediate pieces, why should we encumber
students with them?
I would be pleased to continue this discussion if you wish.
Wayne E. Steinmetz
Carnegie Professor of Chemistry
Woodbadge Course Director
645 North College Avenue
Claremont, California 91711-6338
Email: wsteinmetz .. pomona.edu
> From: owner-chemistry .. ccl.net [mailto:owner-chemistry ..
Sent: Thursday, November 10, 2005 3:12 PM
To: Wayne Steinmetz
Subject: CCL: question on molecular orbitals in CO and NO
Sent to CCL by: Eric Scerri [scerri!^!chem.ucla.edu]
In teaching qualitative molecular orbital theory one encounters the
question of the crossing of the sigma 2p and pi 2p bonding orbitals
in homonuclear diatomics.
This is such that N2 has the pi orbitals of lower energy than sigma
whereas for O2 the energies are reversed.
But what about heteronuclear diatomics where each of the atoms ?
The ordering given in general chemistry textbooks shows a wide
variation. See for example Oxtoby, Zumdahl or Olmsted and Williams,
all three of which contradict each other.
Herzberg's, Spectra of Diatomic Molecules specifically states the
configurations of CO and NO.
For CO the ordering is as in the case of N2 whereas for NO the
ordering is as in O2.
However Herzberg's book is now a little out of date (1950).
Do more recent calculations on these two molecules suggest anything
different? Would anyone be prepared to run these calculations and
tell me the results?
How about experimental evidence such as PES on CO and NO?
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