Thanks. It seems reasonable to expect that the same happens with ions. I have just seen HOMO energies of some ions turn from positive to negative in the presence of a solvent. For these I probably cannot talk of solvation energy, since they are electronically unstable in the gas phase.

-----Original Message-----
From: Toomas Tamm tt-ccl.|a|.kky.ttu.ee <owner-chemistry|a|ccl.net>
To: Phala, Noko <nokophala|a|aim.com>
Sent: Fri, 10 Feb 2006 04:57:03 -0500
Subject: CCL: Aquous electrochemistry

Sent to CCL by: Toomas Tamm [tt-ccl*_*kky.ttu.ee]
Our experience with aqueous cations shows that a significant portion
(up to and, in some cases, more than one elecron) of the charge is
transfered from the ion to the surrounding water molecules. The ions
themselves had +2 or +3 formal charges, which in some cases were
reduced to +1 and below (Mulliken) in the larger water clusters.

The charge resides even not on the ligating water molecules, but a
significant portion of it is spread over the outer solvation shell in
two-shell model clusters. For some numbers, see Uudsemaa and Tamm,
Chemical Physics Letters, 342, 667-672 (2001). Many more numbers are
available from us directly, if you ask.

The same may happen to anions, although we have no practical
experience with anions or ion pairs in water clusters.


On Thu, Feb 09, 2006 at 10:07:44AM -0500, nokophala+*+aim.com wrote:

> just to add again on thuis issue: Some (most?) anions have positive
> homo energies. I suppose this means they do not really exist in
> isolation. I wonder what the correct way of calculating the energis
> in solution for such species would be. I was thinking of I want to
> calculate the energy of an anionic species, A-(aq), then I can
> consider the cycle: AH = A-(aq) +H+(aq). Both AH (which is neutral)
> and H+ obviously hav epositive homos and so can be considered
> stable, so E(AH-H+(aq)) can be considered the true energy of A- in
> solution, and it may/may not differ from a direct calculation of a
> hypothetical A- in solution. But then, the energy of A- can easily
> depend on the nature of the counterion. Is that not a true picture
> of reality, i.e. energy of an anion in solution should depend on the
> nature of the surounding cation (even if it is not strongly bound)?


> I hope the argument that species with positive HOMOs (think HF
> energies to avoid the issue of meaning of DFT orbitals) do not exist
> is true irrespective of the belief about whether orbitals are
> reality or not (i do not want to re-start that debate).

>  
> Noko
>  
-- 
Toomas Tamm                                 e-mail: tt-ccl_+_kky.ttu.ee
Chair of Inorganic Chemistry                voice:  INT+372-620-2810
Tallinn University of Technology            fax:    INT+372-620-2828
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