Subject: Re: redox potentials via HOMO/LUMO?

From: "Jens Spanget-Larsen" <[email-masked]>
Date: Fri, 19 May 1995 10:26:48 +0100

Dear McVey Iain!

the correlations you attempt to establish have certainly been 
investigated for many years.  For example, it is well known that 
polarographic half-wave reduction potentials for aromatic 
hydrocarbons correlate well with Huckel LUMO-energies (Streitwieser, 
"Molecular Orbital Theory for Organic Chemists", Wiley, 1961).  It 
has little sense to include CI in a calculation MO energies.  For an 
ordinary closed-shell molecule, consideration of interaction with 
singly excited configurations does not affect the SCF ground state 
(Brillouin's Theorem). If you include doubly or multiply excited 
configurations, you introduce electronic correlation effects and the 
ground state is stabilized, but the orbital concept "breaks down".

Yours Sincerely, Jens >--<

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JENS SPANGET-LARSEN
Department of Chemistry               Phone:    +45 46757711
Roskilde University (RUC)             Fax:      +45 46757721 
POB 260, DK-4000 Roskilde, Denmark    Internet: [email-masked]
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From: [email-masked] (Jennifer E. Grant)
Date: Fri, 19 May 1995 09:06:18 -0400

Have you tried searching Bigmed
via Entrez? Your idea sounds really terrific and I would 
be very interested to see your paper when it gets published.

Jennifer
Grad student
(Enzyme kinetics)

--
   "Wer denkt, schafft"
Jennifer E. Grant
Department of Biochemistry
Case Western Reserve University
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From: [email-masked]
Date: Thu May 18 08:31:47 1995

Hi Iian,

Sounds like an interesting effort.  I haven't really tried this yet, but I've been
gearing up for it in the context of some transition metal complexes I care
about.  In these cases the solvent-solute interaction is very important to the
potentials, so I've been trying to get that figured out first.

My feeling is that for at least some organic series, you will be able to assume
small or constant solvent contribution to the observed potentials.  This should
allow you to to at least get at *relative* potentials (you will need at least one
experimental point to reference your calculated potentials to).  Keep in mind
that a redox potential is a measurement of the the free energy change between
reactants and products resulting from an electron transfer reaction.  Thus to 
get at it from quantum mechanical (and hence heat of formation) data, you are
assuming a constant entropic term throughout your series.

You may be able to get some decent correlations from simple HOMO/LUMO
data, but I suspect that a more rigorous route will be to calculate heats of
formation for both the oxidized and reduced forms of your couples and looking
at the difference.  I would expect this quatity to be most directly related to the
redox potential (or at least that's my plan of attack at the moment for the calcs
I haven't started yet!).  This will mean doing calcs using the UHF option so 
that the program can geometry optimize the structure of the half of the couple
with an unpaired electron properly.  You would then do a single point calcula
tion within the RHF approximation at the optimized geometry in order to get the
best value for the heat of formation of the paramagnetic species.

If you're not familiar with all this UHF/RHF stuff, the Hyperchem manuals touch
on it and any decent textbook on quantum theory will explain it in detail.

Good luck!  As you can see, I'm interested in this topic and I would very much
appreciate hearing about how it goes for you.  If you end up with any interest-
ing correlations and/or results you wouldn't mind sharing, I'd love to see them.

Best wishes,

                             -- Jeff Curtis at Univ. of San Francisco

                                USF Chem Dept.
                                2130 Fulton St.
                                SF, CA  94117        v)415-666-6391

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