CCL: question on molecular orbitals in CO and NO

 Sent to CCL by: "Wayne Steinmetz" []
 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
 Chemistry Department
 Pomona College
 645 North College Avenue
 Claremont, California 91711-6338
 phone: 1-909-621-8447
 FAX: 1-909-707-7726
 Email: wsteinmetz ..
 -----Original Message-----
 > From: owner-chemistry .. [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!^!]
 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?
 eric scerri
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