CCL: question on molecular orbitals in CO and NO

 Sent to CCL by: Alan Shusterman [alan]-[reed.edu]
 Sengen,
It's an interesting quote, and it's too bad that we don't have more context for it because it can be interpreted in several ways. Probably you should write Professor Hoffmann and ask *him* what he means. His answer would provide "experimental proof". 
 Now for my 2 cent interpretation...
First, I think the quote does not imply orbitals are experimental observables.
Second, a Hoffmann paper that comes to mind (and I hope my memory is not playing tricks on me) is an analysis of through-bond effects on the PE spectra of 1,2-diamines. If I recall correctly, different PE spectra were predicted for N lone pairs that a) do not interact, b) interact through-space, and c) interact through-bond, and Hoffmann's Extended Huckel MO calculations indicated the last option was most likely. I think the PE data supported his model.
So based on this string of questionable memories, my expanded reading of Hoffmann's quote would go like this: I have calculated orbital interactions for molecules, and I have used these orbitals to predict experimental properties that others tested (and ultimately confirmed) using PE spectroscopy.
Third, the on-line article you point to is a celebration of Albert Einstein, especially the photoelectric effect. I think Hoffmann was just trying to say that PES has been useful for his research because it has told him whether his (highly parameterized) models were on a realistic track or not. As such, it shows one more way that orbital models make contact with experiment, but it is very far from a claim of experimentally observed orbitals. 
 -Alan
 Sengen Sun sengensun(~)yahoo.com wrote:
 
 Sent to CCL by: Sengen Sun [sengensun/a\yahoo.com]
 My apologies if I am a bad guy again.
 But in contrast to the recent discussions by Wayne
 Steinmetz and Ulrike Salzner, Roald Hoffmann recently
 said: “Photoelectron spectroscopy has provided me with
 experimental proof of the ways orbitals interact.”
 (See:
 http://acswebcontent.acs.org/chemistry/autumn_2005.pdf)
 
 --
 Alan Shusterman
 Chemistry Department
 Reed College
 Portland, OR 97202-8199
 503-517-7699
 http://academic.reed.edu/chemistry/alan/
 "Yield and overcome; Bend and be straight." Lao Tzu 22