Subject: CCL: Seeing molecular orbitals?

From: "Jens Spanget-Larsen" <[email-masked]>
Date: Thu, 20 Jul 2000 10:43:01 +0100

Dear CCL:

In a recent, most interesting publication ('Seeing molecular orbitals', 
Chem.Phys.Letters 321, 78-82 [2000]) scanning tunneling microscopy is used to 
observe the shapes of single molecular orbitals of the fullerene C60: 

  "..the scanning tunneling microscope (STM) is probing a single molecular 
  orbital (MO)."  

However, this reminds me of previous statements in the literature some time 
ago, associated with the advent of molecular photoelectron spectroscopy, f.i. 

  "Chemists can see the orbital structure of even fairly large molecules and no 
  longer have to rely on the predictions of theoreticians"

  "Photoelectron spectroscopy has demonstrated experimentally to chemists,
  physicists and other sceptics that molecular orbitals really do exist"

I wonder whether this is acceptable terminology, considering the fact 
that molecular orbitals are model constructs; they are not physical 
observables. By using approximations, such as Koopmans' approximation, many 
experimental observations can be conveniently interpreted in terms of 
theoretical MO data. But strictly speaking, this does not turn MOs into 
observable quantities; MOs have no physical existence.

Any comments?

Yours, Jens >--<
                             
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JENS SPANGET-LARSEN         Phone:  +45 4674 2000  (RUC)
Department of Chemistry             +45 4674 2710  (direct)
Roskilde University (RUC)   Fax:    +45 4674 3011 
P.O.Box 260                 E-Mail: [email-masked]
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From: Serguei Patchkovskii <[email-masked]>
Date: Thu, 20 Jul 2000 12:11:38 -0600 (MDT)

>   "..the scanning tunneling microscope (STM) is probing a single molecular 

See the essay by S.-G. Wang and W.H.E. Schwarz "On Closed-Shell Interactions,
Polar Covalences, d Shell Holes, and Direct Images of Orbitals: The Case of
Cuprite", published in Angew. Chem. Intl. Ed. 2000, 39(10), pages 1757-1762.
They do analyse, in quite some detail, that is *actually* seen in this kind
of experiments - and it is not a direct image of a single molecular orbital.

/Serge.P

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