Subject: RE: Is anion energy of every molecule MUST be LOWER than that of neutral one? Cation energy GREATER than neutral energy?

From: "Shobe, Dave" <[email-masked]>
Date: Mon, 25 Sep 2000 11:55:55 -0400

I'm not sure what the "q" in "Alq3" and "Gaq3" represents, but the
inequality
	E(neutral) > E(anion) 
is *not* generally true.  I.e. there are neutral species that don't "want"
another electron.  In which case, you'll never see the anion in the lab,
since A- --> A + e- is a (nearly at least) barrierless process.  Even some
of the alkyl radicals fall into this category, so the above inequality is
not even true for all open-shell E(neutral).

In calculations, you will get an "energy" for unbound anions (anions where
A- --> A + e- is a favorable reaction), but this is a result of the finite
basis set not allowing the electron to "escape" from the rest of the anion.
I would expect calculations of unbound anions to show the following
characteristics:
1. E(anion) > E(neutral)
2. Unusual sensitivity of E(anion) to the addition of diffuse functions
3. High occupancy of the diffuse functions.

On a related note, E(dianion) is generally >> E(anion).

--David Shobe
S㥣-Chemie Inc.
phone (502) 634-7409
fax     (502) 634-7724
email  [email-masked]

Any opinions herein are not necessarily representative of S㥣-Chemie.

-----Original Message-----
From: Kiniu WONG Kin-Yiu [mailto:[email-masked]]
Sent: Monday, September 25, 2000 9:14 AM
To: [email-masked]
Subject: CCL:Is anion energy of every molecule MUST be LOWER than that
of neutral one? Cation energy GREATER than neutral energy?

Hi all,

    I've learned we could estimate the value of HOMO eigen energy and LUMO
eigen energy by calculating the value of Ionization Energy (IE) and Electron
Affinity (EA) respectively. We could obtain HOMO = E(cation) - E(neutral)
and LUMO = E(neutral) - E(anion). So, I believe that E(cation) > E(neutral)
> E(anion).

    However, when I calculated IE and EA of molecule Alq3 (or Gaq3) by
Hartree-Fock (Gaussian 98), surprisingly the value of E(anion) > E(neutral)
with stabilised wavefunction of 6-31G(d). I don't know why. Is it just
because the upper bound of E(anion) is much higher than that of E(neutral),
so finally E(anion) > E(neutral)? Or any other reasons?

    Although, when I changed the basis function to be either 6-31+G(d)
(diffuse function special for anion) or LanL2DZ (ECP basis function) then
both give E(anion) < E(neutral), however, the computed LUMO ( E(neutral) -
E(anion) ) is much smaller than that of the experiment. How could I solve
this problem if I still use Hartree-Fock (HF) ? Any suggestions?

    Since we know HF gives us upper bound of the calculated energy, but we
don't know whether DFT gives us the upper bound or not, so my group still
prefers to use HF. Anyway, could this kind of problem be solved by DFT?
If so, why could DFT handle this kind of computation, but HF couldn't?

    Once you think details of Alq3 or Gaq3, such as their structures,
experimental or computational HOMO & LUMO, could help you answer my
question, I am very pleased to give you at once. Thanks for any help in
advance!

Regards,
Kiniu WONG Kin-Yiu
Undergraduate student
Department of Physics
The Chinese University of Hong Kong
Email: [email-masked]  or  [email-masked]

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