From owner-chemistry@ccl.net Sat Apr 14 13:39:00 2012
From: "David A Case case:_:biomaps.rutgers.edu" <owner-chemistry~~server.ccl.net>
To: CCL
Subject: CCL:G: energy for proton
Message-Id: <-46695-120413082906-32437-UQ/zrgf6ibNKe8gs//Olfg~~server.ccl.net>
X-Original-From: David A Case <case++biomaps.rutgers.edu>
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Date: Fri, 13 Apr 2012 08:28:58 -0400
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Sent to CCL by: David A Case [case()biomaps.rutgers.edu]
On Thu, Apr 12, 2012, Alexander Bagaturyants sasha#photonics.ru wrote:
> 
> That is, it senseless to calculate formally any thermodynamic function of a
> free (individual) proton. 

I think "senseless" is too strong a term here.  If you like, you can think of
reactions like AH -> A- + H+ as half-reactions (as in the corresponding
electron case), and that there will eventually be some proton acceptor in 
a "real" equilibrium experiment.  But this doesn't vitiate the utility of
estimating proton affinities (and gas-phase basicities) as standard
thermochemical quantities.  Computational estimates using the thermodynamics
of the free proton can be remarkably accurate compared to experiment; of
course, the conversion of raw experimental data to standard-state proton
affinities uses the same model.

As a side note, it is hard to recommend just using Gaussian or any other
program to get such values.  The posts that began this thread showed two
outputs, ostensibly from Gaussian:

A.  Temperature   298.150 Kelvin.  Pressure   1.00000 Atm.
...
 Zero-point correction=                           0.000000 (Hartree/Particle)
 Thermal correction to Energy=                    0.001416
 Thermal correction to Enthalpy=                  0.002360
 Thermal correction to Gibbs Free Energy=        -0.010654

B.  Temperature   298.150 Kelvin.  Pressure   1.00000 Atm.

 Zero-point correction=                           0.000000 (Hartree/Particle)
 Thermal correction to Energy=                    0.001416
 Thermal correction to Enthalpy=                  0.002360
 Thermal correction to Gibbs Free Energy=        -0.010000

Note that the Gibbs free energy values differ by a non-negligible amount.
Of course, no one can ever double-check everything that programs print out,
but here one can do the calculation by hand; see, e.g.
http://www.gaussian.com/g_whitepap/thermo.htm.) [I'm not sure where how the
"A" value was computed, although I have some guesses.]

...dave case