From owner-chemistry $#at#$ ccl.net Fri Nov 11 16:48:00 2005 From: "Wayne Steinmetz WES04747*pomona.edu" To: CCL Subject: CCL: question on molecular orbitals in CO and NO Message-Id: <-29948-051111140327-12234-vJiL/gGK9oFU/oCQHQ6T6Q++server.ccl.net> X-Original-From: "Wayne Steinmetz" Content-class: urn:content-classes:message Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset="us-ascii" Date: Fri, 11 Nov 2005 11:02:20 -0800 MIME-Version: 1.0 Sent to CCL by: "Wayne Steinmetz" [WES04747:pomona.edu] You have asked a host of questions around one theme, qualitative MO theory. 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 USA phone: 1-909-621-8447 FAX: 1-909-707-7726 Email: wsteinmetz .. pomona.edu WWW: pages.pomona.edu/~wsteinmetz -----Original Message----- > From: owner-chemistry .. ccl.net [mailto:owner-chemistry .. ccl.net] 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? regards, eric scerrihttp://www.ccl.net/cgi-bin/ccl/send_ccl_messagehttp://www.ccl.net/chemistry/sub_unsub.shtmlhttp://www.ccl.net/spammers.txt------------------------------------------------------------- This message has been scanned by Postini anti-virus software.