Subject: Summary - hydration energy : phosphonate anion

From: <[email-masked]>
Date: Fri, 27 Mar 1998 10:32:58 +0900

Dear all,

   A few days ago, I posted a question about hydration energy
of phosphonate(and/or phosphate) anion.

Several people have reposned to this question and I'm grateful
to them all.

My question was
>>   Does anybody have information about experimental hydration(solvation)
energy
>> of phosphonate anion or phosphate anion?
>> I calculated the solvation energy of CH3PO3(2-) by GB/SA solvation model
implemented
>> in BatchMin6.0 and it gives c.a. -1200 kJ/mol (= -300 kcal/mol).

>>It's quite large compared with a monomeric anion(e.g. experimetal energy
for
>>acetate ion is -80 kcal/mol).
>>M.E.Colvin et al. gives some theoretical results for some phosphorus
containing molecules:
>>         PO4(3-) :-629.2kcal/mol(AM1-SM2), -570.9(PM3-SM3)
>>         HPO4(2-):-299.9kcal/mol(AM1-SM2), -266.2(PM3-SM3)
>>                  (JACS Vol.117,No.15,4357-4362,(1995))
>> I don't have any further information.

1. Cory ([email-masked])
One would expect that the solvation energy would scale as the square of the
charge, from Born's equation, so roughly speaking, anions of charge -2
should
be stabilized roughly 4 times that of an anion of charge -1.
One of the problems of obtaining hydration energies of these kinds of ions
are
that they can often act as strong bases to give other species as HPO42-. To
prevent this one must use very alkaline solutions, but then ionic strength
will
increase and thus the activity coefficients may deviate significantly from
unity.

2. Lars Hemmingsen ([email-masked])
The book "Ion solvation" by Y. Marcus (John Wiley and Sons Limited, 1985)
contains a lot of references and data, for example for solvation of
PO4(3-): -260.5 kcal/mol.

3. Christopher J. Cramer ([email-masked])
You should recall that the free energy of solvation of an ion goes as the
square of the charge (cf. Born formula for charged sphere, delta-G(solv) =
(1 - 1/epsilon) * q^2/alpha, where epsilon is the dielectric constant, q is
the charge, and alpha is the sphere's radius). So, doubly charged ions
should have solvation free energies roughly 4 times those of singly charged
analogs.
Experimental data are, as far as we are aware, unavailable for ions like
those you describe, although "reasonable" estimates can be arrived at by
making certain assumptions about entropies, etc., in free energy cycles
designed to predict the data. The Colvin paper you cite does as good a job
of this as any I've seen.

With best regards,

                    Takanori

***************************************************************
                   Takanori Kanazawa, Ph.D.
                   E-mail : [email-masked]
                   Phone  : +81-797-74-2597
                   FAX    : +81-797-74-2598

                   Drug Discovery Group / CAMM
                   Takarazuka Research Institute
                   Novartis Pharma K.K.
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                   JAPAN
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